Question

2,6-Di-tert-butyl-4-methylphenol, alternatively known as butylated hydroxytoluene (BHT), is used as an antioxidant in foods to "retard spoilage" (Section 8.7). BHT is synthesized industrially from 4 -methylphenol by reaction with 2 -methylpropene in the presence of phosphoric acid. Propose a mechanism for this reaction. C=C(C)C CC1=CC(C(C)(C)C)=C(O)C(C(C)(C)C)C1 4-Methylphenol 2-Methylpropene 2,6-Di-tert-butyl-methylphenol "Butylated hydroxytoluene" ( \(p\)-Cresol) "Butylated hydrox (BHT)

Short answer

Question: Describe the mechanism of synthesizing butylated hydroxytoluene (BHT) from 4-methylphenol and 2-methylpropene in the presence of phosphoric acid, and provide the intermediate structures involved in each step. Answer: The mechanism involves the following steps: 1. Protonation of 2-methylpropene by phosphoric acid to form a carbocation. 2. Attack of the carbocation on the ortho carbon of 4-methylphenol, forming an intermediate with a new carbon-carbon bond. 3. Deprotonation of the intermediate by H2PO4- ion, generating a para-substituted product. 4. Reaction of the para-substituted product with another 2-methylpropene and phosphoric acid, following the same sequence of steps, to eventually form 2,6-Di-tert-butyl-4-methylphenol (BHT).

Step-by-step solution

Identify the starting materials and the product

The starting materials for the reaction are 4-methylphenol and 2-methylpropene. The product of the reaction is 2,6-Di-tert-butyl-4-methylphenol (BHT). 4-Methylphenol: CC1=CC=C(O)C=C1 2-Methylpropene: C=C(C)C BHT: CC1=CC(C(C)(C)C)=C(O)C(C(C)(C)C)C1

Protonation of 2-methylpropene

In the presence of phosphoric acid (H3PO4), 2-methylpropene is protonated to form a carbocation. This is an electrophilic addition reaction. C=C(C)C + H3PO4 -> [C+](C)C-C + H2PO4-

Attack of the carbocation on the 4-methylphenol

The carbocation formed in step 2 attacks the ortho carbon of 4-methylphenol, leading to the formation of an intermediate with a new carbon-carbon bond. [C+](C)C-C + CC1=CC=C(O)C=C1 -> CC1=CC(C(C)C+)=C(O)C=C1

Deprotonation of the intermediate

The intermediate formed in step 3 undergoes deprotonation with the participation of the H2PO4- ion, generating the para-substituted product. CC1=CC(C(C)C+)=C(O)C=C1 + H2PO4- -> CC1=CC(C(C)(C)C)=C(O)C=C1 + H3PO4

Formation of the final product (BHT)

The para-substituted product formed in step 4 undergoes a similar sequence of steps with another molecule of 2-methylpropene and phosphoric acid. This results in the formation of the final product, 2,6-Di-tert-butyl-4-methylphenol (BHT). CC1=CC(C(C)(C)C)=C(O)C=C1 + 2-Methylpropene + H3PO4 -> CC1=CC(C(C)(C)C)=C(O)C(C(C)(C)C)C1 + H2PO4- The proposed mechanism successfully explains the synthesis of BHT from 4-methylphenol and 2-methylpropene in the presence of phosphoric acid.