Question

Write a detailed mechanism for the reduction of acetic acid to ethanol by \(\mathrm{LiALH}_{4}\).

Short answer

The reduction of acetic acid to ethanol by LiAlH₄ involves a series of steps: 1) Formation of hydride ion (H⁻) from LiAlH₄ as a nucleophile, 2) nucleophilic attack of H⁻ on the partially positive carbonyl carbon of acetic acid, breaking the π bond, 3) protonation of the resulting alkoxide intermediate, converting the negative oxygen atom to an alcohol group (-OH), and 4) the formation of ethanol as the final product. The overall reaction is: Acetic acid + LiAlH₄ → Ethanol + LiAl(OH)₃.

Step-by-step solution

Formation of the nucleophile

In this step, we will focus on the formation of the hydride ion (H-) nucleophile from LiAlH4. Lithium aluminum hydride is a strong reducing agent, which can easily donate a hydride ion (H-) to carbonyl groups. When it comes in contact with acetic acid, a hydride ion (H-) is released from the LiAlH4 structure.

Nucleophilic attack

The hydride ion (H-) formed in Step 1 attacks the carbonyl carbon of the acetic acid molecule, making it a nucleophile. The carbonyl carbon is partially positive because of the electronegativity of the oxygen atom double-bonded to it. The nucleophilic attack by the hydride ion breaks the π bond of the carbonyl group and forms a single bond between the carbonyl carbon atom and the hydride ion.

Protonation of the intermediate

The addition of the hydride ion to the carbonyl carbon results in an alkoxide intermediate. This intermediate has a negative charge on the oxygen atom. Next, a proton (H+) available from acidic conditions (which can be provided by another molecule of acetic acid or any protic solvent) will donate a proton to this negatively charged oxygen atom, resulting in the formation of an alcohol group (-OH) in the organic molecule.

Formation of the final product

After the protonation of the intermediate alkoxide, we have successfully replaced the carbonyl group in the acetic acid molecule with an alcohol group, resulting in the formation of ethanol, our final product. The overall reaction is written as: Acetic acid + LiAlH₄ → Ethanol + LiAl(OH)₃ In conclusion, using LiAlH4 as a reducing agent can successfully reduce acetic acid to ethanol through a series of steps including the formation of a nucleophile, nucleophilic attack, protonation of intermediate, and formation of the final product.

Key concepts

LiAlH4 Reduction

Lithium aluminum hydride, abbreviated as \textbf{LiAlH\(_4\)}, plays a pivotal role as a strong reducing agent in transforming various functional groups to alcohols or other less oxidized forms. Specifically, in the context of converting acetic acid to ethanol, LiAlH\(_4\) donates a hydride ion (\textbf{H\(^-\)}), which is essentially a hydrogen atom with an additional electron, acting as a very potent nucleophile. This hydride transfer is the initial crucial step in the reduction process.

LiAlH\(_4\) tends to react vigorously with water and other protic solvents, releasing hydrogen gas, thus it's usually handled in an anhydrous environment to prevent premature reaction. When used to reduce acetic acid, it effectively converts the carbonyl group of the acid to a primary alcohol group. The by-products of the reaction include lithium ions and aluminum hydroxide, which is often depicted as LiAl(OH)\(_3\) or similar variations depending on the exact stoichiometry and conditions.

Nucleophilic Attack

A \textbf{nucleophilic attack} is a cornerstone of many organic reactions, and it is the moment when a nucleophile, carrying a pair of electrons, sets its sights on a positively charged or electron-deficient center. In our case, the hydride ion (\textbf{H\(^-\)}) generated from the LiAlH\(_4\) assumes the role of the nucleophile.

During the attack, the hydride ion targets the carbonyl carbon of acetic acid. The carbonyl carbon is electrophilic due to the polar bond between carbon and oxygen, as the oxygen's strong electronegativity creates a partial positive charge on the carbon atom. The hydride ion's additional electron pair allows it to form a bond with this carbonyl carbon, resulting in the breaking of the carbon-oxygen double bond. This redistributes electrons and is a key step in transforming the carbonyl group into an alkoxide intermediate, setting the stage for the subsequent formation of an alcohol.

Carbonyl Group Reduction

The reduction of the \textbf{carbonyl group} involves converting the carbon-oxygen double bond into a single bond while adding a hydrogen atom to the carbon and a hydroxyl group (-OH) to the oxygen. This reduction is typically achieved through a two-step process. First, as mentioned, a nucleophilic attack occurs, with the hydride ion from LiAlH\(_4\) attacking and bonding to the carbonyl carbon. As a result, we witness the transformation of the double-bonded oxygen into an alkoxide anion with a single bond to carbon.

The second step is \textbf{protonation}, where a proton (H\(^+\)), often sourced from an acid like acetic acid itself, collides with the oxygen atom on the alkoxide to neutralize its charge. As a result of this step, the oxygen atom becomes a hydroxyl group, and the former carbonyl group is now fully reduced to an alcohol functional group, completing the conversion from acetic acid to ethanol.

Mechanism of Alcohol Formation

The \textbf{mechanism of alcohol formation} from a carbonyl compound, such as acetic acid, is an intricate dance that involves careful choreography of electrons. It starts with the formation of a hydride ion from LiAlH\(_4\), which attacks the carbonyl group. Following the nucleophilic attack, the alkoxide intermediate emerges, which consists of an oxygen atom holding a negative charge.

For the finale, a proton source, such as another acetic acid molecule or a protic solvent, donates a proton (H\(^+\)) to the oxygen atom. The negative charge of the alkoxide oxygen pulls the proton towards itself, and upon receiving this proton, the compound stabilizes into a neutral alcohol molecule. The final step reveals the transformation of the carbonyl group of acetic acid into the hydroxyl group of ethanol, thereby creating the desired alcohol.