Question

Explain why the reaction of 1,4-di-t-butylbenzene with t-butyl chloride and AlCl3 gives 1,3, 5-tri-t-butylbenzene

Answer 1

To understand this, you need to understand that friedel crafts alkylation is a reversible process.

Alkylating benzene with t-butylchloride will give you the 1,4 t-butyl product. However, further alkylating it will not give you the 1,2,4 t-butyl product as you might expect. What actually happens is a series of alkylations and dealkylations which ultimately lead to the most stable molecule, the 1,3,5 t-butyl product.

The 1,3,5 product is the most thermodynamically stable of the two molecules, as it has less steric repulsion between the bulky alkyl groups. It's known as the thermodynamic product. The 1,2,4 product is a less stable molecule but it forms much more readily because the alkyl groups electronically activate the molecule to this substitution pattern. This is known as the kinetic product.

In a normal electrophilic aromatic substitution, the kinetic product is the one which dominates. However, Friedel Crafts alkylation is a special case because it's reversible. Thus, even though the kinetic product will dominate initially, its instability allows it to be dealkylated back to a disubstituted species. In contrast, what little thermodynamic product is formed will not be dealkylated due to its better stability. Thus the thermodynamic product gradually accumulates as the reaction proceeds; ultimately becoming the major product.