Question

What represents the best method for converting a carboxylic acid to an aldehyde?

• A. Convert the acid to an acid chloride, and react the acid chloride with water.
• B. Reduce the acid with $LiAlH_{4}.$
• C. Convert the acid to an acid chloride, and reduce the acid chloride with lithium aluminium tri(t-butoxy) hydride.
• D. Convert the acid to an acid chloride, react the acid chloride with a Grignard reagent, and reduce the product with $LiAlH_4$.

Answer 1

Answer: (C)

(i) This method will not work
$R COOH \xrightarrow{ SOCl _{2} / PCl _{3} / PCl _{5}} RCOCl \xrightarrow{ H _{2} O }RCOOH$
Aldehyde will not form
(ii) Carboxylic acid are not reduced by $LiAlH_{4}.$
(iii) This method is best
$RCOOH \xrightarrow{ SOCl _{2} / PCl _{3} / PCl _{5}} RCOCl \xrightarrow{\text { lithium aluminiumtri }(\text { t-butoxy }) \text { hydride. }}RCHO$ (aldehyde)
(iv) This method will produce alcohol rather than aldehyde:
$RCOOH\xrightarrow{S O C l_{2} / P C l_{3} / P C l_{5}}RCOCl\xrightarrow{\left(I\right) R'M g B r}RCOR'\xrightarrow{L i A l H_{4}}3^{\circ}alcohol$