Page 170 - Mechanism and Theory in Organic Chemistry

Substituent Effects on Strengths of Brernsted Acids and Bases 159

Table 3.12 APPROXIMATE pKa VALUES OF CONJUGATE ACIDS OF
SOME SIMPLE HYDRIDES
Group V Group VI Group VII

" See Table 3.2.
Estimated from exchange rate measurements by R. E. Weston and J. Bigeleisen, J. Amer. Chem.
SOC., 76, 3074 (1954).
" E. M. Arnett, R. P. Quirk, and J. J. Burke, J. Amer. Chem. Soc., 92, 1260 (1970).
Relative basicities in the gas phase are consistent with the solution results: D. Holtz and J. L.
Beauchamp, J. Amer. Chem. Soc., 91, 5913 (1969); D. Holtz, J. L. Beauchamp, and J. R. Eyler, J.
Amer. Chem. Soc., 92, 7045 (1970).
D. D. Perrin, Dissociation Constants of Inorganic Acids and Bases in Aqueous Solution, Butterworths,
London, 1969.
J Calculated from the equilibrium:

Table 3.13 SOLUTION pKa VALUES OF CONJUGATE ACIDS OF SOME NITROGEN AND
PHOSPHORUS BASES
Base PKR (BH+) Base PKR (BH+)

NH3 9.24" pH3 .V -13b
CH3NH2 10.6c n-C4HgPH, Od
(CH3) 2NH 10.7" (n-C4Hg)2PH 4.5d
(CH3)3N 9.8" (n-C4H9)3P 8.4d
a See Table 3.2.
See Table 3.12.
" D. D. Perrin, Dissociation Constants of Organic Bases in Aqueous Solution, Butterworths, London, 1965.
Values at 25°C.
C. A. Streuli, Anal. Chem., 32, 985 (1960). Determined by titration in nitromethane and corrected
to water solution.
-
primary > ammonia.94 The phosphines have the same basicity order in the gas
phase as in solution, tertiary >> pho~phine.~~
Arnett has presented an analysis of the solvation thermochemistry of the
amines and their conjugate acids.96 Table 3.14 gives Arnett's data for the four

04 (a) E. M. Arnett, F. M. Jones, 111, M. Taagepera, W. G. Henderson, J. L. Beauchamp, D. Holtz,
and R. W. Taft, J. Amer. ChemSoc., 94, 4724 (1972); (b) D. H. Aue, H. M. Webb, andM. T. Bowers,
J. Amer. Chem. Soc., 94, 4726 (1972); (c) W. G. Henderson, M. Taagepera, D. Holtz, R. T. MeIver,
Jr., J. L. Beauchamp, and R. W. Taft, J. Amer. Chem. Soc., 94, 4728 (1972).
O5 E. M. Arnett, Accts. Chem. Res., 6, 404 (1973). Gas-phase data are reported for (CH3),P and pH3
only, but heat of protonation results indicate that secondary and primary phosphines will fall in that
order between these two.
Oe See note 94 (a).