CHAPTER 18
Practice Exercises
18.1
(a)
(b)
(c)
(d)
1-chloro-3-methylbut-2-ene
1-bromo-1-methylcyclohexane
1,2-dichloropropane
2-chlorobuta-1,3-diene
18.3
(a)
Br + CH3CH2S-Na+
S CH2CH3
(b)
O
Br + CH3CO-Na+
O
OCCH3
Na+Br-
+
+
Na+Br-
18.5
Cl
(a)
CH3
NaOCH2CH3
+ Na+Cl-
CH3CH2OH
Major
product
CH2Cl
(b)
(c)
NaOCH2CH3
+
CH3CH2OH
Cl
CH3
Minor
product
Na+Cl-
NaOCH2CH3
CH3CH2OH
+
+
Na+Cl-
Equal amounts
18.7
Reaction (a) involves the methoxide anion, which is both a strong base and strong nucleophile.
Secondary halides in polar solvents can undergo either substitution or elimination reactions. SN2
reactions are favoured with strong nucleophiles and E2 is favoured with strong bases, therefore
these two reactions are competing in (a). The E2 reaction favours the most stable alkene as the
major elimination product, which is the trans isomer.
Review Problems
18.1
(a)
(b)
(c)
(d)
(e)
(f)
18.3
1,1-difluoroethene
3-bromocyclopentene
1,6-dichlorohexane
2-chloro-5-methlyhexane
dichlorodifluoromethane
3-bromo-3-ethylpentane
(a)
(b)
Br
H
Cl
(c)
H
Br
Br
H
(d)
Br
(e)
Cl
(f)
18.5
Cl
Br
2-Iodooctane and trans-1-chloro-4-methylcyclohexane are 2˚ haloalkanes.
(a)
isobutyl chloride: 1˚ haloalkane
(b)
2-iodoctane: 2˚ haloalkane
18.7
(c)
trans-1-chloro-4-methylcyclohexane: 2˚ haloalkane
(a)
CH2Cl2
(b)
(c)
CH3CH2OH
(d)
(e)
18.9
The carbonyl group of acetone is a polar functional group, so acetone is the most polar of the
three. The oxygen atom of diethyl ether adds polarity to this solvent compared to the hydrocarbon
pentane. The order of increasing polarity is:
pentane
<
diethylether
<
acetone
18.11
(a)
True: both nucleophile and haloalkane are reacting in the rate-determining step.
(b)
True: backside attack at the substitution centre results in inversion of configuration.
(c)
True: SN2 reactions result in an inversion of stereochemistry at the substitution centre,
therefore, optical activity is retained in the product. Therefore, the product is optically
active and rotates plane-polarised light to the same extent, but in the opposite direction of
the original.
(d)
False: steric crowding influences the order of reactivity in SN2 reactions. As the number
of substituent groups on the substitution centre increases, the reaction rate decreases. For
SN2 reactions, the order of reactivity is: methyl > 1° > 2° > 3°.
(e)
False: all nucleophiles are Lewis bases and therefore must have an unshared pair of
electrons. Moderate nucleophiles such as ammonia (NH3) react in SN2 reactions, even
though they do not carry a negative charge.
(f)
True: the nucleophile is involved in the rate-determining step; therefore, as nucleophilic
strength increases, the rate also increases.
18.13
Cl
-HCl
(a)
Major Zaitsev products: cis-trans isomers are possible
Br
-HBr
(b)
Major Zaitsev products: cis-trans isomers are possible
(c)
(d)
Cl
Cl
-HCl
-HCl
No cis-trans isomers are possible
No cis-trans isomers are possible
18.15
The carbocation that needs to be formed for an SN1 reaction mechanism is too unstable for this to
be a viable route to the product.
18.17
The major product is substitution to give 2-ethoxypropane. Elimination to give propene is a minor
side reaction.
18.19
(a)
SN1 (tertiary substrate)
(b)
(c)
(d)
(e)
No. The rate is not dependent on the concentration or strength of the nucleophile.
