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Organic Chemistry. William H. Brown & Christopher S. Foote. Aromatics II. Chapter 20. Chapter 21. Reactions of Benzene. The most characteristic reaction of aromatic compounds is substitution at a ring carbon. Reactions of Benzene. Electrophilic Aromatic Sub.

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organic chemistry

Organic Chemistry

William H. Brown & Christopher S. Foote

aromatics ii
Aromatics II

Chapter 20

Chapter 21

reactions of benzene
Reactions of Benzene
  • The most characteristic reaction of aromatic compounds is substitution at a ring carbon
electrophilic aromatic sub
Electrophilic Aromatic Sub
  • Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile
  • We study
    • several common types of electrophiles
    • how each is generated
    • the mechanism by which each replaces hydrogen
chlorination
Chlorination

Step 1: formation of a chloronium ion

chlorination1
Chlorination

Step 2: attack of the chloronium ion on the aromatic ring

Step 3: proton transfer regenerates the aromatic character of the ring

eas general mechanism
EAS: General Mechanism
  • A general mechanism
  • General question: what is the electrophile and how is it generated?
nitration
Nitration
  • The electrophile is NO2+, generated in this way
nitration1
Nitration

Step 1: attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile)

Step 2: proton transfer regenerates the aromatic ring

nitration2
Nitration
  • A particular value of nitration is that the nitro group can be reduced to a 1° amino group
sulfonation
Sulfonation
  • Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide
friedel crafts alkylation
Friedel-Crafts Alkylation
  • Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group
friedel crafts alkylation1
Friedel-Crafts Alkylation

Step 1: formation of an alkyl cation as an ion pair

Step 2: attack of the alkyl cation on the aromatic ring

Step 3: proton transfer regenerates the aromatic ring

friedel crafts alkylation2
Friedel-Crafts Alkylation
  • There are two major limitations on Friedel-Crafts alkylations

1. carbocation rearrangements are common

friedel crafts alkylation3
Friedel-Crafts Alkylation
  • the isobutyl chloride/AlCl3 complex rearranges to the tert-butyl cation/AlCl4- ion pair, which is the electrophile
friedel crafts alkylation4
Friedel-Crafts Alkylation

2. F-C alkylation fails on benzene rings bearing one or more of these strongly electron-withdrawing groups

friedel crafts acylation
Friedel-Crafts Acylation
  • Friedel-Crafts acylation forms a new C-C bond between a benzene ring and an acyl group
friedel crafts acylation1
Friedel-Crafts Acylation
  • The electrophile is an acylium ion
friedel crafts acylation2
Friedel-Crafts Acylation
    • an acylium ion is a resonance hybrid of two major contributing structures
  • F-C acylations are free of a major limitation of F-C alkylations; acylium ions do not rearrange
friedel crafts acylation3
Friedel-Crafts Acylation
  • A special value of F-C acylations is preparation of unrearranged alkylbenzenes
other aromatic alkylations
Other Aromatic Alkylations
  • Carbocations are generated by
    • treatment of an alkene with a protic acid, most commonly H2SO4, H3PO4, or HF/BF3
other aromatic alkylations1
Other Aromatic Alkylations
  • by treating an alkene with a Lewis acid
  • and by treating an alcohol with H2SO4 or H3PO4
di and polysubstitution
Di- and Polysubstitution
  • Existing groups on a benzene ring influence further substitution in both orientation and rate
  • Orientation:
    • certain substituents direct preferentially to ortho & para positions; others direct preferentially to meta positions
    • substituents are classified as either

