Reductions on the Carboxylic Acid Level


with LiAlH4
LiAlH4 is reactive enough to reduce first to aldehyde and then through to alcohol – not able to stop at the aldehyde unless strictly one equivalent is added at low temperature (-780C).
with DIBAL-H (Di-Iso-Butyl-Aluminium-Hydride)
This reagent is useful because it will reduce an ester to the aldehyde and go no further;

Carboxylic Acids

These are difficult functional groups to reduce using ordinary hydride reagents because they are acidic and so therefore the hydride will react with the H+ and release H2. LiAlH4 will reduce them under vigorous conditions, but then of course it will reduce everything else in the molecule as well – not very useful when selectivity is required. The reagent of choice is diborane (B2H6) because it will reduce carboxylic acids selectively, with the following mechanism;

NB. The ester is left untouched.