Experiment 4: Electrophilic Substitution,
Nitration of Methyl Benzoate
In this experiment we will nitrate methyl benzoate, an ester of benzoic acid. This
reaction is an example of electrophilic aromatic substitution.
O
O
O
O
HNO 3
H2SO 4
NO 2
Background:
Mechanism: Electrophilic Aromatic Substitution
The regiochemistry of an electrophilic aromatic substitution (EAS) reaction, that is,
where on the ring the new group is attached, is determined by the other functional groups
already attached to the ring. These may be ortho-para directing (electron donating), or meta
directing (electron withdrawing).
.
Ortho
substituents
Meta
substituents
Para
substituents
In addition to their directing effect, electron donating groups also activate the aromatic
ring to electrophilic substitution. Resonance structures placing a negative charge at the ortho
and para positions can be drawn and are shown below. The positively charged electrophiles
more likely to react at a center that is more negatively charged and substitution occurs at the
ortho and para positions. Conversely, electron withdrawing groups deactivate the ring to
electrophilic substitution. In this case, the resonance structures drawn below show a positive
charge at the ortho and para positions of the aromatic ring. These are now unattractive sites for
the positively charged electrophile to react. The uncharged meta positions are more reactive by
comparison.
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Ortho-para directing --Functionality is electron donating
X
X
X
X
Meta directing -- Functionality is electron withdrawing
O
O
X
O
X
X
O
X
The mechanism of an electrophilic, aromatic substitution reaction goes through a
tetrahedral intermediate as shown below. In your lab report you will write a full mechanism
using electron pushing arrows.
X
X
A
+ E+
-H+
E
H
E
The actual nitrating agent in nitration of aromatic compounds is the nitronium ion, NO2+
derived from nitric acid by the action of sulfuric acid:
O
HO
OH
S
O
Sulfuric Acid
H2O
N
O
O
+
HO
N
O
O
O
O
S
OH
+ H2O
O
N
O
O
Nitric Acid
O
N
O
+
H2O
Nitronium Ion
Active Electrophile for Nitration
45
Techniques:
Recrystallization:
Purification of a solid by recrystallization from a solvent relies upon the fact that
different substances are soluble to differing extents in various solvents. As well as the fact that
solubility often increases greatly with increasing temperature. In the simplest case, all of the
impurities are much more soluble than the desired compound and present in much smaller
quantities. In this case, the sample is dissolved in just enough of the hot solvent to form a
saturated solution. When the hot solution cools slowly, pure crystals form and may be filtered
off. The impurities do not crystallize out and stay dissolved in the solvent. If insoluble
impurities are present in the sample, they are removed by filtering the hot solution by gravity
before it is allowed to cool.
Flow Chart of Recrystalization Procedure
Material to be Purified
Desired Compound
Soluble Impurities
Insoluble Impurities
Dissolve material in
hot solvent and filter
by gravity filtration.
Filtrate
Desired Compound
Soluble Impurities
Insoluble Material
Removed by the filter
May be discarded
Set aside to cool for
crystallization, collect
crystals by suction filtration
Filtrate
Soluble Impurities
May contain enough
product for a second
crop of crystals
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Crystals of Desired Product
Filtered off, washed and dried.
Check purity by melting point
Choice of Solvent: The choice of solvent is crucial in purification by recrystallization,
but there is no easy way to know which solvent will work best. If you wish to recrystallize a
known compound, the lab manual or the chemical literature may recommend or report the use
of a certain solvent. Sometimes however, you must experimentally determine what solvent will
be suitable.
Dissolving the sample: After a solvent has been chosen a solution of the sample in the
hot solvent must be prepared. You should keep in mind the possibilities that the compound may
dissolve only slowly in the boiling solvent and that “insoluble” impurities may be present in
your sample. Forgetting about this can lead to adding too much solvent and low or no yield.
