UNESCO-NIGERIA TECHNICAL &
VOCATIONAL EDUCATION
REVITALISATION PROJECT-PHASE II
NATIONAL DIPLOMA IN
SCIENCE LABORATORY TECHNOLOGY
ORGANIC CHEMISTRY II
COURSE CODE: SLT 221
YEAR II- SE MESTER II
PRACTICAL
Version 1: December 2008
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TABLE OF CONTENTS
WEEK 1
Preparation of neonerolin………………………………..
WEEK 2
Identification of ethers……………………………………
WEEK 3
Distinguishing between primary, secondary and
tertiary amines by chemical tests………………………..
WEEK 4
Comparison of the chemical properties of phenyl
amine and propyl amine…………………………………
WEEK 5
Preparation of phenacetin by acylation of pPhenetidine……………………………………………….
WEEK 6
Nitration of methylbenzoate……………………………..
WEEK 7
Preparation of sym-tribromoaniline……………………
WEEK 8
Investigation of the solubility of phenols,
alcohols and carboxylic acids……………………………
WEEK 9
Preparation of benzoin by benzoin condensation……...
WEEK 10
Preparation of benzoic acid from benzyl
Chloride…………………………………………………..
WEEK 11
Preparation of methylbenzoate by the reaction
of benzoic acid with thionyl chloride and methanol…...
WEEK 12
Preparation of benzamide from benzoyl chloride
and ammonia…………………………………………….
WEEK 13
Hydrolysis of benzamide………………………………..
WEEK 14
Preparation of aniline by reduction with tin and
Acid………………………………………………………
WEEK 15
Preparation of azo-dye…………………………………
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W EEK 1: PREPARATION OF NEONEROLIN
1.1 Introduction
This preparation illustrates the formation of ether by the Williamson reaction. The ether to be
prepared is neonerolin (iii), alternatively called β -naphthyl ethyl ether, 2-naphthyl ethyl
ether or more systematically ethoxynaphthalene. Unlike aliphatic alcohols, which require
metallic sodium for reaction to form the sodium alkoxide, the corresponding compound can
be formed from 2-naphthol (i) by reaction with potassium hydroxide solution. The potassium
naphthionate (ii) formed is then reacted with ethyl iodide to form the ether. The reaction is a
nucleophillic substitution with the naphthionate acting as the nucleophile. The carbon –
iodine bond in ethyl iodide is polarised, due to the electronegativity difference between
carbon and iodine, with the iodine atom being the centre of negative charge and the carbon
atom one of positive charge. By means of the unshared pair of electrons on the oxygen atom,
the naphthionate ion attacks this partially positively charged carbon atom to form a bond and
the iodide ion is eliminated.
1.2 Experimental procedure
1.2.1 Chemicals required
1.
2.
3.
4.
5.
Potassium hydroxide
2-Naphthol
Distilled water
Methylated spirit
Ethyl iodide
1.2.2 Apparatus required
1.
2.
3.
4.
5.
6.
100cm3 Round-bottomed flask (Quick fit)
Water condenser
Heating mantle
Glass rod
Analytical balance
Melting point apparatus
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7. Sample bottle
1.2.3 Hazards and precautions
Gloves, lab coat, and safety glasses must be worn at all times. Avoid skin contact / inhalation
of reactants. Work in the fume cupboard as much as possible. No naked flames are allowed.
1.2.4 Procedure
Weigh potassium hydroxide (2.0g, 0.04mol) and dissolve it in distilled water (8 cm3) in a
clean, dry 100cm3 round-bottomed flask (Quick fit). Next, accurately weigh 2-naphthol (3.4g,
0.024mol) and transfer it into the flask and add methylated spirit (4 cm3) and gently swirl the
flask until the 2-naphthol is dissolved. In a fume cupboard, add ethyl iodide [3.95g (2 ml),
0.025mol] and a few anti bumping granules to the mixture. Attach an upright water-cooled
condenser to the flask and heat the flask, using a heating mantle set at about 3, for 40
minutes. Remove the heating mantle from the reaction and take it away from the immediate
vicinity. Still working in the fume cupboard, cool the reaction mixture. The crude product
should precipitate - if the product is an oil, it may need "scratching" with a glass rod to
induce crystallisation or if this is not successful you may need to "seed" the solution. Take
the solution to your bench (keep it cold) and filter off the precipitated product by using
suction - do not wash the crystals. Recrystallise it from a mixture of 10 cm3 methylated spirit
/ 2 cm3 water - use the correct procedure for recrystallisation from a flammable solvent and
filter the cooled mixture while it is still cold. Record the weight and melting point of the
crystals. Hand in your product in a labelled sample bottle with your name, the compound's
name and the date clearly visible.
1.3 Assignments
1.3.1 Laboratory report
Record the weight as well as the melting point of the recrystallized product. (Do not quote
commercial chemical catalogues. Quote literature sources such as the Dictionary of Organic
4
Compounds, Handbook of Physics and Chemistry, etc). Submit your laboratory report to the
instructor in a week from the date of completion of the practical work.
Your report should be presented in essay form using reported speech indicating the date and
title of the experiment.
1.3.2 Questions and problems
Answer the following questions and submit answers along with your laboratory report.
1. Calculate the % yield of the product. Present your calculation in a manner that is easy
to follow – marks - will be lost for poor presentation (If in doubt about how yields are
calculated consult one of the references listed below).
2. Draw, by using curly arrow notation, the mechanism of the reaction leading to the
formation of ethoxynaphathalene from 2-naphthol and iodoethane.
3. If tertiary butyl iodide was used instead of ethyl iodide would the reaction mechanism
be the same as in question 2 above?
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W EEK 2: IDENTIFICATION OF ETHERS
2.1 Introduction
Ethers are, in general, inert compounds and their identification presents difficulties. Further
complications also arise with mixed ethers. In spite of this fact however, ethers can be
identified as follows:
1. Iodine dissolves in liquid ethers giving brown solutions.
2. Esters are formed with sulphuric acid and acetic acid
3. Many ethers dissolve in conc. HCl
4. Aliphatic ethers are broken down by heating with ZnCl2
5. Ethers are usually ruptured by hydrogen iodide.
6. Aryl ethers form picrates
In this experiment we shall examine some of these characteristics of ethers.
2.2 Experimental procedure
2.2.1 Chemicals required
1. Iodine
2. Diethyl ether
3. Glacial acetic acid
4. Concentrated sulphuric acid
5. Hydroxylamine hydrochloride
6.
10% Sodium hydroxide solution
7. Dilute hydrochloric acid
8. Ferric chloride solution
9. Anisole
10. Ethanol
2.2.1 Apparatus required
1. Small test tubes
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2. Round-bottomed flask with a reflux condenser and other simple distillation things.
1.2.3 Hazards and precautions
Gloves, lab coat, and safety glasses must be worn at all times. Avoid skin contact / inhalation
of reactants. No naked flames are allowed particularly because of the high inflammability of
ether.
