Page 316 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

However, from Figure 3.17, 297
‡
‡
‡
G − G + G = G −G ‡ (3.40) SECTION 3.4
b a c b a
Electronic Substituent
The product ratio is therefore not determined by G but by the relative energy of the Effects on Reaction
c
Intermediates
‡
‡
two transition states A and B . The conclusion that the ratio of products formed from
conformational isomers is not determined by the conformational equilibrium ratio is
known as the Curtin-Hammett principle. 63 Although the rate of the formation of the
products is dependent upon the relative concentration of the two conformers, because
G b ‡ is decreased relative to G a ‡ to the extent of the difference in the two confor-
mational energies, the conformational preequilibrium is established rapidly, relative
to the two competing product-forming steps. 64 The position of the conformational
equilibrium cannot control the product ratio. The reaction can proceed through a minor
conformation if it is the one that provides access to the lowest-energy transition state.
The same arguments can be applied to other energetically facile interconversions
of two potential reactants. For example, some organic molecules can undergo rapid
proton shifts (tautomerism) and the chemical reactivity of the two isomers may be quite
different. However, it is not valid to deduce the ratio of two tautomers on the basis of
subsequent reactions that have activation energies greater than that of the tautomerism.
Just as in the case of conformational isomerism, the ratio of products formed in
subsequent reactions is not controlled by the position of the facile equilibrium, but by
the E of the competing steps.
a

3.4. Electronic Substituent Effects on Reaction Intermediates

It is often observed that the introduction of substituents changes the rate of organic
reactions, even if the substituent is not directly involved in the reaction. Changes that
‡
affect the G for the rate-determining, or preceding, steps will cause a change in the
observed reaction rate. The difference can result from energy changes in the reactant(s)
or transition state, or both. Shifts in product composition can result from changes in
‡
the relative G of competing reaction paths. In the broadest terms, there are three
kinds of substituent effects, resulting from electronic, steric, and structure-specific
interactions. Steric effects have their origin in nonbonding interactions. Structure-
specific interactions include, for example, intramolecular hydrogen bonding and
neighboring-group participation that depend on location of the substituents that are
involved. In this section, we focus on electronic effects. Electronic substituent effects
can be further subdivided. Substituents can operate by delocalization of electrons
∗
∗
by resonance or hyperconjugation, including − and − , as well as −
delocalization. Resonance and hyperconjugation operate through specific orbital inter-
actions and therefore impose particular stereoelectronic requirements; that is, inter-
acting orbitals must be correctly aligned. Polar effects are electrostatic and include
both the effect of proximate charges owing to bond dipoles and the effects of more
distant centers of charge. Polar effects also may have a geometric component. For
example, the orientation of a particular bond dipole determines how it interacts with
a developing charge at a reaction center elsewhere in the molecule. Polar effects are

63 D. Y. Curtin, Rec. Chem. Prog., 15, 111 (1954); E. L. Eliel, Stereochemistry of Carbon Compounds,
McGraw-Hill, New York, 1962, pp. 151–152, 237–238.
64
For a more complete discussion of the relationship between conformational equilibria and reactivity,
see J. I. Seeman, Chem. Rev., 83, 83 (1983).