📕 Chapter 18 – Aromatic Substitution Reactions

EAS stands for

Electrophilic Aromatic Substitution

Why substitution instead of addition?

Preserves aromaticity

General EAS mechanism

Attack electrophile → sigma complex → deprotonation

Br2/FeBr3

Bromination

Cl2/FeCl3

Chlorination

HNO3/H2SO4

Nitration

Electrophile in nitration

NO2

SO3/H2SO4

Sulfonation

Unique feature of sulfonation

Reversible

R-Cl/AlCl3

Friedel–Crafts alkylation

RCOCl/AlCl3

Friedel–Crafts acylation

Problems with Friedel–Crafts alkylation

Rearrangements, polyalkylation, fails on strongly deactivated rings

Advantages of Friedel–Crafts acylation

No rearrangements; no polyacylation

Electron-donating groups

Activate; ortho/para directors

Electron-withdrawing groups

Deactivate; meta directors

Halogens

Deactivating but ortho/para directing

Strong activators

NH2, NHR, NR2, OH, OR

Weak activators

Alkyl groups

Strong deactivators

NO2, CN, SO3H, CHO, COR, COOH, COOR

Requirements for SNAr

Leaving group + strong EWG ortho/para to leaving group

Meta EWG in SNAr

No reaction

Meisenheimer complex

Intermediate in SNAr

Benzyne reagents

NaNH2 and heat

Benzyne mechanism

Elimination–addition

Nitro group reduction

NO2 → NH2

How to identify mechanism

Electrophile=EAS; Nucleophile+EWG=SNAr; NaNH2/heat=Benzyne