(LAH) cannot be used to convert carboxylic acids (RCOOH) to the corresponding aldehydes because: ?LAH is not sufficiently reactive. ?RCOOH is converted into R
C=O. ?RCOOH is reduced to RCH
OH. ?RCOOH is converted into RCOOLI. ?RCOOH is reduced to RCH
.
The Correct Answer and Explanation is:
Correct Answer: RCOOH is reduced to RCH₂OH.
Explanation:
Lithium aluminum hydride (LiAlH₄ or LAH) is a powerful reducing agent commonly used in organic chemistry to reduce various carbonyl-containing compounds. When it comes to carboxylic acids (RCOOH), LAH does not stop at the aldehyde stage (RCHO) during reduction. Instead, it fully reduces carboxylic acids all the way to the corresponding primary alcohols (RCH₂OH).
The mechanism begins with the activation of the carboxylic acid by the hydride ion (H⁻) provided by LAH. Initially, the acidic proton on the hydroxyl group of the carboxylic acid reacts with LAH to form an aluminate salt. This intermediate is then attacked by another hydride ion, which replaces the hydroxyl group and ultimately leads to the reduction of the carbonyl carbon.
At no point during this process is the aldehyde isolated or stabilized. Aldehydes are reactive intermediates and, in the presence of excess LAH, are themselves rapidly reduced to alcohols. Therefore, the reaction proceeds past the aldehyde stage directly to the alcohol.
The inability of LAH to selectively reduce carboxylic acids to aldehydes lies in its strong nucleophilic hydride, which does not differentiate between the carbonyl groups of carboxylic acids and aldehydes. Once an aldehyde is formed, it is even more electrophilic and is immediately attacked by more hydride ions, completing the reduction to a primary alcohol.
To obtain an aldehyde from a carboxylic acid, a milder and more selective reducing agent must be used. One such method is to convert the carboxylic acid to an acid chloride (RCOCl), followed by reduction with a milder reagent like lithium tri-tert-butoxyaluminum hydride [(LiAlH(O-t-Bu)₃)], which stops at the aldehyde stage.
In conclusion, LAH reduces carboxylic acids completely to primary alcohols, not to aldehydes.
