Reduction of carboxylic acids

Carboxylic acids are reduced through to PRIMARY ALCOHOLS by the powerful reducing agent lithium aluminium hydride (LiAlH4), carried out in dry ether solvent: R-COOH, with LiAlH4/dry ether, gives R-CH2OH. Carboxylic acids can ALSO be reduced by diborane -- and diborane is notably MORE SELECTIVE than LiAlH4 in one specific, useful way: it does NOT reduce an ester group (-COOR), a nitro group (-NO2), or a halo group (-X) that might be present elsewhere in the same molecule, so diborane can reduce a -COOH group cleanly even in a molecule that also carries one of those other, more sensitive groups. By contrast, sodium borohydride (NaBH4) -- despite being a common, comparatively mild reducing agent for other carbonyl groups such as aldehydes and ketones -- …