Q17P - Organic Chemistry (Mcmurry)

Question

Show the mechanism for the reaction of p-methylphenol with 2-methylpropene and ${\mathbf{H}}_3{\mathbf{PO}}_4$ catalyst to yield the food additive BHT.

Step-by-Step Solution

Answer

The mechanism for the reaction of p-methylphenol with 2-methylpropene and ${\mathrm{H}}_3{\mathrm{PO}}_4$  catalyst is:

1.

2.

3.

Step 1: Preparation of BHT

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.

Step 2: The protonation of 2-methylpropene

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  ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$

Step 3: The alkylation of p-cresol

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

Step 4: The formation of BHT

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