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 $LiAl{H}_4.$
• 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 $LiAl{H}_4$.

Answer 1

Answer: (C)

(i) This method will not work
$RCOOH\stackrel{SOC{l}_2/PC{l}_3/PC{l}_5}{\to }RCOCl\stackrel{H_{2}O}{\to }RCOOH$
Aldehyde will not form
(ii) Carboxylic acid are not reduced by $LiAl{H}_4.$
(iii) This method is best
$RCOOH\stackrel{SOC{l}_2/PC{l}_3/PC{l}_5}{\to }RCOCl\stackrel{\text{ lithium aluminiumtri }(\text{ t-butoxy })\text{ hydride. }}{\to }RCHO$ (aldehyde)
(iv) This method will produce alcohol rather than aldehyde:
$RCOOH\stackrel{SOC{l}_2/PC{l}_3/PC{l}_5}{\to }RCOCl\stackrel{\left(I\right)R^{\prime }MgBr}{\to }RCO{R}^{\prime }\stackrel{LiAl{H}_4}{\to }{3}^{\circ }alcohol$