Question

Explain how the percentages just given show that the methyl group exerts an ortho–para directive effect by considering the percentages that would be obtained if the methyl group had no effect on the orientation of the incoming electrophile.

Answer 1

Although there are five hydrogens available to be replaced on the benzene ring of toluene , two of those are directly adjacent (ortho) to the methyl group. Attack at either of these positions gives the same product. Similar considerations apply to the meta position. The result is that only three products are obtained. The actual distribution of these products shows that very little of the meta product is obtained. Instead almost all of the product consists of the ortho and para isomers. For this reason the methyl group is called an "ortho-para directing group." It "directs" the incoming electrophile to attack at the orthoand para positions relative to itself.

How does the methyl group exert this "directive" effect? We can begin to make sense of this by remembering that we explained Markovnikov's rule by considering that the product which is most abundant is formed fastest through a pathway which has the lowest activation energy. That lead us to a consideration of the relative energies of competing transition states and the use of intermediates as useful "stand-ins" for those transition states. We concluded that Markovnikov's rule is explained by the idea that the more abundant products are formed by pathways which go through lower energy intermediates.

If we apply the same reasoning to these directive effects, the question becomes "What makes the intermediates for the formation of ortho and para products more stable than those for para products? As usual, we will look for the answers by comparing the structures of the intermediates.


Each of the intermediates is described by resonance hybrid which includes three carbocations. However, for the intermediates leading to the ortho and para products, one of the contributing carbocationic structures is tertiary. This structure is more stable than the others because the electrons on the methyl group can directly stabilize the electron deficient carbocationic carbon. This stability is passed on to the resonance hybrid, which makes the intermediates for attack at the more stable than that for attack at the meta position. More stable intermediates mean lower energy transition states and faster reactions. A faster reaction means more product is formed through that pathway.