ortho-para directingor meta directing

di and polysubstitution1
Di- and Polysubstitution
  • Rate
    • certain substituents cause the rate of a second substitution to be greater than that for benzene itself; others cause the rate to be lower
    • substituents are classified as activating or deactivating toward further substitution
di and polysubstitution2
Di- and Polysubstitution
  • -OCH3 is ortho-para directing
di and polysubstitution3
Di- and Polysubstitution
  • -NO2 is meta directing
di and polysubstitution5
Di- and Polysubstitution
  • From the information in Table 21.1, we can make these generalizations
    • alkyl, phenyl, and all other groups in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing. All other groups are meta directing
    • all ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating
di and polysubstitution6
Di- and Polysubstitution
  • the order of steps is important
theory of directing effects
Theory of Directing Effects
  • The rate of EAS is limited by the slowest step in the reaction
  • For almost every EAS, the rate-determining step is attack of E+ on the aromatic ring to give a resonance-stabilized cation intermediate
  • The more stable this cation intermediate, the faster the rate-determining step and the faster the overall reaction
theory of directing effects1
Theory of Directing Effects
  • For ortho-para directors, ortho-para attack forms a more stable cation than meta attack
    • ortho-para products are formed faster than meta products
  • For meta directors, meta attack forms a more stable cation than ortho-para attack
    • meta products are formed faster than ortho-para products
theory of directing effects2
Theory of Directing Effects
  • -OCH3; assume meta attack
theory of directing effects3
Theory of Directing Effects
  • -OCH3: assume ortho-para attack
theory of directing effects4
Theory of Directing Effects
  • -NO2; assume meta attack
theory of directing effects5
Theory of Directing Effects
  • -NO2: assume ortho-para attack
activating deactivating
Activating-Deactivating
  • Any resonance effect, such as that of -NH2, -OH, and -OR, that delocalizes the positive charge on the cation intermediate lowers the activation energy for its formation, and has an activating effect toward further EAS
  • Any resonance or inductive effect, such as that of -NO2, -CN, -CO, or -SO3H, that decreases electron density on the ring deactivates the ring toward further EAS
activating deactivating1
Activating-Deactivating
  • Any inductive effect, such as that of -CH3 or other alkyl group, that releases electron density toward the ring activates the ring toward further EAS
  • Any inductive effect, such as that of -halogen, -NR3+, -CCl3, or -CF3, that decreases electron density on the ring deactivates the ring toward further EAS
halogens
Halogens
  • for the halogens, the inductive and resonance effects run counter to each other, but the former is somewhat stronger
  • the net effect is that halogens are deactivating but ortho-para directing
nucleophilic aromatic sub
Nucleophilic Aromatic Sub.
  • Aryl halides do not undergo nucleophilic substitution by either SN1 or SN2 pathways
  • They do undergo nucleophilic substitutions, but by mechanisms quite different from those of nucleophilic aliphatic substitution
  • Nucleophilic aromatic substitutions are far less common than electrophilic aromatic substitutions
benzyne intermediates
Benzyne Intermediates
  • When heated under pressure with aqueous NaOH, chlorobenzene is converted to sodium phenoxide. Neutralization with HCl gives phenol.
benzyne intermediates1
Benzyne Intermediates
  • the same reaction with 2-chlorotoluene gives a mixture of ortho- and meta-cresol
  • the same type of reaction can be brought about using of sodium amide in liquid ammonia
benzyne intermediates2
Benzyne Intermediates
  • -elimination of HX gives a benzyne intermediate, that then adds the nucleophile to give products
nu addition elimination
Nu Addition-Elimination
  • when an aryl halide contains electron-withdrawing NO2 groups ortho and/or para to X, nucleophilic aromatic substitution takes place readily
  • neutralization with HCl gives the phenol
meisenheimer complex
Meisenheimer Complex
  • reaction involves a Meisenheimer complex intermediate
prob 21 7
Prob 21.7

Write a mechanism for each reaction.

prob 21 8
Prob 21.8

Offer an explanation for the preferential nitration of pyridine in the 3 position rather than the 2 position.

prob 21 9
Prob 21.9

Offer an explanation for the preferential nitration of pyrrole in the 2 position rather than in the 3 position.

prob 21 15
Prob 21.15

Predict the major product(s) from treatment of each compound with HNO3/H2SO4.

prob 21 16
Prob 21.16

Account for the fact that N-phenylacetamide is less reactive toward electrophilic aromatic substitution than aniline.

prob 21 17
Prob 21.17

Propose an explanation for the fact that the trifluoromethyl group is meta directing.

prob 21 19
Prob 21.19

Arrange the compounds in each set in order of decreasing reactivity toward electrophilic aromatic substitution.

prob 21 19 cont d
Prob 21.19 (cont’d)

Arrange the compounds in each set in order of decreasing reactivity toward electrophilic aromatic substitution.

prob 21 20
Prob 21.20

Draw a structural formula for the major product of nitration of each compound.

prob 21 21
Prob 21.21

Which ring in each compound undergoes electrophilic aromatic substitution more readily? Draw the product of nitration of each compound.

prob 21 22
Prob 21.22

Propose a mechanism for the formation of bisphenol A.

prob 21 23
Prob 21.23

Propose a mechanism for the formation of BHT.

prob 21 24
Prob 21.24

Propose a mechanism for the formation of DDT.

prob 21 27
Prob 21.27

Propose a mechanism for this reaction.

prob 21 28
Prob 21.28

Account for the regioselectivity of the nitration in Step 1, and propose a mechanism for Step 2.

prob 21 29
Prob 21.29

Propose a mechanism for the displacement of chlorine by (1) the NH2 group of the dye and (2) an -OH group of cotton.

prob 21 31
Prob 21.31

Show how to prepare (a) and (b) from 1-phenyl-1-propanone.

prob 21 33
Prob 21.33

Show how to bring about each conversion.

prob 21 35
Prob 21.35

Propose a synthesis for each compound from benzene.

prob 21 36
Prob 21.36

Propose a synthesis of 2,4-D from chloroacetic acid and phenol.

prob 21 41
Prob 21.41

Propose a synthesis of this compound from benzene.

prob 21 42
Prob 21.42

Propose a synthesis of this compound from 3-methylphenol.

prob 21 43
Prob 21.43

Propose a synthesis of this compound from toluene and phenol.

prob 21 44
Prob 21.44

Propose a mechanism for this example of chloromethylation (introduction of a CH2Cl group on an aromatic ring). Show how to convert the product of chloromethylation to piperonal.

prob 21 45
Prob 21.45

Given this retrosynthetic analysis, propose a synthesis for Dinocap from phenol and 1-octene.

prob 21 46
Prob 21.46

Show how to synthesize this trichloro derivative of toluene from toluene.

prob 21 47
Prob 21.47

Given this retrosynthetic analysis, propose a synthesis for bupropion.

aromatics ii1
Aromatics II

End Chapter 21