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 position on the bezene 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:
|R in C6H5R||% o- product||% m- product||% p- product|
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 makemostly meta-substituted products. here may be small amounts ofortho- and para- products, but don't worry about that. Focus on the bigger picture. Some groups are "meta-directors". These regiochemical effects are very closely related to the activating and directing effects we have already seen. If we want to understand this data, we need to think about things like π-donation, π-acceptance, inductive effects and cation stability.
Show resonance structures for the cationic intermediate that results during nitration of toluene (methylbenzene). Explain why a mixture of ortho- and para- substitution results.
Show resonance structures for the cationic intermediate that results during nitration of chlorobenzene. Explain why a mixture of ortho- and para- substitution results.
Show resonance structures for the cationic intermediate that results during nitration of acetophenone (C6H5COCH3). Explain why mostly meta- substitution results.
Show resonance structures for the cationic intermediate that results during nitration of acetanilide (C6H5NH(CO)CH3). Explain why a mixture of ortho- and para- substitution results.
In general, we can divide these substituents into three groups:
- π-acceptors are meta- directors.
- π-donors are ortho-/para- directors.
- alkyls are ortho-/para- directors.
Note that, once again, we may have two competing effects in one substituent, such as a halogen. In halogens, although the net effect may be to slow the reaction down, that weak π-donation is still enough to tilt the balance of products in favour of ortho- and para- substitution.
Fill in the major organic products of the following reactions.
Given two different substituents on a benzene, there can sometimes be a conflict in predicting which substitution pattern will result. Generally, the group with the stronger activating effect wins out. Predict the major products of the following reactions.