CHAPTER 18

Practice Exercises

18.1 (a) 1-chloro-3-methylbut-2-ene(b) 1-bromo-1-methylcyclohexane(c) 1,2-dichloropropane(d) 2-chlorobuta-1,3-diene

18.3

18.5

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.

+Br CH3CH2S-Na+(a) S CH2CH3 + Na+Br-

(b) Br CH3CO-Na++

O

OCCH3 + Na+Br-

O

(a) CH3

ClNaOCH2CH3

CH3CH2OH+ Na+Cl-

Majorproduct

Minorproduct

(b)

CH2Cl NaOCH2CH3

CH3CH2OH + Na+Cl-

(c)

CH3Cl NaOCH2CH3

CH3CH2OH+ Na+Cl-+

Equal amounts

Review Problems

18.1 (a) 1,1-difluoroethene(b) 3-bromocyclopentene(c) 1,6-dichlorohexane(d) 2-chloro-5-methlyhexane(e) dichlorodifluoromethane(f) 3-bromo-3-ethylpentane

18.3 (a)

(b)

(c)

(d)

(e)

(f)

18.5 2-Iodooctane and trans-1-chloro-4-methylcyclohexane are 2˚ haloalkanes.

(a) isobutyl chloride: 1˚ haloalkane

(b) 2-iodoctane: 2˚ haloalkane

Br

ClH

Br BrH H

Br

C lC l

Br

(c) trans-1-chloro-4-methylcyclohexane: 2˚ haloalkane

18.7 (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

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) No. The rate is not dependent on the concentration or strength of the nucleophile.

(e)

Cl(a)

-HCl

Major Zaitsev products: cis-trans isomers are possibleBr

(b)-HBr

Major Zaitsev products: cis-trans isomers are possible

(c) Cl-HCl No cis-trans isomers are possible

Cl(d)-HCl No cis-trans isomers are possible

Review Problems

18.21

Br2

light

1-chloromethyl-1,3,3-trimethylcyclopentane

(b)Br

Br

Br2-bromo-1,1,3,3-tetramethylcyclopentane

1-bromo-2,2,4,4-tetramethylcyclopentane

18.23

Br2

heat

3,4-diethyl-1-bromohexane

(c)

Br

Br

Br

3,4-diethyl-2-bromohexane

3,4-diethyl-3-bromohexane

Br2

light

3-ethyl-1-bromopentane

(d)

3-ethyl-2-bromopentane

3-ethyl-3-bromopentane

Br

Br

Br

Cl

CH3CO- Na+

O

ethanol(a) +

O

O + Na+Cl-

CH3CHCH2CH3

I

CH3CH2S-Na+

acetone(b) +

S+ Na+I-

acetone(c) +

CH3

CH3CHCH2CH2Br Na+I- + Na+Br-

I

acetone+(d) (CH3)3N CH3I (CH3)4N + I

+methanol

(e) CH2Br CH3O- Na+ CH2OCH3 + Na+Br-

18.25 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.

18.27

ethanol+(f) H3C Cl CH3S- Na+ H3C

SCH3

+ Na+Cl-

(g) +ethanol

NH CH3(CH2)6CH2Cl N(CH2)7CH3

H+ + Cl

+(h) CH2Cl NH3 ethanolCH2NH3

++ Cl

(a) HOCH2CH2NH2 + CH3Iethanol HO

NCH3

H H

++ I

(b)

N

O

H

+ CH3Iethanol

N

O

H CH3

++ I

(c) HOCH2CH2SH + CH3Iethanol HO

SCH3

+ HI

(a) +ethanol

Cl

CH3CHCH2CH3 CH3CH2OH

S enantiomer

OCH2CH3

OCHCH3

Racemic mixture

+ HCl

(b) +methanolCH3OH

Cl

CH3

OCH3

+ HCl

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:

acetic acid+(c) CH3COHCH3CCl

CH3

CH3

O

O

O

+ HCl

(d)methanol

+ CH3OHBr

OCH3

OCH3

+ HBr

Racemic mixture

SN1 reaction mechanism with water as a nucleophile:

E1 reaction mechanism:

18.33 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

CH3

CH3

H3C Clslow

C

CH3

CH3

H3C

C

CH3

CH3

H3C HOCH2CH3 C

CH3

CH3

H3C OCH2CH3

H

+

OH

H

CH3COCH2CH3

CH3

CH3

+ Cl

+ H3O+

C

CH3

CH3

H3C Clslow

C

CH3

CH3

H3C

C

CH3

CH3

H3C C

CH3

CH3

H3C O

H

+

OH

H

CH3COH

CH3

CH3

+ Cl

+ H3O+OH

HH

C

CH3

CH3

H3C Cl C

H2C

H3CCl

CH3

H

OH

HCH3C

CH3

CH2

slow+ H3O+

CNu L

18.35

18.37 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.

(a)

Br CN

NaCN NaBr

(b)Br CN

NaCN NaBr

(c)Br

O

ONaOCCH3

O

NaBr

(d) Br NaSH SH NaBr

(e) Br NaOCH3 OCH3NaBr

(f) Br NaOCH2CH3 ONaBr

(g)Br

NaSH

SH

NaBr

(a) Cl or

Cl

KOH

(b)

CH2Cl KOH

(c) Cl KOH

(d)Cl

orCl

KOH

18.39

18.41 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.

(e)

ClKOH

(a) ClNaOCH2CH3

CH3CH2OH

(b) BrHBr

Cl OH(c)NaOCH2CH3

CH3CH2OH

H2O

H2SO4

(d) BrNaOCH2CH3

CH3CH2OH

(e)

Br NaOCH2CH3

CH3CH2OH

OH

OH

OsO4

ROOH

(f)

Br Br

Br

NaOCH2CH3

CH3CH2OH

Br2

Sodium hydroxide and sodium amide are very strong bases and also very good nucleophiles. Some elimination occurs, but the major product arises from substitution.

Sodium cyanide is a strong nucleophile but a weak base, which favours substitution over elimination.

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.

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.

INaOH

HO

Majorproduct

minorproduct

(a)

INaNH2 H2N

Majorproduct

minorproduct

(b)

INaCN

NC

Onlyproduct

(c)

INaOOCCH3 O

Onlyproduct

(d)

O

H3C

INaI

I

Onlyproduct(racemate)

(e)

INaOC(CH3)3

majorproduct

(f)

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

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.

18.45

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-

CH3CO- K+ CH2ClDMSO

CH3COCH2 +(a) + KCl

CH3

CH3

CH3

CH3

Major product (SN2)

CH2O-K+ CH3CCl CH3COCH2(b) + + KClCH3

CH3

CH3

CH3

Minor product

DMSO

CH2O H2C C

H CH3

CH3

ClE2 H2C C

CH3

CH3

CH2OH

Major product(E2)

Cl CH2CH2 O H OH Cl CH2CH2 O H2C CH2

O

18.51

Cl(a) HCl

CH3CH2CH=CH2 CH3CH2CHCH3(b)

IHI

(c) CH3CH=CHCH3 CH3CHCH2CH3

ClHCl

(d) BrCH3

CH3HBr