2.2.3 Procedure
1. Dissolve 1 crystal of iodine in diethyl ether and note the brown colour. Aromatic
hydrocarbons (e.g. benzene) give purple solutions.
2. Heat under very efficient reflux 1 cm3 of diethyl ether, 4 cm3 of glacial acetic acid and
1 cm3 of concentrated sulphuric acid for 10 minutes. Distil off 2 cm3 of the liquid and
use a few drops of the liquid for the hydroxamic acid test for esters. The hydroxamic
acid test for esters is carried out in the following way: To a few drops of an ester (the
distillate you have collected is suspected to contain an ester), add 0.2g of
hydroxylamine hydrochloride and about 5 cm3 of 10% sodium hydroxide solution and
the mixture gently boiled for 1-2 minutes. Cool and acidify with dilute HCl and then
add a few drops of ferric chloride solution. A violet or deep red-brown colour
develops immediately. Note however that a similar coloration is given by acid
chlorides, acid anhydrides and many amides, but these classes of substances are
readily detected by other means and cannot be confused with ethers.
3. Dissolve 0.1g of anisole in 10 cm3 of hot ethanol and add this solution to a solution of
0.25g of picric acid in 10 cm3 of ethanol. Set aside until separation of the picrate (1:1)
compound is complete. Filter off the solid and recrystallize from ethanol, melting
point, 80oC.
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Assignments
Laboratory report
Submit your laboratory report to the instructor in a week from the date of completion of the
practical work. Your report should be presented in a table of three columns bearing the
headings: Test, Observation and Inference. Reported speech should be used in the report.
1.3.2 Questions and problems
Answer the following questions and submit answers along with your laboratory report.
1. Explain one chemical test that can be used to distinguish between ethanol and diethyl
ether.
2. The boiling point of ethanol is 78oC while dimethyl ether is a gas at room
temperature. Why is this so seeing that they have identical chemical composition?
3. (a) Give all steps of a likely mechanism for the dehydration of an alcohol to an ether.
(b) Is this the only possibility? Give all steps of an alternative mechanism. (c)
Dehydration of n-butyl alcohol gives di-n-butyl ether. Which of your alternatives
appear to be operating here?
4. In ether formation by dehydration, as in other cases of substitution, there is a
competing elimination reaction. What is this reaction, and what products does it
yield? For what alcohols would elimination be most important?
5. (a) Upon treatment with sulphuric acid, a mixture of ethyl and propyl alcohols yield a
mixture of three ethers. What are they? (b) On the other hand a mixture of tert-butyl
alcohol and ethyl alcohol gives a good yield of single ether. What ether is this likely
to be? How do you account for the good yield?
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W EEK 3: DISTINGUISHING BETWEEN PRIMARY, SECONDARY
AND TERTIARY AMINES BY CHEMICAL TESTS
3.1 Introduction
This week we shall examine how primary, secondary and tertiary amines can be distinguished
by means of chemical tests. Chemical test to distinguishing between primary, secondary and
tertiary amines involves the use of nitrous acid as detailed below.
3.1.1 Nitrous acid test
0.2g of the suspected amine is dissolved in 5 cm3 of 2M hydrochloric acid. The mixture is
then cooled in ice and 2 cm3 of ice-cold 10 per cent aqueous sodium nitrite solution is slowly
added by means of a dropper and with stirring until, after standing for 3-4 minutes, an
immediate positive test for nitrous acid is obtained with starch-iodide paper. If a clear
solution is obtained with continuous evolution of nitrogen gas the substance is a primary
aliphatic or aryl alkyl amine.
If there is apparently no evolution of nitrogen from the clear solution, add one half of the
solution to a cold solution of 0.4g of 2-naphthol in 4 cm3of 5 percent sodium hydroxide. The
formation of a coloured (e.g., orange-red) azo-dye indicates the presence of a primary
aromatic amine; in which case warm the other half of the diazotised solution and note the
evolution of nitrogen and the strong phenolic aroma which is produced. If a colourless
solution is obtained which gives an immediate and sustained positive test with starch-iodide
paper when only a little of the sodium nitrite solution has been added, the compound is a
tertiary aliphatic amine.
The presence of a secondary amine is indicated by the formation of a nitrosamine which
usually separates as orange-yellow oils or low-melting solid. Thee formation of a nitrosamine
can be confirmed by the Lieberman Nitroso reaction.
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3.1.2 The Liebermann reaction
A small amount of nitrosophenol is dissolved in a little molten phenol and concentrated
sulphuric acid is added. A magnificent cherry colour is obtained: when the melt is diluted
with water and alkali is added the colour changes to blue.
Since phenol is converted to nitrosophenol by nitrous acid, even when the latter is combined
in the form of the NO-group, the labile nitroso groups may be detected by the Liebermann
reaction.
3.2 Experimental procedure
3.2.1 Chemicals required
1. 2M hydrochloric acid.
2. Aqueous sodium nitrite solution
3. Starch-iodide paper
4. 2-naphthol
5. 5% sodium hydroxide.
6. Phenol
7. Concentrated sulphuric acid
3.2.2 Apparatus required
1. Test tubes
2. Glass rods
3. Spatulas
3.2.3 Hazards and precautions
Gloves, lab coat, and safety glasses must be worn at all times. Avoid skin contact / inhalation
of reactants.
3.2.4 Procedure
You are provided with samples of three amines which are primary, secondary and tertiary and
are labelled A, B and C, but which is which is not known. Carry out the nitrous acid test as
well as the Liebermann test if necessary in order to ascertain which is which. As well as
10
stating the class of each amine, try as much as possible to state also whether the substituents
on the nitrogen atom are alkyl, aryl or alkylaryl.
3.3 Assignments
3.3.1 Laboratory report
Submit your laboratory report to the instructor in a week from the date of completion of the
practical work. Your report should be presented in a table of three columns bearing the
headings: Test, Observation and Inference. Reported speech should be used in the report.
3.3.2 Questions and problems
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WEEK 4: COMPARISON OF CHEMICAL PROPERTIES OF
PROPYLAMINE AND PHENYL AMINE
4.1 Introduction
Primary amines are those amines in which only one hydrogen atom of ammonia has been
replaced. In aliphatic amines an alkyl group is the substituted group while in aromatic amine
it is the phenyl group. In this experiment we shall compare and contrast the properties of
primary aliphatic and primary aromatic amine. Propyl amine represents the aliphatic amine
while aromatic amine is represented by phenyl amine. Some of the general reactions of
amines are:
1. They give the isocyanide (or carbylamine reaction .
2. They form acetyl derivatives.
3. They give benzoyl and toluene-p- sulphonyl derivatives (Schotten Baumann reaction).
4. They form diazonium compounds which couple with alkaline 2-naphthol to give a red
dye.
5. They give coloured oxidation products, depending on the amine and the oxidizing
agent used.
