Page 172 - Mechanism and Theory in Organic Chemistry
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Substituent Effects on Strengths of Brransted Acids and Bases 161
Table 3.16 ENTHALPY AND ENTROPY CONTRIBUT~ONS RELATIVE FREE
TO
ENERGIES SOLUTION AMINES
OF
OF
Amine SAHs(B) - S TAS,(B)
SOURCE: Reprinted with permission from E. M. Arnett, F. M. Jones, 111, M. Taagepera, W. G.
Henderson, J. L. Beauchamp, D. Holtz, and R. W. Taft, J. Amer. Chem. Soc., 94,4724 (1972). Copy-
right by the American Chemical Society. Values in kcal mole-= at 25°C. See text for explanation of
symbols.
Table 3.15 dissects the important GAG,(BHf) term into enthalpy and
entropy contributions. Increasing substitution on BHf makes both of these
quantities less favorable in the gas -+ solution direction. For the bases themselves,
on the other hand, the solution enthalpies and entropies are in opposition (Table
3.16), enthalpies being more favorable to the solution process the more substi-
tuents, but entropies becoming less favorable with more substituents. The
cancelation of these opposing effects leaves the small GAG,(B) values shown in
Table 3.14. As we have noted in Section 2.4, these results are not easily inter-
preted at the molecular level, although the most important effect, that on solution
free energy of BHf , is undoubtedly caused by decreasing opportunity for hydro-
gen bonding as hydrogens are replaced by alkyl groups.97
Oxygen and Sulfur Bases
The oxygen and sulfur bases are weaker than the nitrogen bases, and accurate
solution basicities are not available. Arnett's heat of protonation studies indicate
that the order of decreasing basicity is ROR > ROH > H,0,98 a result that is
in agreement with gas-phase measurement^.^^ Hydrogen sulfide in the gas phase
has basicity comparable to that of water (Table 3.18), and substitution of H by
alkyl produces stronger gas-phase bases just as does similar substitution on oxygen.
Comparisons among the alcohols are difficult to make in solution. Titration
in acetic acid indicates an order of basicity isopropyl alcohol > ethyl > methyl,
but water was found to be a stronger base than any of these alcohols,100 a result
in disagreement with the gas-phase data. In the gas phase (Table 3.18), the
basicity order (strotgesths~ tn &-a>
CH,CH,OH > CH,OH > -H20. Againeore and larger aljddgmqssxm to
uize charge.
Substitution by an aromatic group has a marked effect on solution base
strength. One might be tempted to attribute the low basicity of aniline, di- and
triphenylamines, and phenol compared with reference compounds (Table 3.17)
to partial delocalization of the nonbonded electron pair on the nitrogen or oxygen
into the err orbital system of the ring. But gas-phase results indicate the basicity
97 (a) See note 94 (a), p. 159; (b) R. W. Taft, M. Taagepera, K. D. Summerhays, and J. Mitsky,
J. Amer. Chem. Soc., 95, 381 1 (1973).
E. M. Arnett, R. P. Quirk, and J. J. Burke, J. Amer. Chem. Soc., 92, 1260 (1970).
99 M. S. B. Munson, J. Amer. Chem. Soc., 87, 2332 (1965).
loo I. M. Kolthoff and S. Bruckenstein, J. Amer. Chem. Soc., 78, 1 (1956).