Choose an Erlenmeyer (conical) flask of an appropriate size so that it will be less than
half filled with solution. Place the solid in the flask, add about 75% of the amount of solvent
thought to be required and bring the mixture to a boil using a steam bath or sand bath. If a little
more solvent is needed, add it in small amounts down the side of the flask until all of the solid
has dissolved.
Hot Filtration: Hot filtration is only necessary if insoluble impurities are present.
When the desired substance is in solution in the hot solvent, insoluble impurities
(including dust, pieces of filter paper, glass, or cork) can be separated by gravity filtration.
Vacuum filtration cannot be used, because the reduced pressure in the suction flask can cause
the filtrate to boil and material in solution will be deposited over the walls of the flask and lost.
The main problem in hot filtration is that the hot solution cools a little before it runs
through the filter. This means that some crystallization can take place in the filter or funnel
You can try to avoid this undesired crystallization by warming the funnel (with steam, quickly
wiping it dry with a towel) and pouring only a little of the solution into the filter at a time,
keeping the remainder at the boiling point. When hot filtration is necessary, it is a good idea to
use a small excess (10-25%) of solvent so that the solution will not become saturated until the
temperature has fallen somewhat below the boiling point. A stemless funnel is always
recommended for filtering recrystalizations so that you can avoid the problem of crystallization
and subsequent clogging in the stem.
The filtrate should be reheated, if necessary, to dissolve any crystals that may have
formed and to give a clear solution. Crystals that form when the hot filtrate hits the cold flask
are likely to be less pure than those that separate more slowly from solution.
Cooling: Slow, undisturbed cooling is the key to pure crystals.
The rate of cooling determines the size of the crystals. Slow cooling tends to favor
fewer and larger crystals, fast cooling tends to favor more and smaller crystals. Needles
between 2 and 10 mm in length or prisms 1 to 3 mm in each dimension are a good target size.
The rate of cooling required for good crystals varies greatly from one compound to another.
Setting the hot flask on a cool bench top or cork ring and leaving it to cool to room temperature
is a good place to start. Stirring breaks up crystals and should be avoided. A final cooling to
0°C in an ice bath can greatly increase the yield in some cases, especially if the solvent used is
not very high boiling.
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Be careful not to filter your recrystallizations too soon, this can result in loss of product!
Cold Filtration:
When crystallization is complete, the product is collected by suction filtration (see the
section on filtration). The size of the funnel used should be such that it will not be more than
half filled with crystals. The suction flask should be large enough so that the solution will not
fill it above the tip of the funnel or the side arm.
If the crystals have formed a cake at the bottom of the flask, break it up with a stirring
rod. Swirl the flask around to suspend the crystals in the solvent and quickly pour the slurry
into the funnel. Scrape any remaining crystals into the funnel with a spatula and, if necessary
rinse the remainder into the funnel with a tiny quantity of ice cold solvent or filtrate.
Washing the crystals:
After the crystals have been transferred to the funnel for suction filtration and the liquid
has been drawn off, a small amount of fresh, ice cold, solvent should be poured over the
crystals in order to wash off the liquid that contains the soluble impurities. If this is not done,
the soluble impurities will be deposited on the crystals when the solvent evaporates. If the
product is relatively soluble in the cold solvent, one washing will usually have to suffice. Two
washes are generally appropriate. More thorough washing is sometimes possible if the desired
product is not very soluble in the wash solvent.
The filter cake should be carefully broken up and suspended in the wash solvent so that
the whole surface is washed evenly. To do this well you will probably have to break the
vacuum on to the flask, stir the crystals in the funnel and then reapply the vacuum.
Drying the Crystals:
After you have collected the crystals by suction filtration, remove as much solvent
from the product as possible by continuing to draw air through the crystals while they remain
on the filter paper in the funnel. The last traces of solvent will evaporate when the crystals are
removed from the funnel and spread out to dry.
