Chapter 18: Aromatic Substitution Reactions

Question 1

Sigma complex

Answer 1

Positively charged, 3 resonant structure intermediate generated in an EAS reaction

Question 2

Bromination of benzene

Answer 2

Reagents

1. Br₂
2. FeBr₃ (iron tribromide) or AlBr₃ (aluminum tribromide)

Mechanism

Forms via a sigma complex intermediate and subsequent rearomatization of the benzene ring

Question 3

Chloronation of benzene

Answer 3

Reagents

1. Cl₂
2. FeCl₃ (iron trichloride) or AlCl₃ (aluminum trichloride)

Mechanism

Forms via a sigma complex intermediate and subsequent rearomatization of the benzene ring

Question 4

Sulfonation of benzene

Answer 4

Reagents

Fuming H₂SO₄
H₂SO₄ & SO₃ (sulfur trioxide)

Mechanism

Forms via a sigma complex intermediate and subsequent rearomatization of the benzene ring

Sulfonation is reversible because the reagent is concentration dependent

Question 5

Nitration of benzene

Installation of a nitro group

Answer 5

Mechanism

HNO3 (nitric acid) & H₂SO₄

Mechanism

Sulfuric acid protonates nitric acid and subsequently forms a nitronium ion which performs the nucleophilic attack

Nitration subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

Question 6

Nitration of benzene

Installation of an amine group

Answer 6

Reagents

1. HNO3 (nitric acid) & H₂SO₄
2. Fe or Zn & HCl
3. NaOH

Mechanism

Sulfuric acid protonates nitric acid and subsequently forms a nitronium ion which performs the nucleophilic attack

Nitration subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

Upon treatment with a metal (Fe or Zn) and HCl the nitro group can be reduced resulting in an ammonium ion (RNH₃⁺) that is subsequently deprotonated by a base

Question 7

Friedel-Crafts alkylation

Answer 7

Reagents

AlCl₃ (aluminum trichloride) & alkyl halide

• Secondary and tertiary alkyl halides are readily converted to carbocations but NOT primary halides EXCEPT ethyl chloride
• Alkyl halide α-carbon must be sp³ hybridized
• Beware of possible rearrangements that may occur

Mechanism

The catalyst (AlCl₃) converts the alkyl halide into a carbocation; creating a better electrophile

Alkylation subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

DEACTIVATED rings CANNOT undergo Friedel-Crafts alkylation

Question 8

Friedel-Crafts acylation

Answer 8

Mechanism

AlCl₃ (aluminum trichloride) & acyl halide

Mechanism

Treatment of the acyl halide with AlCl₃ forms a cationic species called an acylium ion

Acylation subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

Question 9

Friedel-Crafts acylation

Installing alkyl groups that are prone to rearrangement

Answer 9

Mechanism

1. AlCl₃ (aluminum trichloride) & acyl halide
2. Zn(Hg) (amalgamated zinc) & HCl, heat

Mechanism

Treatment of the acyl halide with AlCl₃ forms a cationic species called an acylium ion

Acylation subsequently occurs via the formation of a sigma complex intermediate and subsequent rearomatization of the benzene ring

A Clemmensen reduction can subsequently be employed to recude the aryl ketone to an alkyl group

Question 10

Strong activating groups

Answer 10

ortho-para directors

Question 11

Moderate activating groups

Answer 11

ortho-para directors

Question 12

Weak activating groups

Answer 12

ortho-para directors

Question 13

Weak deactivating groups

Answer 13

ortho-para directors

Question 14

Moderate deactivating groups

Answer 14

meta directors

Question 15

Strong deactivating groups

Answer 15

meta directors

Question 16

Answer 16

Strong activator

ortho-para director

Question 17

Answer 17

Strong activator

ortho-para director

Question 18

Answer 18

Strong activator

ortho-para director

Question 19

Answer 19

Strong activator

ortho-para director

Question 20

Answer 20

Strong activator

ortho-para director

Question 21

Answer 21

Moderate activator

ortho-para director

Question 22

Answer 22

Moderate activator

ortho-para director

Question 23

Answer 23

Moderate activator

ortho-para director

Question 24

Answer 24

Moderate activator

ortho-para director

Question 25

Answer 25

Weak **activator** *ortho-para* director

Question 26

Answer 26

Weak **deactivator** *ortho-para* director

Question 27

Answer 27

Moderate **deactivator** *meta* director

Question 28

Answer 28

Moderate **deactivator** *meta* director

Question 29

Answer 29

Moderate **deactivator** *meta* director

Question 30

Answer 30

Moderate **deactivator** *meta* director

Question 31

Answer 31

Moderate **deactivator** *meta* director

Question 32

Answer 32

Moderate **deactivator** *meta* director

Question 33

Answer 33

Moderate **deactivator** *meta* director

Question 34

Answer 34

Strong **deactivator** *meta* director

Question 35

Answer 35

Strong **deactivator** *meta* director

Question 36

Answer 36

Strong **deactivator** *meta* director

Question 37

Identifying directing effects for di- and poly-substituted benzene rings

Answer 37

1. Identify each group 2. Select the **most powerful activator** and identify where it is directing to 3. Identify the unoccupied position as the most likely site for EAS 4. Consider steric effects; identify least sterically hindered location

Question 38

Nucleophilic aromatic substitution

Answer 38

_Reagents_ 1. **Strong Nuc, 70° C** 2. **H₃O⁺** _Mechanism_ Benzene ring is attacked by a strong nucleophile which creates a resonance-stabilized intermediate called a **Meisenheimer complex** that exhibits a *negative* charge throughout the ring _Criteria for reaction to proceed_ 1. Ring must contain a strong electron-withdrawling group; typically a *nitro group* 2. Ring must contain a good leaving group; typically a halogen 3. The leaving group must be either ***ortho*** or ***para*** to the strong deactivating group

Question 39

Elimination-addition reactions

Answer 39

_Reagents_ 1. **NaNH₂** (sodium amide)**, NH₃ (*l*)** 2. **H₃O⁺** _Mechanism_ Elimination-addition reaction occurs through a high energy radical/alkyne intermediate called a **benzyne**

Question 40

Determining reaction type between: * EAS * S_NAr * Elimination-addition

Answer 40