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Lecture Notes 4 Series: Organic Chemistry
Question 4.1
(a)
Predict the shape of the methane molecule.
tetrahedral
(b)
What is the hybridisation at the carbon atom in methane?
sp
3
Question 4.2
Draw a Lewis dot-cross diagram for ethane.
H
x
H C
x
H
x
H
C xH
x
x
x
x
H
Question 4.3
(a)
The next member of the family contains four carbon atoms. Name it.
butene
(b)
An alkene has the formula C8H16. What is its name?
octene
Question 4.4
The first member of this family is ethyne, C2H2. Draw this molecule, clearly showing the orbitals involved in the
bonding.
unhybridized py orbitals
π
π
σ
σ
π
H
π
H
C
C
π orbitals
Question 4.5
(a)
(b)
Classify each of the following molecules as an alkane, alkene or an alkyne
(i)
C2H4 alkene
(ii)
C7H12 alkyne
(iv)
C10H18 alkyne
(v)
C18H38 alkane
(iii)
Consider the following molecule
H
1
2
C
C
H
H
H
C
C
C
3
4
H
H
H
C
C
H
H
5
H
C22H44 alkene
What is the hybridisation at each of the numbered carbons?
C1 and C2 sp, C3 and C4 sp2, C5 sp3
(c)
Complete the following table
Molecule
(d)
Name
Hybridisation at carbon
3
C2H6
Ethane
sp
C2H2
Ethyne
sp
C2H4
Ethene
sp2
Name the following molecules
(i)
C4H6 butyne
(ii)
C10H20 decene
(iii)
C7H14 heptene
Question 4.6
What are the molecular formulae of the following compounds?
C6H14
(i)
(ii)
C9H18
C19H40
(iii)
Question 4.7
Calculate the oxidation number of carbon in the following compounds.
(a)
methane -4
(b)
methanol -2
(c)
methanal 0
(d)
methanoic acid +2
Question 4.8
Name the following molecules.
(a) CH3CH2CH2OH propanol (b) CH3CH2CH2COOH butanoic acid (c) CH3CH2CH2CH2CHO pentanal
Question 4.9
(a)
Name the following substituent groups.
(i)
(b)
-C3H7
propyl
(ii)
-C4H9
butyl
Name the following esters.
(i) CH3CH2CH2COOCH3 methyl butanoate (ii) CH3COOCH2CH2CH3 propyl ethanoate
Question 4.10
Name the following molecules.
(a)
C
C
C
C
C
C
C
C
C
C
C
3-ethyl-4-methyloctane
(b)
C
C
C
C
C
C
2,2,5-trimethylheptane
C
C
C
C
c)
CH3
CH3
CH3
C
C
C
C
3-ethyl-3,4-dimethylhexane
C
CH2
CH3
Question 4.11
(a)
Draw the structure of the following molecules
(i)
3-methylhexane
C
C
C
C
C
C
C
C
C
C
C
C
(ii)
C
3-ethyl-2-methylhexane
C
C
C
(iii)
3-ethyl-4,7-dimethylnonane
C
C
C
C
C
C
C
C
C
C
C
C
C
(b)
Give a systematic name to isobutane.
2-methylpropane
Question 4.12
Name the following compounds.
(a)
CH3CH2
CCH3
butanone
(b)
CH3CHClCH2COOH
3-chlorobutanoic acid
O
(c)
ClCH2COOH
chloroethanoic acid
(d)
ICH3
C(CH3)2
CH2
iodomethane
Question 4.13
Name the following molecule.
Cl
CH2
CH
CH2
COOH
C2H5
5-chloromethyl-3,3-dimethylheptanoic acid
Question 4.14
List the following alcohols as 1o, 2o or 3o alcohols.
CH3
(a)
o
CH3CH2OH 1
(b)
CH3CH2CH(OH)CH3 2
o
CH3
(c)
C
OH 3o
CH3
Question 4.15
In how many ways can you arrange the atoms for a molecular formula of C5H12 i.e. how many isomers can you
draw? three
Question 4.16
(a)
Draw all the constitutional isomers for the molecular formula C4H10O.
(b)
Name all the compounds in part (a).
[Hint: this question combines the three forms of constitutional isomerism mentioned- different
carbon skeletons, different functional groups, and the different position of the functional groups.]
CH3
O
C2H5
C3H7 methylpropylether
O
C
C
C
C
C2H5 diethylether
C
C
C
C
OH butan-1-ol
butan-2-ol
OH
C
C
C
2-methylpropan-2ol
C
OH
C
CH3
O
isopropylmethylether
C
C
Question 4.17
Draw all the possible constitutional isomers for the molecular formula C6H12.
[Hint: remember cycloalkanes have the same carbon:hydrogen ratio as alkenes and you can draw more than
one cycloalkane here!].
There are 25 different isomers
Question 4.18
Write down and name the geometric isomers possible for the following molecules.
