Rate constants k(DO) (M-1 s(-1)) for the deprotonation of cyanoalkanes by deuterioxide ion in D2O at 25 degrees C were determined by following the appearance of the deuterium-labeled cyanoalkanes by H-1 NMR. These data were evaluated to give the following pK(a)＇s in water: CH3CN, 28.9;CH3CH2CN, 30.9;NCCH2CH2CN, 26.6. High level abinitio calculations on cyanoalkanes and alpha-cyano carbanions and combined QM/Monte Carlo calculations of their free energies of solvation were carried out. The interaction between a carbanionic center and an alpha-cyano substituent is concluded to be largely polar. The 5.1-fold difference in alpha-cyano and beta-cyano substituent effects on carbon acidity in water which, nominally, is consistent with significant resonance stabilization of alpha-cyano carbanions is attributed to the differential solvation of cyanoalkanes and cyanocarbanions. The free energy change for the highly unfavorable tautomerization of acetonitrile to ketenimine in water was computed as Delta G(T) = 30.7 kcal/mol. We propose that the large instability of the ketenimine cumulative double bond favors the valence bond resonance form of the alpha-cyanocarbanion in which there is a formal carbon-nitrogen triple bond and the negative charge is localized at the alpha-carbon.