16. Chemistry of Benzene: Electrophilic Aromatic Substitution Part 2 Based on McMurry’s Organic Chemistry, 6 th 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? The orientation will depend on: - directing power of each substituent - steric effects ? ? ?

3 Disubstituted Benzenes: Directing Power Case A If the directing effects of the two groups are the same, the result is additive. -Me is an o/p director -NO 2 is a meta director

4 Case B If 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 more powerful 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 -NR 2 groups Class II: alkyl and halogen Class III: all meta directors Example: 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? The orientation will depend on: - directing power of each substituent - steric effects