Reduction Process: The reduction of carboxylic acids to alcohols using lithium aluminum hydride (LiAlH₄) involves a decrease in the oxidation state of the carbon in the acid.
Alternative Reagent: Borane (BH₃) can also be used for this reduction and is more chemoselective.
Mechanism
Step 1: Acid-Base Reaction
Carboxylic Acid Formation: Start with the carboxylic acid, focusing on the acidic proton on the oxygen.
Lithium Aluminum Hydride: Acts as a strong base; a hydride ion (H⁻) is a hydrogen atom with two electrons.
Formation of Carboxylate Anion: Proton from the carboxylic acid is removed by the hydride, forming a carboxylate ion (negatively charged oxygen) and hydrogen gas (H₂).
Step 2: Formation of Aldehyde
Carboxylate Anion and Aluminum Hydride Reaction:
Formation of bond between oxygen and aluminum (AlH₃), and between carbon and hydrogen.
Results in a tetrahedral carbon structure, forming an aldehyde.
Step 3: Reduction to Alcohol
Further Reduction:
Excess lithium aluminum hydride reduces the aldehyde to an alcohol.
Hydride ion attacks the carbonyl carbon, turning the aldehyde into an alcohol.
Protonation:
The alkoxide ion (formed during reduction) is protonated to yield the alcohol as the product.
Application and Selectivity
Practice Problem
Example: Reduction of a compound with both a carboxylic acid and ketone group using LiAlH₄.
Both functional groups (carboxylic acid to primary alcohol and ketone to secondary alcohol) are reduced.
Borane as a Reducing Agent
Chemoselectivity: Borane selectively reduces only the carboxylic acid group.
Application: Useful when selective reduction is needed to prevent reduction of other functional groups such as ketones.
Conclusion
Lithium aluminum hydride is effective in reducing carboxylic acids to alcohols, but is less selective compared to borane, which is preferred for its chemoselectivity in certain situations.