If the crystals collect as a solid cake, air should be drawn over them in the funnel until
the solvent has almost stopped dripping. Failure to suck the filter cake as dry as possible
before breaking it up and spreading it out to dry is a very common mistake. The filter cake
should be a damp, crystalline solid, not a paste or mush, when spread out to dry.
Simplified Recrystallization Procedure (when insoluble impurities are not present):
1. Dissolve the solid sample in a minimum amount of boiling hot solvent with a boiling chip or
stir bar present.
2. Cool the solution slowly to room temperature without stirring and then in an ice-water bath
to induce crystallization. Scratch the glass if necessary to initiate crystallization.
3. Vacuum filter the mixture to collect the crystals formed.
48
4. Wash the crystals with a small amount of ice cold solvent.
7. Allow the crystals to dry by pulling air through them.
8. Weigh the dried crystals.
Possible Difficulties:
Oiling Out:
Sometimes, during cooling for crystallization, the product separates not as crystals but
as a liquid, an “oil”. This shows up first as cloudiness or opalescence, and then as visible
droplets. An oil is undesirable because it usually contains impurities. When, or if, the oil
finally freezes, the impurities that have dissolved in the oil will remain in the crystals.
Ways to initiate crystallization at the first sign of oiling out include: scratching the
walls of the flask with a stirring rod and vigorous stirring of the solution. The remainder of the
product will usually separate as crystals if the rate of cooling is not too great. If oiling out
cannot be prevented-that is, if most of the product separates as an oil before it can be caused to
solidify, you can hope that recrystallization of the solidified oil will give a better result, you can
try a different solvent, or, probably best, you can purify the product by another method before
attempting to recrystallize it.
Filtration
The separation of insoluble solid materials from a liquid is called filtration. Liquid
passes through a porous barrier (sintered glass or filter paper), which retains any solids. The
liquid can pass through the barrier by gravity alone, in which case the procedure is called a
gravity filtration, or the liquid can pass through due to a combination of gravity and air
pressure, in this case called suction, or vacuum, filtration.
A3.1 Gravity Filtration
Medium to Large Scale
Gravity filtration of larger samples is best done with a piece of filter paper and a conical
glass funnel. To maximize the rate at which liquid flows through the filter paper, fold the paper
as indicated in Figure 5.1. Drop the folded paper into a glass funnel (see Figure 5.2). The
funnel is best supported in an iron ring, as shown. Pour the material to be filtered into the filter
paper cone, in portions if necessary. Use a stemless funnel for hot filtration during
recrystallization, or in removal of a drying agent from a solution.
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Microscale
For small filtrations, as in microscale work, a Pasteur pipette can be used instead of a
glass funnel. A tiny piece of glass wool pushed down into the narrow part of the tip serves as
the filter. This method of gravity filtration is preferred for quickly removing small amounts of
solid from a small volume (< 5 mls) of liquid sample.
A3.2 Vacuum or Suction Filtration
In vacuum or suction filtration, a partial vacuum below the filter causes an increased
rate of flow through the filter. A typical apparatus is illustrated below.
For a suction filtration, use a circle of filter paper just large enough to cover the holes in
the bottom of the Hirsch or Buchner funnel. A common error is to try to use a piece of filter
paper so large that it must be turned up at the edges. Doing this makes it almost impossible to
create a vacuum in the suction flask. Not only will the filtration take much longer, but also any
material that flows over the edge of the filter paper will run down into the suction flask without
being filtered.
To prepare for suction filtration, connect the side arm of a suction flask to a source of
vacuum, which is almost always a water aspirator. When a water aspirator is used, the flask
should be connected to the aspirator through a trap, as shown in the diagram below. The trap
prevents water from the aspirator from being sucked back into the filter flask. Use a rubber
adapter between the funnel and flask to form a seal.
Turn the aspirator on to create a gentle vacuum, and wet the filter paper with a small
portion of the same solvent used in the solution being filtered. Make sure that the paper is being
pushed down over the holes, and pour the mixture to be filtered onto the center of the paper.
