Electrophilic Aromatic Substitution (EAS) : Directing Effects

In addition to exerting an effect on the speed of reaction, substituents on the benzene ring also influence the regiochemistry of the reaction. That is, they control where the new substituent appears in the product.

Remember, there are three different positions on the benzene ring where a new substituent can attach, relative to the original substituent.  Substitution could actually occur on five positions around the ring, but two pairs are related by symmetry. Isomerism in disubstituted benzenes can be described by numbering the substituents (1,2- etc) or by the relationships ortho-, meta– and para-.  There are two positions ortho– to the initial substituent and two positions meta– to it.

Ingold and colleagues investigated the question of regiochemistry in nitration.  They reported the following observations:

In looking at the table, you might see that there are two groups of substituents. One group reacts to make mixtures of ortho– and para– products. There may be different ratios of ortho– to para– and there may be small amounts of meta-, but don’t get bogged down in the details. Focus on the bigger picture. Some groups are “ortho/para directors”.

The other group reacts to make mostly meta-substituted products. Here may be small amounts of ortho– and para– products, but these groups are best described as “meta-directors”. These regiochemical effects are very closely related to the activating and directing effects we have already seen. If we want to understand these data, we need to think about things like π-donation, π-acceptance, inductive effects and cation stability.