4.2 Experimental procedure
4.2.1 Chemicals required
1. Chloroform
2. Phenyl amine
3. Propylamie
4. Alcoholic NaOH solution alcoholic NaOH solution
5. Conc. HCl
6. Acetic acid
7. Acetic anhydride
8. Methylated spirit
9. Sodium nitrite
10. 2-Naphthol
11. 10% NaOH solution
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4.2.2 Apparatus required
1. Test tubes
2. Small flask with reflux condenser
4.2.3 Hazards and precaution
Gloves, lab coat, and safety glasses must be worn at all times. Avoid skin contact / inhalation
of reactants. Work in the fume cupboard as much as possible. No naked flames are allowed.
Isocyanides are quite poisonous; considerable care must be taken in noting the odour.
4.2.4 Procedure
1. The isocyanide reaction. Add a few drops of chloroform to about 0.2 g of the
substance, and then 2-3 cm3 of alcoholic NaOH solution. Mix well and warm gently:
the foul odour of isocyanide (carbylamine) is produced. Immediately the odour of
isocyanide is detected, cool the tube and ad carefully an excess of conc. HCl: the
isocyanide is thereby hydrolysed to the odourless amine.
2. Acetylation. Place 1 cm3 of the substance in a small flask fitted with a reflux
condenser, add 5 cm3 of an acetic anhydride-acetic acid mixture (equal volumes) and
reflux gently for 15 minutes. Pour into water: the solid anilide separates. Filter off,
wash with water and recrystallize from water or dilute methylated spirit. Note the
melting point.
3. Diazotisation. Dissolve 0.2 g of the substance in 1 cm3 of conc. HCl. Dilute with
about 3 cm3 of water, cool in ice and add a few drops of sodium nitrite solution. Next
add this cold diazonium solution slowly to a cold solution of 2-naphthol in a
considerable excess of 10% NaOH solution: A brilliant red dye is produced.
4.3 Assignments
4.3.1 Laboratory report
Submit your laboratory report to the instructor in a week from the date of completion of the
practical work. Your report should be presented in a table of three columns bearing the
headings: Test, Observation and Inference. Reported speech should be used in the report.
4.3.2 Questions and problems
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W EEK 5: PREPARATION OF PHENACETIN BY ACYLATION OF pPHENETIDINE
5.1 Introduction
Preparation of pheneacetin involves treating an amine with an acid anhydride to form an amide.
In this case, p-phenetidine (p-ethoxyaniline), the amine is treated with acetic anhydride to form
pheneacetin, the amide.
5.2 Experimental procedure
Chemicals Required
1. p-Phenetidine
2. Hydrochloric acid
3. Decolourizing charcoal
4. Sodium acetate
5. Acetic anhydride
Apparatus required
1.
2.
3.
4.
5.
Spatula
Beakers
Filter papers
Erlenmeyer flask
Apparatus for low pressure filtration
Hazards and Precautions
• To avoid spills or contact with the skin, use disposable pipette to transfer the pphenetidine to the flask
14
Place p-phenetidine (2g, 0.015mol) in 35 cm3 of water and add hydrochloric acid (1.5 cm3).
This should dissolve the amine completely. If some amine remains undissolved, add a few
more drops of concentrated hydrochloric acid. Do not be concerned if some undissolved
material remains. Add a spatula of decolorizing charcoal to the solution, swirl the solution on
a steam bath for a few minutes, and remove the charcoal by gravity filtration, using fluted
filter.
Prepare a solution for use as a buffer by dissolving sodium acetate (CH3COONa.3H2O).
(2.3g, 0.02mol) in 7.5 cm3 of water and clarify the solution by gravity filtration. Set this
solution aside to use later use.
Transfer the p-phenetidine hydrochloride solution to 125 cm3 Erlenmeyer flask and warm it
on a steam cone. Add acetic anhydride (2 cm3) while swirling the solution. Add the sodium
acetate buffering solution all at once, and swirl the solution vigorously to ensure mixing.
Cool the reaction mixture by immersing the flask in an ice water-bath, and stir the mixture
vigorously until the crude pheneacetin crystallizes. Collect the crystals in a Buckner funnel
by vacuum filtration. Wash the crystals with a portion of cold water. Weigh the crude crystals
and calculate the yield.
Crystallize the pheneacetin by dissolving the crude product (1g) (solubility 1.1 g per 100
cm3) in the minimum amount of boiling water. Allow the solution to cool slowly. When the
first crystals appear, immerse the flask in an ice bath for 15-20 minutes. Collect the crystals
by vacuum filtration, using a Buckner funnel and then dry them. Record the weight of the
pheneacetin obtained in the experiment, and finally, the melting point of the recrystallized
sample. Submit the sample of the recrystallized material to the instructor in a labeled vial.
5.3 Assignments
5.3.1 Laboratory report
Record the weight as well as the melting point of the recrystallized product. (Do not quote
commercial chemical catalogues. Quote literature sources such as the Dictionary of Organic
15
Compounds, Handbook of Physics and Chemistry, etc). Submit your laboratory report to the
instructor in a week from the date of completion of the practical work.
Your report should be presented in essay form using reported speech indicating the date and
title of the experiment.
Questions and problems
Answer the following questions and submit answers along with your laboratory report.
QUESTIONS
1. p-Phenetidine is purified by adding concentrated hydrochloric acid and decolorizing
the resulting solution. Write the chemical equation for the reaction of p-Phenetidine
with hydrochloric acid. Explain why the p-Phenetidine dissolves.
2. p-Phenetidine is basic but pheneacetin is not. Explain the difference.
3. Acetaminophen has the structure shown. How might it be prepared?
4. Calculate the theoretical yield of pheneacetin if 10g of p-Phenetidine is allowed to
react with excess acetic anhydride.
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WEEK 6: NITRATION OF METHYL BENZOATE
6.1 Introduction
The nitration of methyl benzoate to prepare methyl m-nitrobenzoate is an example of
aromatic electrophillic substitution reaction, in which a proton of the aromatic ring is
replaced by a nitro group.
Many such substitution reactions are known to occur when an aromatic substrate is allowed
to react with a suitable electrophillic reagent, and many other groups besides nitro may be
introduced into the ring.
You may recall that alkenes (which are electron-rich due to an excess of electrons in the π π
system) can react with an electrophillic reagent. The intermediate formed is electron
deficient. It reacts with the nucleophile to complete the reaction. The overall sequence is
called electrophillic addition. Addition of HX to cyclohexene is an example.
Aromatic compounds are not fundamentally different from cyclohexene. They can also react
with electrophiles. However, due to resonance in the ring, the electrons of the π system are
17
generally less available for addition reactions since an addition will mean loss of the
stabilization that resonance provides. In practice this means that aromatic compounds react
only with powerfully electrophillic reagents, usually at somewhat elevated temperatures.
Benzene, for example can be nitrated at 50oC with a mixture of concentrated nitric and
sulphuric acids; the electrophiles is NO2+ (Nitronium ion), whose formation is promoted by
action of concentrated sulphuric acid on nitric acid:
The nitronium ion thus formed is sufficiently electrophillic to add to the benzene ring
temporarily interrupting ring resonance.
