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Physical Organic Chemistry 243

TABLE XI Substituent Constants Derived Formanyreactionsitispossibletorecognizeananalogy
from pK a of Benzoic Acids, XC 6 H 4 COOH either to acid dissociation of substituted benzoic acids or to
X σ X X σ X other model reactions. Among these models are acidities
of phenols, XC 6 H 4 OH, or aliphatic acids, XCH 2 COOH,
p-MeO −0.28 p-Br 0.23 rates of solvolysis of XC 6 H 4 C(CH 3 ) 2 Cl, and even the abil-
p-Me −0.17 m-Br 0.39 ity of a solvent to stabilize the ground electronic state
m-Me −0.06 p-NC 0.70 of a dyestuff. Then the effects of substituents on both
H = 0.00 m-O 2 N 0.71 equilibrium constants and rate constants can be correlated
m-MeO 0.11 p-O 2 N 0.78
quantitatively with the substituent effects in those model
reactions. No analogy is perfect, but one may provide an
understanding of a new reaction in terms of a more familiar
one.
constant ρ and whose intercept is c (which equals log k H
10
in an ideal case).
Figure 20 shows such a plot for the reaction of substi-
tuted benzonitriles with HS : SEE ALSO THE FOLLOWING ARTICLES
−
−
XC 6 H 4 CN + HS → XC 6 H 4 C( S)NH 2 . (57) CHEMICAL KINETICS,EXPERIMENTATION • CHEMICAL
THERMODYNAMICS • HYDROGEN BOND • KINETICS
HS δ- ‡
(CHEMISTRY) • ORGANIC CHEMICAL SYSTEMS,THEORY
δ-
N • ORGANIC CHEMISTRY,SYNTHESIS • ORGANOMETAL-
X
LIC CHEMISTRY • PERIODIC TABLE (CHEMISTRY) • PHO-
179
TOCHEMISTRY,MOLECULAR • POTENTIAL ENERGY SUR-
The slope is 2.1. The positive value reflects the fact that FACES • QUANTUM CHEMISTRY • STEREOCHEMISTRY
electron-withdrawing substituents accelerate the reaction.
This is consistent with a transition state 179 that carries
a negative charge. This transition state is not the same BIBLIOGRAPHY
as a carboxylate anion, but it is analogous, in that any
substituent that stabilizes the carboxylate also stabilizes BERNASCONI,C.F.(ED.). (1986). “INVESTIGATION OF RATES AND
the transition state. If the slope had been negative, as in MECHANISMS OF REACTIONS,” 4TH ED., WILEY,NEW YORK.
CAREY, F. A., AND SUNDBERG, R. J. (2000/1990). “ADVANCED OR-
some reactions, it would mean that electron-donating sub-
GANIC CHEMISTRY,” PART A, 4TH ED. (2000), PART B, 3RD ED. (1990),
stituents accelerate the reaction, consistent with the devel- KLUWER/PLENUM,NEW YORK.
opment of a positive charge in the transition state. CARROLL, F. A. (1998). “PERSPECTIVES ON STRUCTURE AND MECHA-
NISM IN ORGANIC CHEMISTRY,” BROOKS/COLE,PACIfiC GROVE, CA.
LOWRY, T. H., AND RICHARDSON, K. S. (1987). “MECHANISM AND THE-
ORY IN ORGANIC CHEMISTRY,” 3RD ED., HARPER &ROW,NEW YORK.
MARCH, J. (1992). “ADVANCED ORGANIC CHEMISTRY:REACTIONS,
MECHANISMS, AND STRUCTURE,” 4TH ED., WILEY,NEW YORK.
MASKILL, H. (1985). “THE PHYSICAL BASIS OF ORGANIC CHEMISTRY,”
OXFORD UNIVERSITY PRESS,NEW YORK.
MILLER, B. (1998). “ADVANCED ORGANIC CHEMISTRY:REACTIONS AND
MECHANISMS,” PRENTICE HALL,UPPER SADDLE RIVER, NJ.
MULLER,P.(ED.). (1994). “GLOSSARY OF TERMS USED IN PHYS-
ICAL ORGANIC CHEMISTRY,” Pure Appl. Chem. 66, 1077–1184
(HTTP://WWW.CHEM.QMW.AC.UK/IUPAC/GTPOC/).
PROSS, A. (1995). “THEORETICAL AND PHYSICAL PRINCIPLES OF OR-
GANIC REACTIVITY,” WILEY,NEW YORK.
STOWELL, J. C. (1994). “INTERMEDIATE ORGANIC CHEMISTRY,” WILEY,
NEW YORK.
TIDWELL, T. T., RAPPOPORT, Z., AND PERRIN,C.L.(EDS.). (1997).
“PHYSICAL ORGANIC CHEMISTRY FOR THE 21ST CENTURY (ASYM-
POSIUM IN PRINT).” Pure Appl. Chem. 69, 210–292 (HTTP://WWW.
FIGURE 20 Hammett plot for reaction of XC 6 H 4 CN with HS . IUPAC.ORG/PUBLICATIONS/PAC/SPECIAL/0297/INDEX.HTML).
−
Substituents X from left to right are p-CH 3 O, p-CH 3 ,H, p-I, WILLIAMS, A. (2000). “CONCERTED ORGANIC AND BIO-ORGANIC
p-Br, p-Cl, and m-Br. MECHANISMS,” CRC PRESS,BOCA RATON, FL.