Review Problems
18.21
Br
(b)
Br2
light
1-chloromethyl-1,3,3-trimethylcyclopentane
Br
2-bromo-1,1,3,3-tetramethylcyclopentane
Br
1-bromo-2,2,4,4-tetramethylcyclopentane
(c)
Br
Br2
heat
Br
3,4-diethyl-1-bromohexane
3,4-diethyl-2-bromohexane
Br
3,4-diethyl-3-bromohexane
(d)
Br2
Br
light
Br
3-ethyl-1-bromopentane
3-ethyl-2-bromopentane
Br
3-ethyl-3-bromopentane
18.23
Cl
(a)
O
O
+ CH3CO- Na+
I
+ CH3CH 2S-Na+
(b) CH3CHCH2CH3
CH3
(c) CH3CHCH2CH2Br
(d) (CH 3)3N
(e)
+
+ CH 3I
CH2Br
Na+I-
acetone
+ CH3O- Na+
S
acetone
acetone
(CH 3)4N
methanol
Na+Cl-
+
O
ethanol
+
Na+I-
+
Na+Br-
I
+
I
CH2OCH3 + Na+Br-
SCH3
Cl
(f) H3C
(g)
CH3S- Na+
NH + CH3(CH2)6CH2Cl
(h)
18.25
+
CH2Cl + NH3
ethanol
+N
ethanol
H
(CH2)7CH3
+
CH2NH3
ethanol
+
H3C
+
+
Cl
Cl
When comparing nucleophilicity of atoms in the same row of the periodic table, as in reactions
(a) and (b), nucleophilicity increases with basicity of the atom (oxygen is less basic than
nitrogen). For reaction (c), when comparing atoms in the same column of the periodic table,
nucleophilicity increases from top to bottom; therefore, sulfur is more nucleophilic than oxygen.
H H
(a) HOCH 2CH2NH 2
+ I
N
+ CH 3I
ethanol
HO
+ CH 3
O
(b)
O
+ CH3I
N
ethanol
H
(c) HOCH2CH2SH
+ CH 3I
+
+
H
ethanol
N
I
CH3
S
HO
CH3
+
18.27
OCH2CH3
Cl
(a) CH3CHCH2CH3
+ CH3CH2OH
S enantiomer
ethanol
+
OCHCH3
Racemic mixture
(b)
Na+Cl-
Cl
+ CH3OH
CH3
methanol
OCH3
+ HCl
HCl
HI
CH3
(c) CH3CCl
CH3
O
+ CH3COH
O
O
acetic acid
+
HCl
OCH3
Br + CH3OH
(d)
+
methanol
HBr
OCH3
Racemic mixture
18.29
The order of reactivity for haloalkanes in SN1 reactions is:
3° > 2° > 1° > methyl
(a)
(b)
(c)
18.31
All of the reaction products shown can be produced from the same carbocation intermediate that
results from the ionisation of the carbon-halogen bond. The reaction proceeds in an ionising
solvent, there are no good nucleophiles present and the 3˚ carbocation is a very stable
intermediate. All of these factors are very favourable for SN1 and E1 reaction mechanisms.
SN1 reaction mechanism with ethanol as a nucleophile:
CH3
H3 C C
CH3
slow
Cl
CH3
H
CH3
CH3
H 3C C
+ Cl
H3 C C
HOCH2CH3
O
CH3 H
H
+
H3C C OCH2CH3
CH3
CH3
CH3
CH3COCH2CH3 + H3O+
CH3
SN1 reaction mechanism with water as a nucleophile:
CH3
H3C C
CH3
slow
Cl
+ Cl
H3C C
CH3
H
CH3
CH3
CH3 H
H3C C +O H
H
O
H3C C
CH3
H
O
H
CH3
CH3
CH3COH + H3O+
CH3
E1 reaction mechanism:
CH3
H3C C
slow
Cl
Nu
Cl
H 3C C
CH3
CH3
18.33
H 2C
H
H
O
H
CH3
CH3C CH2 + H3O+
The reacting carbon in an SN2 reaction transition state is sp2 hybridised, with the substituents
bonding to sp2 hybridised orbitals and the incoming nucleophile and leaving groups each partially
bonded to an unhybridised p orbital. The molecular shape of this transition state is trigonal
bipyramidal.
