Question

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

Short answer

(a) p-Methylbenzoic acid (unchanged), (b) Terephthalic acid, (c) p-Methylbenzyl alcohol.

Step-by-step solution

Understand the Reaction Types

Identify the reagents involved in each reaction and what type of reaction they are likely to undergo with the given starting material, which is p-methylbenzoic acid. (a) In the presence of methylmagnesium bromide (CH₃MgBr), a Grignard reagent, followed by acid work-up, we can predict a reaction involving the acidic proton of the carboxylic acid. (b) With potassium permanganate (KMnO₄) and the acidic condition (H₃O⁺), a strong oxidizing reaction is expected. (c) Lithium aluminum hydride (LiAlH₄), a potent reducing agent, followed by acid work-up is often used to reduce carboxylic acids to alcohols.

Evaluate Reaction (a)

The addition of the Grignard reagent (CH₃MgBr) typically attacks the electrophilic carbon of the carboxyl group in benzoic acids leading to the formation of a ketone intermediate. However, since benzoic acids are deprotonated by the Grignard reagent before nucleophilic addition occurs, the intermediate product is a carboxylate salt. Upon subsequent acid work-up (H₃O⁺), there will be no further reaction with the methyl added Grignard reagent. Therefore, the product is simply the original p-methylbenzoic acid after quenching the Grignard reagent.

Evaluate Reaction (b)

KMnO₄ under acidic conditions fully oxidizes any aromatic side chain to carboxylic acids. The methyl group in p-methylbenzoic acid is oxidized to a carboxylic acid group. As a result, the reaction produces terephthalic acid ( meta-consumer friendly name: 1,4-benzenedicarboxylic acid) with two carboxylic acid groups at the para positions.

Evaluate Reaction (c)

LiAlH₄ reduces carboxylic acids to primary alcohols via intermediate aldehyde formation. In this reaction, the carboxylic group of the p-methylbenzoic acid will be reduced to a primary alcohol. Therefore, the product is p-methylbenzyl alcohol, with the structure where the original carboxylic group is reduced.

Key concepts

Grignard Reagents

Grignard reagents, like (CH₃MgBr), are organometallic compounds used widely in organic chemistry. These reagents are highly reactive and act as nucleophiles, meaning they donate an electron pair to form a new chemical bond.
Grignard reagents are typically used to create carbon-carbon bonds, extending the carbon chain in molecular structures.

• They react with carbonyl compounds such as aldehydes and ketones to form alcohols.
• In reaction with carboxylic acids, however, Grignard reagents first deprotonate the acid, leading to the formation of a carboxylate salt.
• Upon acid work-up, such as with water or (H₃O⁺), any added Grignard reagent is quenched without further reaction.

This quenching does not allow the Grignard reagent to attack other parts of the molecule, and hence, in the presence of enough acid, the starting carboxylic acid is often recovered without alteration.

Oxidation Reactions

Oxidation reactions in organic chemistry involve the increase of oxygen content or the removal of hydrogen from molecules. Potassium permanganate (KMnO₄) is a powerful oxidizing agent often invoked to achieve these transformations. In aromatic chemistry, oxidation of side chains is a typical reaction.
Specifically, in the case of methyl groups attached to aromatic rings like in p-methylbenzoic acid,

• KMnO₄ will oxidize the methyl group to a carboxylic acid group.
• Multiple oxidations can occur, leading to compounds with increased acidic functional groups and conjugation.
• This particular reaction transforms p-methylbenzoic acid into terephthalic acid, providing another carboxylic acid group at the para position to the existing one.

This process highlights the ability of strong oxidizing agents to alter molecular structures extensively, turning hydrocarbon sections into oxygen-rich derivatives.

Reduction Reactions

Reduction reactions involve gaining hydrogen or losing oxygen. Lithium aluminum hydride (LiAlH₄) is a potent reducing agent, used frequently in reducing carbonyl compounds.
It is known for its ability to provide thorough reductions, efficiently converting more oxidized groups to less oxidized forms.

• LiAlH₄ can reduce carboxylic acids to primary alcohols, passing through an intermediate aldehyde stage.
• This reaction involves adding hydrogen atoms to the carbonyl carbon first, then completely reducing the carbon to obtain the alcohol.
• In the case of p-methylbenzoic acid, treatment with (LiAlH₄) results in the formation of p-methylbenzyl alcohol.

LiAlH₄ is particularly useful in synthetic organic chemistry for converting functional groups into forms more amenable for further chemical transformations or use in biological systems.

Carboxylic Acids and Derivatives

Carboxylic acids and their derivatives are key functional groups in organic chemistry, exhibiting diverse reactions and applications. They are characterized by the presence of a carbonyl group (C=O) bonded to a hydroxyl group (OH).
In transformations, carboxylic acids can undergo various reactions due to the reactivity of the acidic hydrogen and the electrophilic carbon.

• They react with bases to form carboxylate salts, showcasing acid-base chemistry.
• Oxidative cleavage experiences of side chains, as seen with KMnO₄, produce new carboxylic acid groups.
• Reductive processes with agents like LiAlH₄ convert them to alcohols efficiently.

Carboxylic acids serve as starting materials for creating esters, amides, anhydrides, and more, paving the way for constructing more complex organic molecules. Understanding their reactivity patterns helps elucidate many organic transformations and syntheses.