Question

The substitution reaction of toluene with Br2 can, in principle, lead to the formation of three isomeric bromotoluene products. In practice, however, only o- and p-bromotoluene are formed in substantial amounts. The meta isomer is not formed. Draw the structures of the three possible carbocation intermediates, and explain why ortho and para products predominate over meta products.

Answer 1

Answer:

See explanation and image attached

Explanation:

Aromatic hydrocarbons undergo electrophillic substitution. Usually, substituted benzene is more or less reactive to electrophillic substitution compared to unsubstituted benzene.

Substituents on the benzene ring tend to direct the incoming electrophile during electrophillic substititution. The presence of the -CH3 group on toluene directs the incoming Br electrophile to the ortho/para position.

Where the incoming electrphile E is Bromine, we can see that in the ortho/ para product, the electron pushing -CH3 stabilizes the resonance structure formed and increases electron density at the ortho/para position via resonance compared to the meta product as we can see from the image attached. Hence, the ortho and para products predominate over meta products.

Image credit: Chemistry steps