Acidic Nature of Nitroalkanes

The alpha-hydrogen of a primary or secondary nitroalkane shows acidic character because the strongly electron-withdrawing -NO2 group pulls electron density away, stabilising the conjugate base once that H is removed -- so much so that nitroalkanes are MORE acidic than aldehydes, ketones, esters and cyanides bearing a comparable alpha-H. This is why nitroalkanes dissolve in aqueous NaOH to form a salt (the aci-nitro salt), and why the aci-nitro tautomer, once formed, is itself more acidic than the parent nitro form. Acidity falls as more alkyl groups are attached to the alpha-carbon, since each alkyl group's electron-donating (+I) effect opposes the electron-withdrawing pull of -NO2: the measured order is CH3-NO2 (nitromethane, no alkyl substitution on the alpha-carbon beyond the H's themselves) > CH3CH2-NO2 (nitroethane, one ethyl-type substitutio …