Question

Formation of tert-butyl ethanoate by direct esterification goes very poorly: Explain why the reaction fails, and indicate the products you actually expect to form on heating a mixture of ethanoic acid and tert-butyl alcohol with sulfuric acid as a catalyst.

Short answer

The reaction fails due to the stability of the tert-butyl carbocation, leading to isobutylene formation instead of tert-butyl ethanoate.

Step-by-step solution

Understanding the Reaction

To form tert-butyl ethanoate by direct esterification, a carboxylic acid needs to react with an alcohol. In this case, ethanoic acid (acetic acid) is reacting with tert-butyl alcohol in the presence of sulfuric acid.

Importance of Carbocation Stability

In acidic conditions, the alcohol may protonate and lose water to form a carbocation. Tertiary carbocations like tert-butyl cations are highly stable, favoring carbocation formation. This suggests possible pathways for tert-butyl alcohol under these conditions.

Observation of Competing Reactions

Due to the stability of the tert-butyl carbocation, it tends to undergo rearrangement or elimination reactions rather than form the ester. This leads to side reactions such as dehydration of tert-butyl alcohol to form isobutylene (an alkene).

Expected Products Formation

With sulfuric acid acting not only as a catalyst but also as a dehydrating agent, the reaction primarily yields isobutylene and may only produce small amounts of tert-butyl ethanoate, if at all. Direct esterification is inefficient for tertiary alcohols due to these competitive reactions.

Key concepts

Carboxylic Acids

Carboxylic acids are a versatile and important functional group in organic chemistry. They are characterized by the presence of a carboxyl group (-COOH). This structure consists of a carbonyl group (C=O) bonded to a hydroxyl group (O-H). Carboxylic acids have distinct acidic properties because the hydrogen of the hydroxyl group can be easily donated as a proton (H⁺).

In esterification reactions, such as the attempted formation of tert-butyl ethanoate, carboxylic acids react with alcohols to form esters and water. The specific carboxylic acid involved here is ethanoic acid, commonly known as acetic acid. It plays a crucial role in the reaction by donating its hydrogen ion, which participates in the formation of the ester linkage.

However, having tertiary alcohol like tert-butyl alcohol complicates the reaction, as seen in this case, due to competing processes.

Carbocation Stability

Carbocations are positively charged ions with a carbon atom bearing the positive charge. They play a key role in many organic reactions, including the one discussed here. The stability of a carbocation is crucial in determining the pathways and products of reactions. Generally, the stability of carbocations increases as you move from primary to secondary to tertiary carbocations. This stability is due to the ability of surrounding carbon atoms to donate electron density through hyperconjugation and induction, stabilizing the positive charge.

In the reaction of ethanoic acid and tert-butyl alcohol, under acidic conditions, the alcohol can lose water after being protonated, forming a carbocation. Tertiary carbocations like the tert-butyl cation are particularly stable. Because of their stability, they can easily undergo side reactions such as rearrangement or elimination, rather than forming esters as initially intended. Thus, even though the formation of a carbocation might initially seem beneficial for esterification, it actually leads to alternate, competing reactions.

Acid Catalysis

In esterification, acid catalysis is a method to speed up the reaction without being consumed by it. Sulfuric acid serves as the catalyst in the reaction between ethanoic acid and tert-butyl alcohol. It donates a proton to the reaction mixture, helping to protonate the alcohol and facilitate the loss of water, which is critical for the initial formation of the carbocation.

However, in this specific reaction, sulfuric acid also serves as a dehydrating agent, which can promote the formation of alkenes through elimination reactions. This dual role can lead to side products such as isobutylene, rather than the desired tert-butyl ethanoate. The role of the acid, therefore, is not only to catalyze the esterification but also to drive other possible dehydration pathways that compete with the esterification process.

Tertiary Alcohols

Tertiary alcohols, such as tert-butyl alcohol, contain a hydroxyl group (-OH) bonded to a carbon atom that is connected to three other carbon atoms. This structure generally makes them less reactive in certain organic reactions, such as esterification. Their steric hindrance and the stable formation of carbocations make them prone to undergo dehydration over esterification.

In this reaction, instead of forming tert-butyl ethanoate, the tert-butyl alcohol frequently dehydrates due to the presence of an acid like sulfuric acid. This results in the production of isobutylene, a more energetically favorable pathway compared to ester formation. Thus, the strong tendency of tertiary alcohols to undergo side reactions underlies the inefficiency of this esterification process.