Answer to Question #94546 in Organic Chemistry for Natasha Prasad

Question

Question 2
Heating any of the three isomeric dimethylbenzenes with HF in the presence of BF3 to 80oC leads to the equilibrium mixture shown below. Formulate a mechanism for these isomerizations, starting with 1,2-dimethylbenzene and simply using H+ to represent the acid. Why is the equilibrium concentration of the 1,2-isomer the lowest?

Answer 1

O-Xylene isomerization proceeds via sequential intermolecular methyl shifts to form m-xylene and p-xylene. The unimolecular isomerization pathways involve only protonation-deprotonation pathways with interspersed methyl shifts.

m-xylene and p-xylene are more thermodinamically stable as methyl groups are further apart.