Once the mixture has been added, the vacuum can be increased. Sometimes you may need to
press your funnel down on the neck of the suction flask to speed filtration.
50
When using the water aspirator, be sure to break the vacuum by disconnecting the tubing
attached to the side arm of the filter flask before turning off the water.
Melting Point Determination
The melting point of a solid is a good way of confirming its identity and purity. Unlike Rf,
melting points are constant and easily reproducible under most laboratory conditions. If the
product of your reaction has the same melting point as the expected product, it is very likely
that your reaction was successful. The normal melting point of a solid is defined as the
temperature at which the solid and liquid are in equilibrium at a total pressure of 1 atmosphere.
Since the melting point of a solid can be easily and accurately determined with small amounts
of material, it is a very common physical property used for the identification and
characterization of solids.
Experimental Determination of the Melting Point
There are several methods by which melting points can be determined, and the choice of
method depends mainly upon how much material i available. We will focus on capillary
melting points, the technique used in this laboratory.
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Capillary Melting Points:
Capillary melting points, either in an oil bath or a melting point apparatus, are most often
used for the determination of the melting point of a solid. A few crystals of the compound are
placed in a thin-walled capillary tube 10-15 cm long, about 1 mm in inside diameter, and closed
at one end. The capillary, which contains the sample, and a thermometer are then suspended so
they can be heated slowly and evenly. The temperature range over which the sample is observed
to melt is taken as the melting point.
The thermometer and sample must be at the same temperature while the sample melts, so
the rate of heating must be slow as the melting point is approached (about 1 degree per
minute). Otherwise, the temperature of the thermometer bulb and the temperature of the
crystals in the capillary may not be the same.
If the approximate temperature at which the sample will melt is not known, determine a
preliminary melting point determination by allowing the temperature of the sample to rise
quickly. Then carry out a more accurate determination on a different sample from the same
source, heating slowly as the temperature nears the melting point.
Filling a Capillary Tube: Usually, the melting point capillary can be filled by pressing the
open end into a small pile of the crystals of the substance to get some into the tube, turning the
capillary open end up, and tapping the closed end on the tabletop to move the crystals to the
bottom of the tube. If this does not work, drop the tube, open end up, down a length of glass
tubing about 1 cm in diameter and a meter long onto a hard surface. The solid should be tightly
packed to a depth of 2-3 mm. Too much solid will cause the melting point range to be
artificially broadened.
In the melting point apparatus that you will be using, the sample and thermometer are
both supported in an electrically heated metal block and the sample in the capillary, up to three
at a time, can be observed through a magnifying glass. You can heat the block rapidly when the
temperature is well below the melting point, and slowly as the melting point is approached.
Sometimes compounds decompose upon heating, rather than simply melting. If a compound
begins to decompose near the melting point, the capillary with the sample should be placed in
the melting point apparatus after the temperature has been raised to within 10 degrees of the
expected melting point, so as to minimize the length of time that the sample is heated.
Melting Ranges: Although there should be a single temperature at which a pure solid and a
liquid are in equilibrium, most samples appear to melt over a small temperature range, usually
2-3 degrees. This happens because, with capillary or block melting points, the temperature of
the bath or block rises a little during the time it takes the sample to melt. The presence of
impurities in the sample can also cause the sample to melt over a range of temperatures. Thus,
the “melting point” will usually be reported as a melting range, the temperatures between which
the sample melted.
Melting Point as a Criterion of Purity: Melting points are lowered by impurities, this is
why salting an icy sidewalk in winter can melt the ice. Impurities can also cause the melting
range to broaden. Therefore, a sharp melting point (actually, a melting range of less than about
1-2°C) is often taken as evidence that the sample is fairly pure, and a wide melting range is
evidence that it is not pure.