The intermediate first formed is somewhat stabilized by resonance and does not rapidly
undergo reaction with a nucleophile; in this behaviour, it is different from the unstabilized
carbonium ion formed from cyclohexene plus an electrophiles. In fact aromaticity can be
restored to the ring if elimination occurs instead. (Recall that elimination is often a reaction of
carbonium ions). Removal of a proton probably by HSO4-, from the sp3-ring carbon restores
the aromatic system and yields a net substitution wherein a hydrogen has been replaced by a
nitro group. Many similar reactions are known, and they are called electrophillic aromatic
substitution reactions.
The substitution of a nitro group for a ring hydrogen occurs with methyl benzoate in the same
way it does with benzene. In principle, one might expect that any hydrogen on the ring could
be replaced by a nitro group. However, the carbomethoxy group directs the aromatic
substitution preferentially to those positions that are meta to it. As a result, methyl mnitrobenzoate is the principal product formed. Additionally one might expect the nitration to
occur more than once on the ring. However, both the carbomethoxy group and the nitro group
that has just been attached to the ring deactivate the ring against further substitution.
Consequently, the formation of a methyl dinitrobenzoate product is much less favourable
than the formation of the mononitration product. While the products described above are the
principal ones formed in the reaction, it is possible to obtain as impurities in the reaction
small amounts of the ortho and para isomers of methyl m-nitrobenzoate and also of the
dinitration products. These side products are removed when the desired product is purified by
crystallization. Water has a retarding effect on the nitration since it interferes with nitric acidsulphuric acid equilibria that form the nitronium ions; therefore the smaller the amount of
water present, the more active the nitrating mixture. Also, the reactivity of the nitrating
18
mixture can be controlled by varying the amount of sulphuric acid used. This acid must
protonate nitric acid, which is a weak base, and the larger the amount of acid available, the
more numerous the protonated species (and hence NO2+) in the solution. Water interferes
since it is a stronger base than sulphuric acid or nitric acid. Temperature is also a factor in
determining the extent of nitration. The higher the temperature, the greater will be the
amounts of dinitration products formed in the reaction.
6.2 Experimental procedure
6.2.1 Chemicals required
6.2.2 Apparatus required
6.2.3 Hazards and precautions
6.2.4 Procedure
Cool concentrated sulphuric acid (12 cm3) in a 150 cm3 beaker to about 0oC and add methyl
benzoate (6.1g). Using an ice-salt bath, cool the mixture to 0oC or below and add, VERY
SLOWLY, using a Pasteur pipette a cool mixture of concentrated sulphuric acid (4 cm3) and
concentrated nitric acid (4 cm3). During the addition of the acids, stir the mixture
continuously and maintain the temperature of the reaction below 15oC. If the mixture rises
above this temperature, the formation of the by-product increases rapidly, bringing about a
decrease in the yield of the desired product.
After all the acid has been added, warm the mixture to room temperature. After 15 minutes,
pour the acid mixture over 50g of crushed ice in a 250 cm3 beaker. After the ice has melted,
isolate the product by vacuum filtration through a Buckner funnel and wash it with 25 cm3
portions of cold water and then with two 10 cm3 of ice-cold methanol. Weigh the product
and recrystallized it from an equal weight of methanol. The melting point of the recrystallized
product should be78oC. Submit the product to your instructor in a labelled vial.
6.3 Assignments
6.3.1 Laboratory reports
6.3.2Problems and questions
QUESTIONS
1. Why is methyl m-nitrobenzoate formed in this reaction instead of the ortho or para
isomer?
19
2. Why does the amount of dinitration increase at high temperature?
3. Indicate the products formed onnitration of each of the following compounds:
benzene, toluene, chlorobemzene, and benzoic acid.
20
WEEK 7: PREPARATION OF SYM-TRIBROMOANILINE
7.1 Introduction
Aniline undergoes very ready nuclear substitutions by bromine even in the cold, the bromine
atoms entering the two ortho positions and the para position with the formation of symmetric
or 2,4,6-tribromoaniline.
C6H5NH2 + 3Br2
C6H2(NH2)Br3 + 3HBr
The presence of the bromine atoms in tribromoaniline reduces considerably the basic
properties of the amino group.
7.2 Experimental procedure
7.2.1 Chemicals required
1. Aniline
2. Dilute hydrochloric acid
3. Bromine
4. Methylated spirit
7.2.1 Apparatus required
1.
500 cm3 Buckner flask
2. 150 cm3 distilling flask
3. Rubber tubings
4. Delivery tubes
7.2.3 Hazards and precautions
Gloves, lab coat, and safety glasses must be worn at all times. Avoid skin contact / inhalation
of reactants. Work in the fume cupboard as much as possible. No naked flames are allowed.
7.2.4 Procedure
A 500 cm3 Buckner flask is fitted with a cork through which passes a glass delivery tube
reaching nearly to the bottom of the Buckner funnel. The delivery tube is the connected by a
21
short piece of rubber tubing to the side arm of a 150 cm3 distilling flask, care being taken to
ensure that the two glass tubes touch one another inside the rubber connections. A narrower
glass tube is then fitted so that it reaches within 2-3 cm of the bottom of the 150 cm3 distilling
flask.
Dissolve 4 cm3 of aniline in 10 of dilute hydrochloric acid in the Buckner funnel and then
dilute the solution with 200 cm3 of water. Next place 6.4 cm3 (20.5g) of bromine (CARE) in
the distillation flask and cover it with about 40 cm3 of cold water. Adjust the position of the
tube inside the distillation flask until it nearly touches the surface of the bromine layer.
Connect the Buckner funnel to a suction water-pump, so that a steady stream of bromine
vapour is carried over from the distillation flask to the Buckner funnel, where the greyishwhite tribromoaniline soon begins to separate. Shake the contents of the Buckner funnel from
time to time to ensure an even distribution of the tribromoaniline. When the evaporation of
the bromine has caused the water in the distillation flask to become almost colourless (about
40 minutes), stop the current of air and filter off the tribromoaniline at the pump, wash well
with water and drain. Recrystallize from methylated spirit, using animal charcoal. The
tribromoaniline is obtained as colourless crystals, m.p. 120oC. Yield 8.5g.
7.3 Assignments
7.3.1 Laboratory report
Record the weight as well as the melting point of the recrystallized product. (Do not quote
commercial chemical catalogues. Quote literature sources such as the Dictionary of Organic
Compounds, Handbook of Physics and Chemistry, etc). Submit your laboratory report to the
instructor in a week from the date of completion of the practical work.
Your report should be presented in essay form using reported speech indicating the date and
title of the experiment.
