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The food additive BHT is prepared from p-cresol, which undergoes Friedel-Crafts alkylation with 2-methylpropene, and this process takes place in the presence of an acid catalyst.It occurs in three steps, protonation of 2-methylpropene by acid, then alkylation of p-cresol, and at last, BHT as a product is formed.

First of all, phosphoric acid (${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$) protonates the 2-methylpropene compound and forms a tert-butyl carbocation along with ${\text{H}}_{\text{2}}{\text{PO}}_{\text{3}}{\text{O}}^{\text{-}}$.

Protonation of 2-methylpropene by ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$

In the second step, the tert-butyl carbocation formed above acts as an electrophile and alkylates p-cresol. The alkylation process occurs at the ortho position to the OH group because of steric and electronic reasons.

Alkylation of p-cresol

In the third step, the product obtained in the second step again undergoes alkylation by second tert-butyl carbocation. This leads to the formation of the food additive BHT.

Formation of BHT