O2N−CH2−COOH→PO2N−CH2−CH2OH. P is

LiAlH4 and B2H6 can reduce –COOH-to-CH2OH, LiAlH4 reduces –NO2 group to –NH2 group. B2H6 cannot reduce -NO2 group.∴O2N−CH2−COOH→B2H6O2N−CH2−CH2OHO2N−CH2−COOH→LiAlH4H2N−CH2−CH2OH