7.3.2 Questions and problems
22
W EEK 8: INVESTIGATION OF THE SOLUBILITY OF ALCOHOLS,
PHENOLS AND CARBOXYLIC ACIDS
8.1 Introduction
Qualitative analysis is one aspect of the work of the organic chemist. In this kind of analysis
the organic chemist attempts to establish the qualitative nature of organic compounds by
finding out the kinds of elements that are present in the compound, whether the compound is
aliphatic or cyclic and the kinds of functional group(s) present in the compound. Standard
procedures have been established for both elemental analysis and the elucidation of the types
of functional groups present in a compound. In view of
the large number of organic
compounds known and coupled with their great diversity and variety it will be extremely
difficult, when confronted with the analysis of an unknown compound, to decide on which
tests to carry out in order to establish the family the compound belongs to. Preliminary tests
are always necessary in order to narrow down the areas of investigation of an unknown
compound. One such preliminary test is to study the solubility behaviour of the compound in
different solvents; and this is the subject of this week’s experiment. This experiment will be
restricted to the investigation of the solubility behaviour of three classes of organic
compounds: alcohols, phenols and carboxylic acids, we shall therefore consider this
behaviour theoretically first before investigating it practically.
8.1.1 Solubility behaviour of alcohols, phenols and carboxylic acids
In our discussion of solubility we shall consider the solubility of the families listed in the
following solvents:
1. Water
2. Ether
3. Dilute hydrochloric acid
4. Dilute sodium hydroxide
5. Dilute sodium hydrogencarbonate
6. Concentrated sulphuric acid
23
8.1.1.1 Solubility in water
Water is a polar solvent and it is therefore expected that salts, which are by nature extremely
polar, to dissolve in water. On the other hand non-polar substances such as hydrocarbons are
expected to be insoluble in water. Alcohols, phenols and carboxylic acids fall between these
two extremes. Each of these families contains a functional group that is polar and thus
capable of interacting with water by means of hydrogen-bonding, and thus dissolving in it.
All organic molecules can be viewed as consisting of two parts: a functional group attached
to a hydrocarbon residue. While the functional group portion of a molecule may aid water
dissolution, the hydrocarbon portion definitely opposes it. In the families under consideration,
the functional group portions aid water solubility, but as the series is ascended the
hydrocarbon portion increases while the functional group portion remains substantially
unchanged. Thus lower members of these families dissolve in water while solubility
decreases as the series is ascended.
8.1.1.2 Solubility in ether
Non-polar and slightly polar and slightly polar compounds will, in general, dissolve in ether
because they are largely unassociated. The solubility of a polar compound in ether will
depend upon the influence of the polar group relative to that of the non-polar part of the
molecule. Usually compounds that have one polar group per molecule will dissolve in ether
unless they are highly associated or of extreme polarity.
Many organic compounds that are insoluble in water dissolve in ether. If a compound is both
soluble in water and ether, it probably (i) is non-ionic, (ii) contains five or less carbon atoms
(iii) has a functional group that is polar and capable of forming hydrogen bonds and (iv) does
not contain more than one strongly polar group. If a compound dissolves in water but not in
ether, it may (i) be ionic (a salt) or (ii) contains two or more polar groups but not more than
four carbon atoms per polar group. There are exceptions to these statements.
8.1.1.3 Solubility in dilute hydrochloric acid
Most compounds that dissolve in dilute hydrochloric acid contain a basic nitrogen atom
possessing an unshared pair of electrons.
24
8.1.1.4 Solubility in dilute sodium hydroxide solution
Carboxylic acids and phenols dissolve in dilute sodium hydroxide solution since they contain
an acidic group of sufficient strength to react with the alkali. The hydrogen atoms of the
hydroxyl groups of simple alcohols are not acidic enough to react with alkali.
8.1.1.5 Solubility in dilute sodium hydrogencarbonate solution
Carboxylic acids are soluble in dilute sodium hydrogencarbonate solution with evolution of
carbon dioxide. Simple phenols are too weakly acidic to dissolve in sodium
hydrogencarbonate solution.
8.1.1.6 Solubility in concentrated sulphuric acid
Solubility in cold concentrated sulphuric acid is used to characterize further those compounds
which by virtue of the results of previous solubility tests are considered to be neutral. The
most important groups of compounds to exhibit solubility in this reagent are those containing
oxygen. The initial solubility of these compounds is due to the basic character of one or more
oxygen atoms that are present in the molecule, and results from oxonium ion formation.
8.2 Experimental procedure
8.2.1 Chemicals required
8.2.2 Apparatus required
8.2.3 Procedure
All solubility determinations are carried out at the laboratory temperature in small test tubes
(e.g. 100 x 12 mm) but of sufficient size to permit vigorous shaking of the solvent and the
solute.
An arbitrary ratio of 0.10g of solid or 0.20 cm3 of liquid for 3.0 cm3 of solvent shall be used
in the experiment. Weigh out 0.10g of the finely powdered solid to the nearest 0.01g. After
some experience subsequent tests with the same compound may be estimated by eye.
25
Measure out 0.20 cm3 of the liquid either with a calibrated dropper or with a small graduated
pipette. Use either a calibrated dropper or a graduated pipette to deliver 3.0 cm3 of solvent.
Treat a o.10g portion of the solid with successive 1.0cm3 of water, shaking vigorously after
each addition until 3.0 cm3 has been added. When dealing with a liquid, add 0.2 cm3 of the
compound to 3.0 cm3 of water and shake. In either case test the contents of the small test tube
with Universal indicator paper. When the experiment with water is finished repeat the whole
exercise with the other solvents.
You are provided with a sample of an alcohol, another sample of a carboxylic acid and a
sample of phenol, labelled respectively A, B and C, but which is which is not indicated.
Carry out solubility tests and try as much as possible to say which is which.
8.3 Assignment
8.3.1 Laboratory report
8.3.2 Problems and questions
26
WEEK
9:
PREPARATION
OF
BENZOIN
BY
BENZOIN
CONDENSATION
9.1 Introduction
Benzaldehyde does not possess alpha hydrogens and therefore does not undergo a self-aldol
condensation. In fact in strongly basic solution, Benzaldehyde, like other aldehydes that lack
alpha hydrogens, undergoes the cannizzaro reaction, yielding benzyl alcohol and sodium
benzoate:
2C6H5CHO + NaOH
C6H5CH2OH + C6H5COO-Na+
In the presence of cyanide ion, however, Benzaldehyde undergoes a unique self-condensation
reaction, called the benzoin condensation, yielding an alpa hydroxyketone called benzoin. In
this experiment we use the benzoin condensation to synthesize benzoin.
The complete mechanism for the reaction is shown below. The fist step is the formation of
the cyanohydrin 1, which in the basic reaction medium immediately forms its conjugate anion
2. The conjugate anion 2, is stabilised by resonance that involves both the cyano group and
the aromatic ring. In a second step of the reaction, the cyanohydrin anion makes a
nucleophillic addition to a second molecule of Benzaldehyde, to give the adduct 3. After a
proton transfer to form the anion, 4, cyanide is expelled, forming benzoin, an alpha hydroxyl
ketone 5.
The reaction is carried out in 95& aqueous ethanol, and the product, which is sparingly
soluble, crystallizes from the reaction mixture on cooling. The product is collected by
vacuum filtration and recrystallized fro 95% ethanol.