(a)
1,2-dichloroethene
H
Cl
C
H
Cl
C
Cl
C
Cl
trans-1,2-dichloroethane
H
C
H
cis-1,2-dichloroethane
(b)
1-bromopropene
(c)
pent-2-ene
Same as 4.18(a) above
Question 4.19
(a)
Draw the two geometric isomers: trans-1,2-dimethylcyclopropane and cis-1,2-dimethylcyclopropane.
CH3
CH3
CH3
cis-
trans-
CH3
(b)
Does the molecule 2-methylbut-2-ene exhibit geometric isomerism? If so, draw the isomers.
No
(c)
Is there a possibility of geometric isomerisation for the molecule 1,2-dichlorocyclohexane?
Yes
Question 4.20
Consider the following molecule
CH3CH(NH2)CH2CHCHCH(CH3)CH3
(a)
Draw an extended structure for the molecule, i.e. show the bonds.
NH2
H 3C
C*
CH2
H
H
H
C
C
C
H
CH3
CH3
(b)
Identify the functional groups present in the molecule. amine, alkene
(c)
Will this molecule display geometric isomerism? Explain your answer. Yes: we have the possibility of
cis/trans isomerism around the double bond.
(d)
Does this molecule have a chiral centre(s)? Indicate them clearly on your drawing of the structure.
Yes: see* in structure.
Question 4.21
How many electrons are in the following bonds?
C
(i)
C
2
(ii)
C
C
4
(iii)
C
C
6
Question 4.22
(a)
How many electrons are around carbon in each of the following species
(i)
(b)
CH3• 7 (ii)
CH3- 8 (iii)
CH3+ 6
Would you expect these species to be reactive? Explain your answer. If carbon doesn’t form 4 bonds
then it will be reactive.
Question 4.23
(a)
Identify the nucleophile and the electrophile in the previous reaction.
NH3 is the nucleophile and BCl3 is the electrophile
(b)
Identify each of the following species as an electrophile or a nucleophile.
(i)
-
I
-
-
(ii)
HO
-
(iii)
-
+
CH3
+
(iv)
CH3
-
(v)
H
+
(iv)
AlCl3
+
I , OH and CH3 are nucleophiles; CH3 , H and AlCl3 are electrophiles
Question 4.24
If the bond energy of Br2 is 229 kJ mol-1, what is the wavelength of the lowest energy photon which can
dissociate Br2? 522 nm
Question 4.25
Identify areas of positive charge and areas of negative charge by using δ+ and δ- terminology in the following
molecule.
+
δ
δ
δ
δ
H2N C C CH CH2 CH CH2
Clδ
Question 4.26
Will alkenes and alkynes be susceptible to nucleophilic or electrophilic attack? Explain your answer.
Electrophilic attack; they are areas of electron density
Question 4.27
Consider the following molecule.
H
H
C
Br
H
Suggest ways that this molecule might undergo bond breaking.
H
H
C
H
Br
or
H
H
C
Br
H
Question 4.28
Arrange the following carbocations in order of increasing stability. Label each of the carbocations in the
question above as 1o, 2o or 3o.
CH3+
CH3+
CH3CH2+
<
methyl
CH3CH2+
(CH3)3C+
<
1o
(CH3)2CH+
(CH3)2CH+
<
(CH3)3C+
2o
3o
Question 4.29
Which of the following carbocations is the more stable?
ClCH2CH2
+
or
CH3CH2
+
Explain your answer clearly.
CH3CH2+ is more stable as the electronegative Cl-substituent in the other carbocation removes electron density
from the carbocation centre destabilising it.
Question 4.30
Are carbon radicals more like carbocations or carbanions? Thus, does their stability sequence resemble that of
carbocations or carbanions?
The radicals are electron-deficient (see Q6.2) and therefore their relative stabilities will mimic the
carbocations.
Question 4.31
The previous reaction can also give rise to dichloromethane. Write a suitable reaction to form this product.
Cl2 + hυ → Cl•
CH4 + Cl• → CH3• + HCl
CH3• + Cl2 → CH3Cl + Cl•
CH3Cl + Cl• → CH2Cl• + HCl
CH2Cl• + Cl2 → CH2Cl2 + Cl•
Question 4.32
(a)
o
o
o
What order of stability would you predict for 1 , 2 and 3 carbanions? Justify your answer.
3
o
< 2
o
< 1
o
Carbanions have an electron-rich centre. Any electron-donating groups will thus make it less stable.
(b)
What would be the favoured product of the reaction between isobutane and Cl2? Again, justify your
answer.
Cl
H3C
C
H
rather
than
CH3
CH3
ClH2C
C
CH3
CH3
(think about the stability of the electron-deficient radical intermediate)
(c)
The reaction between Cl2 and butane also gives rise to 3,4-dimethylhexane as one of the minor
products. Suggest a reaction mechanism which will give rise to this product.
