SolutionsReduction
1. LiAlH4 or BH3
Give the product of treating the monoethyl ester of glutaric acid, EtOOC–(CH2)3–COOH: a) with excess LiAlH4 and then H3O+; b) with BH3 in THF and then water.
- 1,5-Pentanediol (and ethanol): LiAlH4 reduces both the acid and the ester.
- Ethyl 5-hydroxypentanoate, HOCH2(CH2)3COOEt: borane reduces the acid much faster than the ester.
2. Extra hydride
Why is more LiAlH4 needed to reduce a carboxylic acid than to reduce an ester? How is this related to the warning on the page?
The first hydride does not reduce: it acts as a base, removes the acidic OH proton and releases H2. Only then is the carboxylate reduced. This acid-base reaction is very fast and exothermic and releases hydrogen, a flammable gas; hence the risk of fire or explosion.