9.2 Experimental procedure
9.2.1 Chemicals required
9.2.2 Apparatus required
9.2.3 Procedure
Using a 100 cm3 round-bottomed flask, assemble an apparatus for heating under reflux. Place
95% ethanol (20 cm3), pure Benzaldehyde (15.0g) and a solution of sodium cyanide (1.5g in
15 cm3 of water) in the flask. Heat the mixture gently under reflux for 30 minutes and then
27
cool the flask in an ice bath (leave the condenser attached). The product should precipitate.
Collect the crude benzoin by vacuum filtration using a Buckner funnel. Wash the product
well with several portions of cold water to remove all the sodium cyanide. Set the benzoin
aside to dry. A second batch of benzoin can be obtained by concentrating the filtrate. This
should be done in a beaker in the fume cupboard. It will be necessary to use a hot plate to boil
the solution. The crystals obtained should be colourless or pale yellow. Weigh the crude
material and record the weight.
Recrystallize the crude benzoin from 95% ethanol (about 8 cm3 peer gram of crude crystal)
to yield pure benzoin (mp 134-135OC). Weigh the purified material, calculate the yield, and
determine the melting point. Place the sample in a labelled vial and submit it with your
report.
28
WEEK 10: PREPARATION OF BENENZOIC ACID FROM BENZYL
CHLORIDE
10.1 Introduction
When an aromatic compound having an aliphatic side chain is subjected to oxidation, fission
of the side chain occurs between the first and second carbon atoms from the benzene ring,
the first carbon atom thus becoming part of a carboxyl ( – COOH) . For example:
1. C6H5CH3
C6H5COOH
2. C6H5COCH3
C6H5COOH
3. C6H5CH2CH3
C6H5COOH
4. C6H5CH = CHCOOH
C6H5COOH
Such oxidations are frequently important for determining the position of a side chain relative
to other substituents in the benzene ring. The oxidations are usually carried out with a
mixture of potassium permanganate and sodium carbonate in aqueous solution, or
alternatively with dilute nitric acid (1:1) by volume. These oxidations are, however, often
very slow particularly if the side chain is a simple alkyl group. To overcome this difficulty
the alkyl group is frequently chlorinated in order to increase its susceptibility to oxidation.
Thus the side chain in toluene, C6H5CH3, is only very slowly oxidised by either of the above
reagents, whereas that in benzyl chloride is rapidly oxidised. This rapid oxidation is due to
the fact that with an aqueous oxidising agent, the benzyl chloride is first hydrolysed to benzyl
alcohol which then undergoes the normal oxidation of a primary alcohol to the corresponding
carboxylic acid.
In the following preparation, the oxidation of benzyl chloride instead of toluene is therefore
given in order to reduce the time required. It should be borne in mind, however, that the
procedure is otherwise independent of the nature of the side chain
10.2 Experimental procedure
10.2.1 Chemicals required
1. Sodium carbonate
2. Potassium permanganate
3. Benzyl chloride
4. Concentrated hydrochloric acid
29
5. Sodium sulphite
10.2.2 Apparatus required
1.
500 cm3 bolt-head flask
2. Reflux water-condenser
10.2.3 Hazards and precaution
Gloves, lab coat, and safety glasses must be worn at all times. Avoid skin contact / inhalation
of reactants. Work in the fume cupboard as much as possible. No naked flames are allowed.
Gloves, lab coat, and safety glasses must be worn at all times. Avoid skin contact / inhalation
of reactants. Work in the fume cupboard as much as possible. No naked flames are allowed.
10.2.3 Procedure
To 200 cm3 of water contained in a 500 cm3 bolt-head flask, add in turn anhydrous sodium
carbonate (5g), potassium permanganate (10g) and finally benzyl chloride [5.5g (5 cm3)]. Fit
the flask with a reflux water-condenser, and boil the mixture gently for 1-1.5 hours, i.e., until
the reaction is complete and the liquid running down from the condenser contains no oily
drops of unchanged benzyl chloride. During this boiling the permanganate is slowly reduced,
and manganese dioxide separates as a dark brown precipitate. Next cool the flask and add
concentrated hydrochloric acid (about 50 cm3) cautiously until the mixture is strongly acid,
and all the benzoic acid has been precipitated. Then add aqueous solution of crystalline
sodium sulphite (about 100 cm3 of 20% solution) slowly with shaking until the manganese
dioxide is completely dissolved and only the white precipitate of benzoic acid remains. When
the mixture is quite cold, filter off the benzoic acid at the pump, and wash well with water.
Recrystallize from boiling water. The benzoic acid is obtained as colourless needles, m.p.
121oC and yield of about 4.5g.
10.3 Assignment
10.3.1 Laboratory report
10.3.2 Problems and question
30
WEEK 11:
PREPARATION OF METHYL BENZOATE BY THE
REACTION OF BENZOIC ACID WITH THIONYL CHLORIDE AND
METHANOL
11.1 Introduction
One of the general methods for the preparation of acid chloride is the action of phosphorus
pentachloride on the corresponding carboxylic acid:
RCOOH + PCl5
RCOCl + POCl3 +HCl
One disadvantage of this method is that it is sometimes difficult to separate the acid chloride
sharply from the phosphorus oxychloride by fractional distillation, and unless the boiling
points of these two substances are fairly wide apart, traces of oxychloride will occasionally
pass over in the vapour of the acid chloride. If, however, thionyl chloride is used instead of
phosphorus pentachloride, this difficulty does not arise, as the acid chloride is the only liquid
product of this reaction.
RCOOH + SOCl2
RCOCl + SO2 +HCl
11.2 Experimental procedure
11.2.1Chemicals required
11.2.2 Apparatus required
11.2.3 Hazards and precautions
11.2.4 Procedure
This preparation must be performed in an efficient fume cupboard. To the neck of a 150 cm3
distillation flask attach a reflux single-surface water-condenser closed at the top by a calcium
chloride tube. The side arm of the distillation flask carries a cork which fits the end of the
condenser where the calcium chloride tube is for subsequent distillation. The side arm of the
distillation flask is meanwhile plugged by a small rubber coke, or by a short length of glass
rod.
Place 20g of dry powdered benzoic acid in the distillation flask, add 15 cm3 (25g, i.e., a 30%
excess) of thionyl chloride and some fragments of porcelain, and then clamp the apparatus on
a boiling water bath so that no liquid can collect in the side arm of the distillation flask. Heat
31
the distillation flask for one hour (with occasional gentle shaking), by which time the
evolution of gas will be complete. Cool the distillation flask, detach the condenser and fit it to
the side arm for distillation using a 360oCthermometer for the neck of the distillation flask.
