Carey - Organic Chemistry - sgchapt12

Carey - Organic Chemistry - sgchapt12

(Parte 1 de 4)

CHAPTER 12

SOLUTIONS TO TEXT PROBLEMS 12.1The three most stable resonance structures for cyclohexadienyl cation are

The positive charge is shared equally by the three carbons indicated. Thus the two carbons ortho to the sp3-hybridized carbon and the one para to it each bear one third of a positive charge ( 0.3). None of the other carbons is charged. The resonance picture and the simple MO treatment agree with respect to the distribution of charge in cyclohexadienyl cation.

12.2Electrophilic aromatic substitution leads to replacement of one of the hydrogens directly attached to the ring by the electrophile. All four of the ring hydrogens of p-xylene are equivalent; so it does not matter which one is replaced by the nitro group.

CH3

CH3 p-Xylene

HNO3 H2SO4

CH3

CH3 NO2

1,4-Dimethyl-2- nitrobenzene

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280REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION

12.3The aromatic ring of 1,2,4,5-tetramethylbenzene has two equivalent hydrogen substituents. Sulfonation of the ring leads to replacement of one of them by @SO3H.

12.4The major product is isopropylbenzene.

Aluminum chloride coordinates with 1-chloropropane to give a Lewis acid/Lewis base complex, which can be attacked by benzene to yield propylbenzene or can undergo an intramolecular hydride shift to produce isopropyl cation. Isopropylbenzene arises by reaction of isopropyl cation with benzene.

12.5The species that attacks the benzene ring is cyclohexyl cation, formed by protonation of cyclohexene.

The mechanism for the reaction of cyclohexyl cation with benzene is analogous to the general mechanism for electrophilic aromatic substitution.

12.6 The preparation of cyclohexylbenzene from cyclohexene and benzene was described in text Section12.6. Cyclohexylbenzene is converted to 1-phenylcyclohexene by benzylic bromination, followed by dehydrohalogenation.

Benzene Cyclohexene Cyclohexylbenzene

H2SO4 NaOCH2CH3N-Bromosuccinimide (NBS), benzoyl peroxide, heat

1-Phenylcyclohexene1-Bromo-1- phenylcyclohexane

Benzene Cyclohexylcation Cyclohexadienyl cationintermediate Cyclohexylbenzene

H HO SO2OH

CyclohexeneSulfuric acidCyclohexyl cationHydrogen sulfate ion OS O2OH

CH3Isopropyl cation

CH2 Cl AlCl3 hydridemigration CH3 CH CH3 AlCl4

CH3CH2CH2Cl1-Chloropropane

AlCl3

Propylbenzene (20% yield)

CH2CH2CH3

Benzene Isopropylbenzene (40% yield)

CH3H3C 1,2,4,5-Tetramethylbenzene

SO3 H2SO4

2,3,5,6-Tetramethylbenzenesulfonic acid

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12.7Treatment of 1,3,5-trimethoxybenzene with an acyl chloride and aluminum chloride brings about Friedel–Crafts acylation at one of the three equivalent positions available on the ring.

12.8Because the anhydride is cyclic, its structural units are not incorporated into a ketone and a carboxylic acid as two separate product molecules. Rather, they become part of a four-carbon unit attached to benzene by a ketone carbonyl. The acyl substituent terminates in a carboxylic acid functional group.

12.9(b)AFriedel–Crafts alkylation of benzene using 1-chloro-2,2-dimethylpropane would not be a satisfactory method to prepare neopentylbenzene because of the likelihood of a carbocation rearrangement. The best way to prepare this compound is by Friedel–Crafts acylation followed by Clemmensen reduction.

12.10(b)Partial rate factors for nitration of toluene and tert-butylbenzene, relative to a single position of benzene, are as shown:

The sum of these partial rate factors is 147 for toluene, 90 for tert-butylbenzene. Toluene is 147 90, or 1.7, times more reactive than tert-butylbenzene. (c)The product distribution for nitration of tert-butylbenzene is determined from the partial rate factors.

