Definition

Addition of lithium aluminum hydride (LiAlH4) to carboxylic acids leads to the formation of primary alcohols (after addition of acid).

Structural reaction scheme for Reduction [LiAlH4], diagram 1 of 4
Figure 1. Reduction [LiAlH4]

Explanation

Lithium aluminum hydride abbreviated LAH, is capable of reducing carboxylic acids all the way to the corresponding primary alcohols, which are obtained upon aqueous acidic work-up.

Although the exact mechanism of this transformation is not completely understood, the first step is likely a proton transfer. Lithium aluminum hydride is a powerful nucleophile and a strong base which may deprotonate carboxylic acid.

Structural reaction scheme for Reduction [LiAlH4], diagram 2 of 4
Figure 2. Reduction [LiAlH4]

One of the possibilities for the rest of the mechanism is a reaction of the carboxylate ion with AlH3 followed by elimination to form an aldehyde.

Structural reaction scheme for Reduction [LiAlH4], diagram 3 of 4
Figure 3. Reduction [LiAlH4]


This aldehyde cannot be isolated and it is further attacks by LAH to form an alkoxide ion, which is then protonated.

Structural reaction scheme for Reduction [LiAlH4], diagram 4 of 4
Figure 4. Reduction [LiAlH4]