To the lower end of the condenser fit a small conical flask by a cork carrying also a calcium
chloride tube. Distil the contents of the distillation flask by heating carefully over a gauze. A
small initial fraction of unchanged thionyl chloride boiling at 78-80oC comes over and the
temperature rises to 194o. Directly this happens stop the distillation, allow the condenser to
drain thorougly
32
WEEKS 12: PREPARATION OF BENZAMIDE FROM BENZOYL
CHLORIDE AND AMMONIA
12.1 Introduction
In the laboratory amides and esters are usually prepared form the acid chloride rather than
from the acid itself. Both the preparation of the acid chloride and its reaction with ammonia
or an alcohol are rapid, essentially irreversible reactions. It is more convenient ions take place
to carry out these two steps than the single slow, reversible reaction with the acid.
Like other acid derivatives, acid chlorides typically undergo nucleophillic substitution.
Chlorine is expelled as chloride ion or hydrogen chloride, and its place is taken by some other
basic group. Because of the carbonyl group these reactions take place much more rapidly
than the corresponding nucleophillic substitution reactions of the alkyl halides. Acid
chlorides are the most reactive of the derivatives of carboxylic acids.
In this experiment we are going to prepare benzamide by the reaction of benzoyl chloride and
aqueous ammonia.
12.2 Experimental procedure
12.2.1 Chemicals required
1. Concentrated ammonia (d. 0.880)
2. Benzoyl chloride
12.2.2 Apparatus required
1. 25 cm3 conical flask
12.2.3 Hazards and precautions
12.2.4 Procedure
Place a mixture of concentrated (d. 0.880) ammonia solution (10 cm3) and water (5 cm3) in a
25 cm3 conical flask for which a well-fitting cork is available (The large excess of ammonia
solution is employed chiefly to prevent too great a rise in temperature during the reaction).
Next add benzoyl chloride (2.4g, 2 cm3) and cork the flask securely. Shake the flask
vigorously and note that the mixture becomes hot and for this reason, throughout the shaking,
hold the cork tightly in position, with the flask pointing away from the operator (and from
33
neighbouring students!). At intervals, release the pressure by cautiously removing the cork.
After vigorous shaking for 15 minutes, no trace of oily benzoyl chloride should remain. Filter
off the fine flakes of benzamide, wash with cold water and then recrystallize from hot water.
Yield 1.5g, colourless crystals, m.p., 130oC.
12.3 Assignment
12.3.1 Laboratory report
12.3.2 Problems and questions
34
WEEK 13: HYDROLYSIS OF BENZAMIDE
35
WEEK 14: PREPARATION OF ANILINE BY REDUCTION OF
NITROBENZENE WITH TIN AND ACID
14.1 Introduction
Both aliphatic and aromatic nitro compounds can be readily reduced in acid solution to
the corresponding primary amine. Thus when a mixture of nitrobenzene and tin is treated
with hydrochloric acid the tin dissolves to produce stannous chloride, SnCl4, which in
these circumstances then reacts with more acid to give stannic chloride, SnCl4 and the
nascent hydrogen produced from these sources reduce the nitrobenzene to aniline.
3Sn + 6H+ + C6H5NO2
3Sn2+ + C6H5NH2 + 2H2O
3Sn2+ + 6H+ + C6H5NO2
3Sn4+ + C6H5NH2 + 2H2O
The stannic chloride combines with the excess of hydrochloric acid to give the complex
chlorostannic acid H2SnCl6, with which the aniline forms a salt, aniline chlorostannate,
(C6H5NH2)2H2SnCl6. The crude product is therefore made strongly alkaline with sodium
hydroxide, which liberates the free base with the formation of sodium stannate and the aniline
can then be removed by steam distillation.
(C6H5NH2)2H2SnCl6(C6H5NH2)2H2SnCl6 + 8NaOH
2 C6H5NH2 + Na2SnO3 + 6NaCl + 5H2O
Experimental procedure
Place nitrobenzene [25g (21 cm3)] and granulated tin (50g) into a 600 cm3 bolt head flask
fitted with a reflux water-condenser. Pour concentrated hydrochloric acid (20 cm3) down the
condenser and shake the contents of the flask steadily. If the heat of reaction causes the
contents of the flask to boil too vigorously, moderate the action by immersing the flask
temporarily in cold water. Then as the reaction slackens, pour another 20 cm3 of hydrochloric
acid down the condenser, and shake the flask to ensure good mixing, again cooling the flask
if the flask if the action becomes too violent. Continue in this way until a total of 100 cm3 of
hydrochloric acid has been added; then heat the flask on briskly-boiling water-bath for 20
minutes. By these means the reduction is completed by the stannous chloride present. At the
end of this time, the odour of nitrobenzene should be barely perceptible. Cool the flask in
36
water, and slowly add a solution of sodium hydroxide (75g in 100 cm3 of water) thus making
the solution strongly alkaline and liberating the aniline. Equip the flask for steam distillation
and steam distil the mixture until about 175 cm3 of distillate have been collected. The aniline
is only moderately soluble in water (giving an approximately 3% solution), and the greater
part therefore separates as oily drops in the aqueous distillate. In order to reduce further the
solubility of the aniline in water, add about 30g of powdered salt to the entire distillate and
shake thoroughly until all the salt has dissolved. In spite of the decreased solubility of the
aniline in the aqueous distillate, an ether extraction is still advisable to ensure efficient
isolation of the aniline. Therefore transfer the distillate to a separating funnel, add ether (40
cm3) and shake vigorously
Fit a dry flask (about 300 cm3 capacity) with a two-holed stopper. Fit the stopper with a
thermometer (preferably enclosed-scale type) which almost touches the bottom of the flask.
The vacant hole functions as the pressure release during the nitration. Into the flask put
benzene (25 or 22g). Notice that, by mass, the proportions used are benzene 1: concentrated
nitric acid 2: concentrated sulphuric acid 3.
To the benzene add a little (say 10 cm3) of the nitrating mixture. Shake well to emulsify the
liquids. It will be found that the temperature rises. This is permissible but only up to the limit
of 55oC. Rise above this limit must be prevented by cooling under the tap, because, at higher
temperatures, there is a tendency to produce unwanted dinitrobenzene. The ideal is to keep
the temperature at 45 – 55oC when nitration is as rapid as possible, but there is no risk of
producing dinitrobenzene. When falling temperature shows that nitration is finishing, a
further small batch of the nitrating mixture is added and so on until the whole of it has been
added. Throughout, shaking to emulsify the liquids, is necessary throughout the nitration
process.
The flask is then heated on a steam-bath for 20 minutes or so with shaking at intervals, but
the temperature must not exceed 55oC as before. This completes the nitration.
The Reduction Stage
37
Fit a round resistance flask (about 400 cm3 capacity) with a reflux air-condenser. In the flask
put nitrobenzene (20cm3) and granulated tin (45g). Measure out concentrated hydrochloric
acid (105 cm3) into a measuring cylinder.
