Question

Predict the product of the reaction of \(p\) -methylbenzoic acid with each of the following: (a) \(\mathrm{LiAlH}_{4},\) then \(\mathrm{H}_{3} \mathrm{O}^{+}\) (b) \(N\) -Bromosuccinimide in \(\mathrm{CCl}_{4}\) (c) \(\mathrm{CH}_{3} \mathrm{MgBr}\) in ether, then \(\mathrm{H}_{3} \mathrm{O}^{+}\) (d) \(\mathrm{KMnO}_{4}, \mathrm{H}_{3} \mathrm{O}^{+}\)

Short answer

(a) p-methylbenzyl alcohol, (b) p-(bromomethyl)benzoic acid, (c) p-(1-hydroxyethyl)benzene, (d) benzoic acid.

Step-by-step solution

Understanding the Reactants

We need to predict the products of reactions involving p-methylbenzoic acid. The reactants given are: \( LiAlH_4 \) with \( H_3O^+ \), \( N \)-Bromosuccinimide in \( CCl_4 \), \( CH_3MgBr \) in ether with \( H_3O^+ \), and \( KMnO_4, H_3O^+ \). Each reactant affects p-methylbenzoic acid differently.

Reaction with LiAlH4 and H3O+

\( LiAlH_4 \) is a strong reducing agent that converts carboxylic acids to primary alcohols. Therefore, p-methylbenzoic acid will be reduced to \( p \)-methylbenzyl alcohol.

Reaction with N-Bromosuccinimide (NBS) in CCl4

\( N \)-Bromosuccinimide (NBS) facilitates allylic bromination. With p-methylbenzoic acid, NBS will lead to the bromination at the benzylic position (next to the methyl group) to produce p-(bromomethyl)benzoic acid.

Reaction with CH3MgBr in Ether then H3O+

\( CH_3MgBr \) is a Grignard reagent that adds to carbonyl groups. The carboxylic acid group in p-methylbenzoic acid reacts to eventually form a tertiary alcohol, following an initial formation of a ketone intermediate. The final product is p-(1-hydroxyethyl)benzene.

Reaction with KMnO4, H3O+

\( KMnO_4 \) is a strong oxidizing agent. When p-methylbenzoic acid is treated with \( KMnO_4 \) and \( H_3O^+ \), the entire methyl side chain is oxidized to a carboxyl group, leading to the formation of benzoic acid.

Key concepts

LiAlH4 reductions

Lithium aluminum hydride ( LiAlH_4 ) is a potent reducing agent used in organic chemistry to reduce various functional groups. This reagent is widely utilized for transforming carboxylic acids, esters, and ketones into their corresponding alcohols. In the case of a carboxylic acid like p -methylbenzoic acid, LiAlH_4 breaks the C=O bond. The result is a primary alcohol as a product. Here, p-methylbenzoic acid is reduced to p-methylbenzyl alcohol. The intermediate once the reaction is complete with LiAlH_4 becomes an alkoxide, which is later protonated by an acid like H_3O^+ to form the final alcohol product.

Steps generally involve:

• Formation of a new C-H bond by hydride ( H^- ) transfer.
• Converting carboxyl (-COOH) group to a hydroxyl (-OH) group.

Grignard Reagent

Grignard reagents, represented by the general formula R-Mg-X , where R is an alkyl or aryl group and X is a halogen, are pivotal in forming carbon-carbon bonds. For p-methylbenzoic acid, using a Grignard reagent like CH_3MgBr facilitates a crucial step in organic synthesis, involving the addition of an R group to a carbonyl carbon.
In this reaction, the Grignard reagent initially transforms the carboxylic acid group into a ketone. Subsequently, another Grignard molecule adds to the ketone, converting it into a secondary or tertiary alcohol. When p-methylbenzoic acid reacts with CH_3MgBr , the outcome is a tertiary alcohol after hydrolysis with H_3O^+ , specifically producing p-(1-hydroxyethyl)benzene.

• Carbon-carbon bond formation.
• Replacement of hydroxyl with a Grignard type alkyl group.
• Formation of a stable alcohol product.

Bromination using NBS

N-bromosuccinimide (NBS) is a versatile brominating agent prominently used for selective halogenation. It is particularly efficient for allylic and benzylic bromination due to its ability to generate bromine radicals. When applied to p-methylbenzoic acid, NBS tends to brominate the position adjacent to methyl, referred to as the benzylic position.
In this reaction, the presence of NBS and an inert solvent like CCl_4 facilitates the substitution of hydrogen with a bromine atom, resulting in the formation of p-(bromomethyl)benzoic acid. This specific bromination follows a radical substitution mechanism, which proceeds via a benzylic radical intermediate.

• Generation of bromine radicals by NBS.
• Substitution at the benzylic position.
• Formation of a brominated benzoic acid.

Oxidation with KMnO4

Potassium permanganate ( KMnO_4 ) is a strong oxidizing agent frequently employed to oxidize organic compounds. When KMnO_4 comes into contact with p-methylbenzoic acid, it targets the alkyl side chain, converting it into a carboxyl group. This transformation is essential for changing the -C(H_x)- moieties into carboxylic acids.
For p-methylbenzoic acid, the specific result of KMnO_4 treatment is demethylation. More precisely, the methyl group is oxidized fully to form an additional carboxyl group, resulting in benzoic acid.

• Conversion of methyl side chains to carboxyl groups.
• Oxidative cleavage of alkyl substituents.
• Ultimately forming a purified benzoic acid product.