Acid Halides to Alcohols: LiAlH₄ Reduction

Lithium aluminum hydride requires two equivalents because the reduction occurs in two distinct stages. The first equivalent converts the acid halide to an aldehyde through nucleophilic attack and halide departure. The second equivalent then attacks the aldehyde intermediate, forming an alkoxide that is protonated to yield the primary alcohol product.

The hydride ion acts as a nucleophile, attacking the carbonyl carbon of the acid halide to form a tetrahedral intermediate. This nucleophilic attack initiates the reduction process, and a second hydride attack on the resulting aldehyde generates an alkoxide intermediate that ultimately yields the primary alcohol.

Lithium tri(tert-butoxy) aluminum hydride is a milder reducing agent due to its bulky tert-butoxy groups, which create steric hindrance and reduce reactivity. This selectivity allows it to reduce acid halides to aldehydes without further reduction to alcohols, whereas lithium aluminum hydride proceeds to the primary alcohol product.

A tetrahedral intermediate forms when the hydride ion attacks the carbonyl carbon. This intermediate has four bonds to the central carbon and represents the transition state before the carbon-oxygen π bond reforms and the halide ion departs as a leaving group.

The halide ion departs as a leaving group during the second step of the mechanism. After the hydride attacks the carbonyl carbon and a tetrahedral intermediate forms, the carbon-oxygen π bond reforms, and the halide ion leaves, generating the aldehyde intermediate.

The alkoxide intermediate is protonated in the final step of the reduction mechanism. This protonation converts the negatively charged alkoxide ion into a neutral primary alcohol product, completing the overall reduction of the acid halide to yield the desired alcohol.

Milder reducing agents like lithium tri(tert-butoxy) aluminum hydride have reduced reactivity due to steric bulk from bulky tert-butoxy groups. This lower reactivity allows them to attack the acid halide carbonyl but prevents the second nucleophilic attack on the resulting aldehyde, stopping the reaction at the aldehyde stage.