AlCl3Zn(Hg), HCl

Benzene2,2-Dimethylpropanoyl chloride

2,2-Dimethyl-1- phenyl-1-propanone

AlCl3

Benzene 4-Oxo-4-phenylbutanoic acid

CCH2CH2COH O O

Succinic anhydride

AlCl3

1,3,5-Trimethoxybenzene OCH3

3-Methylbutanoyl chloride

Isobutyl 1,3,5-trimethoxyphenyl ketone OCH3

OCH3

REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION281

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12.11The compounds shown all undergo electrophilic aromatic substitution more slowly than benzene. Therefore, @CH2Cl, @CHCl2, and @CCl3are deactivatingsubstituents.

The electron-withdrawing power of these substituents, and their tendency to direct incoming electrophiles meta to themselves, will increase with the number of chlorines each contains. Thus, the substituent that gives 4% meta nitration (96% ortho para) contains the fewest chlorine atoms

12.12(b)Attack by bromine at the position meta to the amino group gives a cyclohexadienyl cation intermediate in which delocalization of the nitrogen lone pair cannot participate in dispersal of the positive charge.

12.13Electrophilic aromatic substitution in biphenyl is best understood by considering one ring as the functional group and the other as a substituent. An aryl substituent is ortho, para-directing. Nitration of biphenyl gives a mixture of o-nitrobiphenyl and p-nitrobiphenyl.

Methyl benzoate

Methyl m-nitrobenzoate (isolated in 81–85% yield) o-Nitrobiphenyl(37%) Biphenyl

NO2

BrH BrH BrH BrH

NH2

Br H

BrH Br H

Deactivating, ortho, para-directing

CH2Cl

Deactivating, ortho, para-directing

CHCl2

Deactivating, meta-directing

CCl3

Benzyl chloride CH2Cl (Dichloromethyl)benzene

CHCl2

(Trichloromethyl)benzene CCl3

282REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION

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(c)The acyl group in 1-phenyl-1-propanone is meta-directing; the carbonyl is attached directly to the ring. The product is 1-(m-nitrophenyl)-1-propanone.

This unshared pair is responsible for the powerful activating effect of an group. On the

12.16The reaction is a Friedel–Crafts alkylation in which 4-chlorobenzyl chloride serves as the carbocation source and chlorobenzene is the aromatic substrate. Alkylation occurs at the positions ortho and para to the chlorine substituent of chlorobenzene.

12.17(b)Halogen substituents are ortho, para-directing, and the disposition in m-dichlorobenzene is such that their effects reinforce each other. The major product is 2,4-dichloro-1-nitrobenzene. Substitution at the position between the two chlorines is slow because it is a sterically hindered position.

(c)Nitro groups are meta-directing. Both nitro groups of m-dinitrobenzene direct an incoming substituent to the same position in an electrophilic aromatic substitution reaction. Nitration of m-nitrobenzene yields 1,3,5-trinitrobenzene.

Both nitro groups of m-dinitrobenzene direct electrophile to same position.

NO2

NO2

1,3,5-Trinitrobenzene (principal product of nitration of m-dinitrobenzene)

Most reactive positions in electrophilic aromatic substitution of m-dichlorobenzene

2,4-Dichloro-1-nitrobenzene (major product of nitration)

Cl NO2

AlCl3ChlorobenzeneCl 4-Chlorobenzyl chloride

ClCH2 Cl

1-Chloro-4-(4 -chlorobenzyl)- benzene

Cl CH2 Cl

1-Chloro-2-(4 -chlorobenzyl)- benzene

CH2 Cl Cl

CH3

CH3 nitration1-Phenyl-1-propanone

1-(m-Nitrophenyl)-1-propanone (isolated in 60% yield)

REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION283

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(d) A methoxy group is ortho, para-directing, and a carbonyl group is meta-directing. The open positions of the ring that are activated by the methoxy group in p-methoxyacetophenone are also those that are meta to the carbonyl, so the directing effects of the two substituents reinforce each other. Nitration of p-methoxyacetophenone yields 4-methoxy-3-nitroacetophenone.

