Question

Predict the product, if any, of reaction between methyl acetate and the following reagents: (a) \(\mathrm{LiAlH}_{4}\), then \(\mathrm{H}_{3} \mathrm{O}^{+}\) (b) \(\mathrm{CH}_{3} \mathrm{MgBr}\), then \(\mathrm{H}_{3} \mathrm{O}^{+}\) (c) \(\mathrm{NaOH}, \mathrm{H}_{2} \mathrm{O}\) (d) Aniline

Short answer

(a) Ethanol and methanol; (b) Tertiary-butyl alcohol; (c) Sodium acetate and methanol; (d) No reaction.

Step-by-step solution

Understanding Reaction (a)

Methyl acetate ( (CH₃COOCH₃ )) is reacted with LiAlH₄ followed by H₃O⁺ . LiAlH₄ is a strong reducing agent that converts esters into primary alcohols. The ester group of methyl acetate is reduced to methanol (primary alcohol). Thus, the product is ethanol and methanol after hydrolysis.

Understanding Reaction (b)

Methyl acetate reacts with CH₃MgBr (Grignard reagent) followed by H₃O⁺ . Grignard reagents react with esters to produce tertiary alcohols. The reaction will involve the nucleophilic addition of the Grignard reagent to the carbonyl carbon, leading to the formation of a tertiary alcohol, specifically, tertiary-butyl alcohol after hydrolysis.

Understanding Reaction (c)

Methyl acetate is reacted with NaOH (a strong base) and H₂O (water). This is a saponification reaction where the ester is converted into a carboxylate salt (sodium acetate) and methanol. The ester linkage in methyl acetate is cleaved during the reaction resulting in sodium acetate and methanol as the products.

Understanding Reaction (d)

Methyl acetate's reactivity with aniline (a nucleophile and weak base) is generally limited because aniline doesn't typically undergo nucleophilic substitutions with esters under normal conditions without the use of a catalyst. Therefore, there is generally no reaction expected between methyl acetate and aniline under standard conditions.

Key concepts

LiAlH4 reactions

Lithium aluminium hydride, often expressed as \( \mathrm{LiAlH}_4 \), is renowned for its strong reducing capabilities. It is commonly used in organic chemistry for the reduction of various functional groups. When esters, such as methyl acetate, come in contact with \( \mathrm{LiAlH}_4 \), they undergo a transformation into primary alcohols.
To illustrate, in the reaction involving methyl acetate \((\mathrm{CH}_3\mathrm{COOCH}_3)\), \( \mathrm{LiAlH}_4 \) targets the ester functional group. This reaction results in the cleavage of the ester bond followed by the addition of hydrogens, yielding methanol \((\mathrm{CH}_3\mathrm{OH})\).
Following the reduction by \( \mathrm{LiAlH}_4 \), an acidic work-up, usually using \( \mathrm{H}_3\mathrm{O}^+ \), is performed. This final step ensures the protonation of the oxygen leading to full conversion of the ester into alcohol. Therefore, from methyl acetate, one can expect methanol and ethanol as the primary products following reduction with \( \mathrm{LiAlH}_4 \).

Grignard reagents and esters

Grignard reagents, typically represented as R-MgX, where R is an alkyl or aryl group and X is a halide, serve as excellent sources of nucleophilic carbon. These reagents are pivotal in forming carbon-carbon bonds. In reactions with esters, such as methyl acetate, Grignard reagents lead to the formation of tertiary alcohols.
When methyl acetate reacts with a Grignard reagent like \( \mathrm{CH}_3\mathrm{MgBr} \), the process begins with the nucleophilic attack on the carbonyl carbon of the ester. This step forms a ketone intermediate, which is further attacked by another equivalent of the Grignard reagent. This dual addition results in the formation of a tertiary alcohol after hydrolysis.
Specifically, in the case of methyl acetate, the product is tertiary-butyl alcohol. The reaction pathway underscores the Grignard reagent's efficacy in synthesizing complex alcohol structures from simple ester precursors.

Saponification of esters

Saponification refers to the hydrolysis of esters under basic conditions, usually employing a strong base like \( \mathrm{NaOH} \). This reaction is classically used in soap making and results in the breaking of the ester bond to form an alcohol and a carboxylate salt.
In the case of methyl acetate, saponification begins with the nucleophilic attack by the hydroxide ion \(( \mathrm{OH}^- )\) on the carbonyl carbon of the ester. This process leads to the breaking of the ester linkage and the formation of a sodium carboxylate salt, known as sodium acetate, along with methanol.
This reaction is significant in organic synthesis due to its ability to efficiently cleave esters and provide useful acid and alcohol components. In practical applications, saponification is a widely employed process, especially in industries pertaining to detergent and soap production.