CH3
CH2
CH
HC
CH3
CH3
CH2
CH2
CH3
CH3
CH2
CH
CH3
CH3
CH2
CH3
3,4-dimethylhexane
Question 4.33
(a)
Which is the more stable carbocation in the previous example, A or B? B
(b)
Which will be the preferred product of this reaction, 1-bromopropane or 2-bromopropane? [Note we
will get both products but one will be the major product and the other the minor product]
2 – bromopropane
(c)
1-bromopropane and 2-bromopropane are isomers. Which type of isomers are they?
constitutional isomers
Question 4.34
How many products will be observed in the following reactions? Justify your answer by drawing suitable
reaction mechanisms for the formation of each product. If more than one constitutional isomer is predicted
indicate which will be the major product.
(a)
2-methylbut-2-ene + HBr
3 products
C
C
CH3
H
Br
H
CH3
H 3C
C
CH2
CH3
CH3
CH3
C
CH3
CH
CH
CH3
Br-
H
C
CH3
C
H3C
H
C
H CH3
H
Br
-
Br
CH3
C
Br
CH3
CH3 CH CH* CH3
CH2CH3
Br
major
C
+ HBr
CH3
H5C2
CH3
C
C
H3C
CH3
H
CH3 H
Br
H5C2
C
C
CH3
CH3
-
Br
H
CH3
CH3
C
C
C2H5
CH3
-
H
H3C
C
C2H5 Br
minor
Br
CH3
C
Br
C * CH3
R & S - racemate
CH3
C
CH2
H
Br
minor (R & S)
CH3CH2
H3C
H
H3C
CH3
(c)
trans - but-2-ene + HBr
H
CH3
H 3C
(b)
CH3
CH3
H5C2
C
Br
H
* C
CH3
major (R & S)
CH3
Question 4.35
Ozonolysis of alkene A gave the following aldehydes. Specify the reaction conditions and deduce the structure
of alkene A.
A → CH3CH2C(CH3)2CHO and (CH3)3CCHO
CH3CH2C(CH3)2CH
CHC(CH3)3
O3
products
Zn/CH2Cl2
Question 4.36
Assign an oxidation number to carbon in the following compounds
(i)
CO +2 (ii)
CH3OH -2
(iii)
CH2O
0
(iv)
HCOOH +2
Question 4.37
Name the product when propan-1-ol is treated with CrO3/H+.
propanal
Question 4.38
If we react a tertiary alcohol with CrO3/H+, what product would you predict (use tert butylalcohol as an
example)? Hint: be careful!
It won't react!
Question 4.39
In fact, if we treat a 1o alcohol with excess CrO3/H+ then it will be oxidised all the way through to the
corresponding carboxylic acid. However, it we treat a 2o alcohol with excess CrO3/H+ the reaction stops at the
formation of the ketone. Explain.
o
1 alcohol → aldehyde → carboxylic acid
2o alcohol → ketone → would require to break a C – C to form a carboxylic acid
Question 4.40
o
o
o
Designate what type of carbocation (1 , 2 or 3 ) the following alkyl halides will give rise to if they undergo an
SN1 type of reaction.
o
CH3CH2I 1
CH3C(CH3)2CH2I
1
o
CH3CH2C(CH3)2I
3
o
CH3CH2CH(CH3)I 2
o
Question 4.41
How many reactants are involved in the formation of the reaction intermediate in an SN2 type reaction? two
Question 4.42
-
(a)
Is the anion R – C ≡ C an electrophile or a nucleophile?
(b)
Is the anion R – C ≡ C an acid or a base?
-
Question 4.43
Would you predict the above reaction to go via an SN1 or an SN2 mechanism? Explain your answer.
SN2:
the intermediate carbocation for an SN1 mechanism is an unstable 1o carbocation
Question 4.44
Predict the products from reaction of hex-1-yne with H2 over
(a) Lindlar catalyst hex-1-ene
(b) Pd/C catalyst hexane
Question 4.45
Predict the products from reaction of dec-5-yne with
(a) H2, Lindlar catalyst
cis – dec–5–ene
(b) H2, Pd/C decane
Question 4.46
Predict the products from reaction of pentan-1-ol with
(a) CrO3 pentanoic acid
(b) PCC in CH2Cl2 pentanal
Question 4.47
In Example 4.8 we broke the C4-C5 bond to choose our starting materials.
(a)
Still by breaking the C4-C5 bond, choose another alkyl halide and another alkyne which will give the
same product.
CH3CH(CH3)C
(b)
C
H + C2H5Cl
Do a retrosynthetic analysis to make 2-methylhexane by breaking the C5-C6 bond.
You should get down to the fact that you need something like CH3Cl and 4-methylpent-1-yne to start
with.
Question 4.48
If we start with hex-2-ene, what would be the products upon vigorous treatment with KMnO4?
Ethanoic acid and butanoic acid