(e)The methoxy group of p-methylanisole activates the positions that are ortho to it; the methyl activates those ortho to itself. Methoxy is a more powerful activating substituent than methyl, so nitration occurs ortho to the methoxy group.

(f)All the substituents in 2,6-dibromoanisole are ortho, para-directing, and their effects are felt at different positions. The methoxy group, however, is a far more powerful activating substituent than bromine, so it controls the regioselectivity of nitration.

12.18The product that is obtained when benzene is subjected to bromination and nitration depends on the order in which the reactions are carried out. Anitro group is meta-directing, and so if it is introduced prior to the bromination step, m-bromonitrobenzene is obtained.

Bromine is an ortho, para-directing group. If it is introduced first, nitration of the resulting bromobenzene yields a mixture of o-bromonitrobenzene and p-bromonitrobenzene.

Benzene BrBromobenzene

Br NO2 o-Bromonitrobenzene

NO2 p-Bromonitrobenzene

Benzene m-BromonitrobenzeneNitrobenzene Br

NO2NO2

Br2 FeBr3

OCH3

Br Br1 2

NO2 Br Br

Methoxy directs toward C-4; bromines direct toward C-3 and C-5. 2,6-Dibromo-4-nitroanisole (principal product of nitration)

4-Methyl-2-nitroanisole (principal product of nitration)

CH3 OCH3 NO2

Methyl activates C-3 and C-5; methoxy activates C-2 and C-6.

CH3 OCH3

Positions ortho to the methoxy group are meta to the carbonyl.

4-Methoxy-3-nitroacetophenone

284REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION

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12.19 A straightforward approach to the synthesis of m-nitrobenzoic acid involves preparation of benzoic acid by oxidation of toluene, followed by nitration. The carboxyl group of benzoic acid is meta-directing. Nitration of toluene prior to oxidation would lead to a mixture of ortho and para products.

12.20The text points out that C-1 of naphthalene is more reactive than C-2 toward electrophilic aromatic substitution. Thus, of the two possible products of sulfonation, naphthalene-1-sulfonic acid should be formed faster and should be the major product under conditions of kinetic control. Since the problem states that the product under conditions of thermodynamic control is the other isomer, naphthalene-2-sulfonic acid is the major product at elevated temperature.

Naphthalene-2-sulfonic acid is the more stable isomer for steric reasons. The hydrogen at C-8 (the one shown in the equation) crowds the group in naphthalene-1-sulfonic acid.

12.21 The text states that electrophilic aromatic substitution in furan, thiophene, and pyrrole occurs at C-2. The sulfonation of thiophene gives thiophene-2-sulfonic acid.

NO2Nitrobenzene

NO2

NO2 m-Dinitrobenzene

Benzene

NO2 Nitrobenzene

Thiophene Thiophene-2- sulfonic acid

Naphthalene Naphthalene-1-sulfonic acid major product at 0 C; formed faster

HS O3H

Naphthalene-2-sulfonic acid major product at 160 C; more stable

H SO3H

CH3

Toluene CO2HBenzoic acid

NO2 m-Nitrobenzoic acid

REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION285

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(d)Trifluoromethyl is deactivating and meta-directing.

Sulfur trioxide could be added to the sulfuric acid to facilitate reaction. The para isomer is the predominant product. ( f ) Acetanilide is quite similar to anisole in its behavior toward electrophilic aromatic substitution.