C
L
18.35
Br
CN
(a)
NaBr
NaCN
Br
(b)
CN
NaCN
O
Br
(c)
O
NaOCCH3
(d)
Br
(e)
Br
(f)
SH
NaSH
NaOCH3
NaBr
OCH 3
NaBr
NaBr
O
SH
Br
(g)
18.37
NaBr
O
NaOCH2CH3
Br
NaBr
NaBr
NaSH
When elimination reactions can give two or more possible alkenes, Zaitsev’s rule predicts that the
most stable alkene (the most substituted alkene) will be the major product.
Cl
(a)
KOH
or
Cl
Cl
(b)
(c)
CH2
KOH
KOH
Cl
Cl
(d)
KOH
or
Cl
Cl
KOH
(e)
18.39
(a)
NaOCH2CH3
Cl
CH3CH2OH
HBr
(b)
(c)
(d)
Cl
Br
(e)
Br
18.41
NaOCH2CH3
H2O
CH3CH2OH
H2SO4
OH
NaOCH2CH3
CH3CH2OH
Br
(f)
Br
OH
NaOCH2CH3
OsO4
CH3CH2OH
ROOH
OH
Br2
Br
NaOCH2CH3
CH3CH2OH
Br
Primary haloalkanes react with bases/nucleophiles to give predominantly substitution products.
With strong bases, such as hydroxide ion and ethoxide ion, a percentage of the product is formed
by an E2 reaction, but it is generally small compared with that formed by an SN2 reaction. With
strong, bulky bases, such as the tert-butoxide ion, the E2 product becomes the major product.
(a)
(b)
I
I
NaOH
NaNH2
HO
Major
product
minor
product
Major
product
minor
product
H 2N
Sodium hydroxide and sodium amide are very strong bases and also very good nucleophiles.
Some elimination occurs, but the major product arises from substitution.
(c)
I
NaCN
NC
Only
product
Sodium cyanide is a strong nucleophile but a weak base, which favours substitution over
elimination.
(d)
I
NaOOCCH3
H3C
O
O
Only
product
Sodium acetate is a weak base and so little elimination occurs. It is a good nucleophile and so the
predominant product is the substitution of the iodine atom by the acetate group.
(e)
I
NaI
I
Only
product
(racemate)
Sodium iodide is a very weak base and so no elimination occurs. The predominant product is the
substitution of the iodine atom by another iodine atom so that a racemic mixture results.
(f)
I
NaOC(CH3)3
major
product
The tert-butoxide amion is a strong base which favours elimination. It is also sterically bulky
which hinders substitution. The major product is propene with only minor amounts of the ether
formed.
Additional Exercises
18.43
CH3
(a) CH3
CO-
CH3
K+
+
CH2Cl
CH3
(b)
CH2
O-K+
CH3
+ CH3CCl
CH3
DMSO
DMSO
CH3COCH2
CH3
Major product (SN2)
CH3
CH3COCH2
+ KCl
+ KCl
CH3
Minor product
Reaction (a) gives the best yield of ether product because it involves favourable conditions for an
SN2 reaction, involving a 1˚ halide substrate and a good nucleophile. The substrate also precludes
E2 reactions with the absence of -protons. Reaction (b) predominately undergoes an E2 reaction
with a strong base deprotonating the -proton on a sterically hindered 3˚ halide to produce an
alkene as the major product.
H
CH2O
H2C
CH3
C
CH3
Cl
E2
CH3
H2C C
CH3
Major product
(E2)
CH2OH
18.45
O
Cl CH2CH2 O H
OH
Cl CH2CH2 O
H2C
CH2
18.47
Under these SN2 conditions, the bromide nucleophile inverts the stereochemistry of the
stereocentre. After 50% of the starting material has reacted, racemisation is attained.
18.49
Although alkoxides are poor leaving groups, the release of strain in the three-
18.51
HCl
(a)
(b) CH3CH2CH=CH2
(c) CH3CH=CHCH3
(d)
CH3
HI
HCl
HBr
Cl
I
CH3CH2CHCH3
Cl
CH3CHCH2CH3
CH3
Br