16. Chemistry of Benzene: Electrophilic Aromatic Substitution Part 2

Based onMcMurry’s Organic Chemistry, 6th edition, Chapter 16

2

Reaction of Disubstituted Benzenes

When you have two or more substituents, where on the benzene ring will the substitution reaction occur?

NH2

+ Br2FeBr3 ?

.

.

The orientation will depend on:- directing power of each substituent- steric effects

?

?

?

3

Disubstituted Benzenes: Directing PowerCase A

If the directing effects of the two groups are the same, the result is additive.

-Me is an o/p director

-NO2 is a meta director

4

Case BIf directing effects of the groups oppose each other, then the more powerful activating group decides the outcome.(Mixtures of products may occationally result.)

-OH is-OH ismoremore

powerfulpowerful

Disubstituted Benzenes: Directing Power

5

Directing Power – Which groups control?

The strongest activators are also the strongest directors.

Strongest directing groups

Weakest directing groups

6

Directing Power – Which groups control?We will classify substituents into three groups, according to

their ability to direct the site of reaction.

Class I: -OH, -OR, and -NR2 groups

Class II: alkyl and halogen

Class III: all meta directors

Example: NH2

CH3

NH2

CH3

Br

+ Br2FeBr3

The amino substituent is in Class I, so it controls the orientation.(The methyl group is in Class II.)

STRONGEST

WEAKEST

7

Reaction of Disubstituted Benzenes

When you have two or more substituents, where on the benzene ring will the substitution reaction occur?

NH2

+ Br2FeBr3 ?

.

.

The orientation will depend on:- directing power of each substituent- steric effects