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Melting Point as a Means of Identification and Characterization: If two samples of
the same molecular formula have different melting points, their molecules must differ either in
structure or in configuration. They must be either structural isomers or diastereomers. If the
melting points of two samples with the same molecular formula are the same, the structures of
their molecules must be the same, although they might have enantiomeric configurations.
These statements apply only to pure substances, and do not take into account the fact that some
substances can exist in different crystalline forms that have different melting points.
Melting Point and Molecular Structure: Systematic variations of melting point with
changes in structure are not as obvious or predictable as are the variations in boiling point.
However, there are some general trends. Differences in molecular weight cannot be relied
upon to predict melting point. As with boiling points, compounds with polar functional groups
generally have higher melting points than compounds with nonpolar functional groups. In
contrast to the case with boiling points, highly branched or cyclic molecules (relatively
symmetrical molecules) tend to have higher melting points than their straight chain isomers.
Procedure:
O
O
O
O
HNO 3
H2SO 4
NO 2
Think about it: Do you expect ortho/para or meta substitution for this reaction? Why?
Safety Precaution: Use care in handling concentrated sulfuric and nitric acids as both are
strong acids. Nitric acid reacts vigorously with many organic compounds. Additions of
nitric acid to organic compounds must be done under very carefully controlled
conditions. If you get any acid on yourself notify your TA immediately. Any acid spills
should be neutralized with sodium bicarbonate powder and cleaned up. Wear old
clothing to lab, strong acid leaves holes.
Using an Eppendorf pipette, place methyl benzoate (0.219 grams, 0.200 mL, 1.6
mmole) in a 150 mm test tube along with a micro stir bar. Cool the liquid by placing the test
tube in an ice bath on a magnetic stir plate to provide continuous stirring. Check to make sure
that your stir bar has enough space to stir in the test tube. “Calibrate” a Pasteur pipette by
sucking up 1 mL of water, as measured by your graduated cylinder, marking the pipette and
expelling the water. The pipette should be about half full for 1.0 mL. In the same way,
calibrate a pipette for 0.5 mL.
53
Add 1.0 mL of concentrated sulfuric acid using the “calibrated” Pasteur pipet. After
allowing the mixture to cool for a minute, keeping it in the ice bath, use the second calibrated
Pasteur pipet to carefully add 0.5 mL of concentrated nitric acid very slowly, dropwise with
continuous rapid stirring. The addition should take about 3 minutes. After the addition is
complete, let the reaction stir in the ice bath for 15 minutes, and then allow it to warm to room
temperature for another 15 minutes.
Pour the reaction mixture onto about 2.5 grams of ice in a small beaker. After stirring
for approximately 5 minutes, filter (by suction filtration) the mixture using a Hirsch funnel.
Wash the solid with a small amount of cold water to remove any remaining acid by gently
pouring the water through the solid in the Hirsch funnel. Air dry the sample for several
minutes by pulling air through it in the Hirsch funnel. Carefully transfer the air-dried product
to a small (10 or 25 mL) clean, tared (pre-weighed) Erlenmeyer flask. Weigh the product to
determine your yield of crude product. Save a small sample of the crude solid as a backup in
case you need it for melting points and NMRs later, just a spatula tip full in a vial should do.
Recrystallize the solid from a small amount (about 3 mL or less) of methanol. Weigh
the recrystallized product to determine your final yield, and measure the melting point. Prepare
a 1H NMR sample (review the technique from Experiment 2 if necessary) and submit it to
obtain an NMR spectrum on your product.
In Your Lab Report:
Report your crude and purified yields. What sources of error could have reduced or
artificially increased your yield? Does your melting point match the literature value?
Analyze your NMR spectrum. Is your product ortho, meta, or para substituted? How
can you tell? Assign all of the signals in the spectrum to protons in the product by drawing the
structure on the spectrum and lettering the hydrogens and peaks (hydrogen A is peak A, etc.).
Identify any impurities that you can.
Write and discuss the mechanism of the reaction. Explain the results of the experiment
in terms of this mechanism.
54