To the nitrobenzene and tin add about 20 cm3 of the acid. Rapid effervescence will occur and
the mixture will become very hot. This is quite permissible, but cooling under the tap should
be used to check actual boiling. The air-condenser will reflux any small amount of liquid
vaporised. (Short of actual boiling, the hotter the mixture becomes the better, because the
reduction is more rapid at high temperature. Open boiling however, will cause loss of
nitrobenzene by vapourisation.) When each batch of acid is used up, as shown by the cooling
of the liquid, about 20 cm3 more are added. The flask should be shaken vigorously all the
time but with a circular motion so that the tin swirls round the sides of the flask and does not
break it When all the acid has been added., heat the flask over steam for about twenty
minutes, with occasional shaking, to complete the reduction. Then the liquid should be
homogeneous and contain no oily drops of nitrobenzene. Reduction has been brought about
partly by the conversion of tin to the tin (II) ion and partly by its further conversion to the tin
(IV) ion. In both these changes the tin is oxidised; correspondingly the nitrobenzene is
reduced. Aniline is left in solution as its chloride, excess acid being available.
Liberation of Free Aniline
Cool the flask and add to it water (50 cm3). Then, slowly, with shaking and cooling under the
tap, add sodium hydroxide solution (75g in 100 cm3 of water). The mixture should then be
strongly alkaline and the aniline should be liberated as brown oil floating on the alkaline
liquid.
At first sodium hydroxide precipitates tin (IV) hydroxide as a gelatinous white precipitate,
3SnCl4 + 4NaOH
Sn(OH)4 +4NaCl
The precipitate dissolves in excess of the alkali
Sn(OH)4 + 2NaOH
Na2SnO3 + 3H2O
Aniline is then liberated from its chloride by more alkali.
38
C6H5NH3+Cl- + Na+OH-
Na+Cl- + H2O + C6H5NH2
Purification of Aniline
39
W EEK 15: P R E P A R A T I O N O F A Z O-D Y E
15.1 Introduction
In this experiment the azo-dye methyl orange is prepared by the diazo coupling reaction. It is
prepared from sulphanilic acid and N,N-dimethylaniline. The first product obtained from the
coupling is the bright red acid form of methyl orange, called helianthin. In base helianthin is
converted to the orange sodium salt, called methyl orange.
Although sulphanilic acid is insoluble in acid solutions, it is nevertheless necessary to carry
out the diazotisation reaction in an acid (HNO2) solution. This problem can be avoided by
precipitating sulphanilic acid from a solution in which it is initially soluble. The precipitate is
a fine suspension and reacts instantly with nitrous acid. The first step is to dissolve
sulphanilic acid in basic solution.
When the solution is acidified during diazotisation to form nitrous acid,
The sulphanilic acid is precipitated out of solution as a finely divided solid, which is
immediately diazotized. The finely divided diazonium salt is allowed to react immediately
with dimethylaniline in the solution in which it was prepared.
Methyl orange is often used as an acid-base indicator. In solutions that are more basic than
pH 4.4, methyl orange exists almost entirely as a yellow negative ion. In solutions that are
more acidic than pH 3.2 it is protonated to form a red dipolar ion.
The methyl orange can be used as an indicator for titrations that have their end points in the
pH 3.2 – 4.4 region. The indicator is usually prepared as a 0.01% solution in water. In higher
concentrations in basic solution, of course, methyl orange appears orange.
Azo compounds are easily reduced at the nitrogen double bond by reducing agents. Sodium
hydrosulphite, Na2s2o4, is often used to bleach azo compounds:
Other good reducing agents, such as stannous chloride in concentrated hydrochloric acid, will
also work.
15.2 Experimental procedure
40
15.2.1 Chemicals required
15.2.2 Apparatus required
15.2.3 Procedure
Diazotized sulphanilic acid
Anhydrous sodium carbonate (1.1g) is dissolved in water (50 cm3) in a 125 cm3 Erlenmeyer
flask. Sulphanilic acid monohydrate (4.0g) is added to the solution and the mixture heated on
a steam bath until the sulphanilic acid dissolves. A small amount of suspended material may
render the solution cloudy. As a remedy a small amount activated charcoal may be added to
the solution; the still hot solution should then be gravity filtered, using fluted paper moistened
with hot water. Rinse the filter paper with a little (2-5 cm3) hot water. Cool the filtrate to
room temperature, add sodium nitrite (1.5g), and stir until solution is complete. Pour the
mixture, with stirring, into a 400 cm3 beaker containing 25 cm3 of ice water to which
concentrated hydrochloric acid 9 5cm3) has been added. The diazonium salt of sulphanilic
acid should soon separate as a finely divided white precipitate. Keep this suspension cooled
in an ice bath until it is to be used.
Methyl orange
In a test tube mix together dimethylaniline (2.7cm3) and glacial acetic acid (2.0 cm3). Add
this solution to the cooled suspension of of diazotised sulphanilic acid in the 400 cm3 beaker.
Stir the mixture vigorously with a stirring rod. In a few minutes, a red precipitate of
helianthin should form. Keep the mixture cooled in an ice bath for about 15 minutes to
ensure completion of the coupling reaction. Next add aqueous sodium hydroxide solution (30
cm3, 10%). Do this slowly and with stirring, as you continue to cool the beaker in an ice bath.
Check with litmus or pH paper to make sure the solution is basic. If it is not, add extra base.
Heat the mixture to boiling with a Bunsen burner for 10-15 minutes to dissolve most of the
newly formed methyl orange. When all (or almost all) the dye is dissolved, add sodium
chloride (10g), and cool the mixture in an ice bath. The methyl orange should crystallize.
When the solution has cooled and the precipitation appears complete, collect the product by
vacuum filtation, using Buckner funnel. Rinse the beaker with two cold portions of a
saturated sodium chloride solution ( 10 cm3 each) and wash the filter cake with these rinse
solutions.
41
Chemicals Required
1. Sodium nitrite
2. Sodium hydroxide
3. 2-naphthol
4. Methylated spirit
5. Sodium acetate
6. Concentrated hydrochloric acid
7. Benzidine
8. Apparatus Required
Procedure
In a small conical flask, dissolve sodium nitrite (3.6g,) in water (20 cm3). This is sodium
nitrite solution. Put the sodium nitrite solution aside. In a 500 cm3 beaker, dissolve sodium
hydroxide (3.83g) in the minimum quantity of water and add 2-naphthol (13.80g,) followed
by 10 cm3 of methylated spirit. Gently swirl the beaker to dissolve the 2-naphthol. Weigh
sodium acetate (20g) and add it to the solution of the 2-naphthol, stir to dissolve and increase
the volume of solution to 250 cm3 with water. This is sodium naphthionate solution. Put the
sodium naphthionate solution aside.
In another 500 cm3 beaker, mix concentrated
hydrochloric acid (12 cm3,) and water (100 cm3,) and heat the mixture to 80oC. To the
mixture, while still hot, add benzidine (4.6g,) to dissolve. Add more water (150 cm3) and cool
the clear solution to 2-3 oC. Add the sodium nitrate solution, with stirring, to the mixture
dropwise within a space of 1 minute. Benzidine is diazotized and a tetraazo derivative is
formed in solution. Leave the tetraazo solution for five minutes and then pour it, with stirring,
into the sodium naphthionate solution.
42
CONGO RED
43
COUPLING OF ANILINE DYE
44