Page 126 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

CH COH, but the order is exactly the opposite in the gas phase. 169 The reverse order 105
3 3
in the gas phase attracted a good deal of interest when it was discovered, since it is
TOPIC 1.5
contrary to the general expectation that more highly substituted alkyl groups are better
electron donors than methyl and primary groups. Both qualitative 170 and quantitative 171 Application of Density
Functional Theory
treatments have identified polarizability, the ability to accept additional charge, as the to Chemical Properties
major factor in the gas phase order. Note also that the order is predicted by the HSAB and Reactivity
relationship since the softer (more substituted) alkoxides should bind a hard proton
more weakly than the harder primary alkoxides.
Another study examined the acidity of some halogenated alcohols. The gas phase
acidity order is ClCH OH > BrCH OH > FCH OH > CH OH. The same Cl > Br > F
2 2 2 3
order also holds for the di- and trihalogenated alcohols. 172 The order reflects competing
effects of electronegativity and polarizability. The electronegativity order F > Cl > Br
is reflected in the size of the bond dipole. The polarizability order Br > Cl > F indicates
the ability to disperse the negative charge. The overall trend is largely dominated by the
polarizability order. These results focus attention on the importance of polarizability,
especially in the gas phase, where there is no solvation to stabilize the anion. The
intrinsic ability of the substituent group to accommodate negative charge becomes
very important.
The role of substituents has been investigated especially thoroughly for substi-
tuted acetic and benzoic acids. Quantitative data are readily available from pK a
measurements in aqueous solution. Considerable data on gas phase acidity are also
available. 173 EWG substituents increase both solution and gas phase acidity. In the
gas phase, branched alkyl groups slightly enhance acidity. In aqueous solution, there
is a weak trend in the opposite direction, which is believed to be due to poorer
solvation of the more branched anions. Geerling and co-workers have applied DFT
concepts to substituent effects on acetic acids. 174 The Fukui functions and softness
descriptors were calculated using electron density and Mulliken population analysis
(see Topic 1.5.2). The relative correlation of these quantities with both solution and
gas phase acidity was then examined. In both cases, the best correlations were with
the Mulliken charge. In the case of gas phase data, the correlations were improved
somewhat by inclusion of a second parameter for group softness. The picture that
emerges is consistent with the qualitative concepts of HSAB. The reactions in question
are hard-hard interactions, the transfer of a proton (hard) to an oxygen base (also
hard). The reactions are largely controlled by electrostatic relationships, as modeled
by the Mulliken charges. The involvement of softness in the gas phase analysis
suggests that polarizability makes a secondary contribution to anionic stability in the
gas phase.

169
J. I. Brauman and L. K. Blair, J. Am. Chem. Soc., 92, 5986 (1976).
170 W. M. Schubert, R. B. Murphy, and J. Robins, Tetrahedron, 17, 199 (1962); J. E. Huheey, J. Org.
Chem., 36, 204 (1971).
171 F. De Proft, W. Langenaeker, and P. Geerlings, Tetrahedron, 51, 4021 (1995); P. Pérez, J. Phys. Chem.
A, 105, 6182 (2001).
172
S. Damoun, W. Langenaeker, G. Van de Woude, and P. Geerlings, J. Phys. Chem., 99, 12151 (1995).
173 C. Jinfeng, R. D. Topsom, A. D. Headley, I. Koppel, M. Mishima, R. W. Taft, and S. Veji, Theochem,
45, 141 (1988).
174
F. De Proft, S. Amira, K. Choho, and P. Geerlings, J. Phys. Chem., 98, 5227 (1994).