FeCl3 Cl2

BrBromobenzene

Br Cl o-Bromochlorobenzene p-Bromochlorobenzene

Acetanilide o-Acetamidobenzenesulfonic acid p-Acetamidobenzenesulfonic acid o-Methoxybenzene-sulfonic acid p-Methoxybenzenesulfonic acid

Br2 FeBr3

CF3

(Trifluoromethyl)- benzene

CF3 m-Bromo(trifluoromethyl)- benzene

Br2 FeBr3

CH3

Toluene

CH3 Br o-Bromotoluene p-Bromotoluene

CH3 Br

286REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION

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(h)Anisole is a reactive substrate toward Friedel–Crafts alkylation and yields a mixture of o- and p-benzylated products when treated with benzyl chloride and aluminum chloride.

CH3H Br

CH3

CH3

CH3

CH3

CH3

HBr H Br

KOHdiethylene glycol C

Benzophenone Diphenylmethane CH2

Zn(Hg)HClC O

Benzophenone Diphenylmethane CH2

HNO3

Benzophenone m-Nitrobenzophenone

NO2

AlCl3 Benzene

CCl O

Benzoyl chloride

Benzophenone

AlCl3

CH2ClBenzyl chloride OCH3Anisole o-Benzylanisole

OCH3

REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION287

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Any one of these resonance forms is a satisfactory answer to the question. Because of its tertiary carbocation character, this carbocation is more stable than the corresponding intermediate formed from benzene. (b)Chlorination of m-xylene will give predominantly 4-chloro-1,3-dimethylbenzene.

The intermediate shown (or any of its resonance forms) is more stable for steric reasons than

The cyclohexadienyl cation intermediate leading to 4-chloro-1,3-dimethylbenzene is more stable and is formed faster than the intermediate leading to chlorobenzene because of its tertiary carbocation character.

An acyl group is electron-withdrawing and destabilizes a carbocation to which it is attached. The most stable carbocation intermediate in the nitration of acetophenone is less stable and isformed more slowly than is the corresponding carbocation formed during nitration of benzene.

CCH3O H

O2Nmore stable thanor H

CH3

ClH ClH

CH3H Cl

CH3

Less stable cyclohexadienyl cation

CH3

CH3 m-Xylene

CH3

Cl CH3

4-Chloro-1,3- dimethylbenzene

CH3

ClH CH3

More stable cyclohexadienyl cation

Cl2 via

288REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION

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(d)The methoxy group in anisole is strongly activating and ortho, para-directing. For steric reasons and because of inductive electron withdrawal by oxygen, the intermediate leading to para substitution is the most stable.

Of the various resonance forms for the most stable intermediate, the most stable one has eight electrons around each oxygen and carbon atom.

This intermediate is much more stable than the corresponding intermediate from acylation of benzene. (e)An isopropyl group is an activating substituent and is ortho, para-directing. Attack at the ortho position is sterically hindered. The most stable intermediate is

or any of its resonance forms. Because of its tertiary carbocation character, this cation is more stable than the corresponding cyclohexadienyl cation intermediate from benzene. (f)Anitro substituent is deactivating and meta-directing. The most stable cyclohexadienyl cation formed in the bromination of nitrobenzene is

This ion is less stable than the cyclohexadienyl cation formed during bromination of benzene. (g) Sulfonation of furan takes place at C-2.The cationic intermediate is more stable than the cyclohexadienylcationformedfrombenzenebecauseitisstabilizedbyelectronreleasefromoxygen.

Furan Furan-2- sulfonic acid

SO3HH SO3H

H Br

Most stable resonance form

H O CCH3

OCH3 more stable thanslightly more stable than

OCH3

O H CCH3

OCH3

REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION289

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12.24(a)Toluene is more reactive than chlorobenzene in electrophilic aromatic substitution reactionsbecause a methyl substituent is activating but a halogen substituent is deactivating. Bothare ortho, para-directing, however. Nitration of toluene is faster than nitration of chlorobenzene.

Faster:

Slower:

(b)Afluorine substituent is not nearly as strongly deactivating as a trifluoromethyl group. The reaction that takes place is Friedel–Crafts alkylation of fluorobenzene.

(Parte 1 de 4)

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