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224 Physical Organic Chemistry

In such reactions the atom that carries the electron pair
is called a nucleophile since it uses its electron pair to form
a bond to some other nucleus. The partner with which the
nucleophile reacts is called an electrophile since it forms
a bond with that electron pair. In some contexts these are
called Lewis bases and Lewis acids, respectively. One of
the unifying features of physical organic chemistry is the
recognition that a very large proportion of all reactions can
be classified as ones where a nucleophile (or Lewis base)
reacts with an electrophile (or Lewis acid). The vastness of
the possible chemistry then arises from the great diversity
of nucleophiles and electrophiles.
FIGURE 12 Effects of energy modifications on rate of reaction of

A relative to A via transition state ‡ or ‡ : (a) A destabilized or ‡

stabilized, (b) A stabilized or ‡ destabilized.
structure. To do so, it is necessary to know how the mod-
ification affects energies, just as for equilibria above. For
a general reaction written as follows, the rate constant is
still given by Eq. (23):
A → ‡ → B. (24)
For the following modified reaction (distinguished with
a prime), if the modification stabilizes the transition state

‡ (not the product B ), then it follows from Eq. (23) that
k > k and that the reaction is faster:

A → ‡ → B . (24 )
Likewise, if the modification destabilizes the reactant A
relative to A, the reaction is again faster. If the modifica-

tion stabilizes reactant A or destabilizes transition state This method is also applicable to multistep reac-
tions. For example, the hydrolysis of phenyl acetate,
‡ , then k < k and the reaction is slower. These conclu-

−
CH 3 C( O) OPh, with hydroxide to form CH 3 CO plus
sions are made graphic in Fig. 12, which is very similar to 2
PhOH does not proceed via transition state 68, involving
Fig. 10.
simultaneous breaking and making of the various bonds.
Instead it is a three-step reaction, proceeding via three
D. Electron Pushing sequential transition states, 69–71, each of which can be
generated by electron pushing.
Because the transition state has an electronic structure de-
scribable as a resonance hybrid, electron pushing, which
provided a convenient method for generating additional
resonance forms, also provides a method for generating
transition states. An electron pair is delocalized toward
another atom so as to form a new bond, while also remov- Between those transition states there are reaction inter-
ing an electron pair from that adjacent atom if necessary mediates 72 and 73. These are ordinary chemical species,
to avoid violating the octet rule. The electron pair can not transition states, but they are not very stable and they
come from a lone pair or from a multiple bond or a single do not persist. Figure 13 shows how the energy varies dur-
bond. For example, this method permits the generation of ing the course of the reaction, as measured by a reaction
the transition states for the methoxide-induced E2 elimi- coordinate that is a composite of the various bond dis-
nation of HBr from ethyl bromide (63), the nucleophilic tances. The transition states are at local maxima and the
substitution of ammonia on methyl iodide (64), the elec- intermediates are at local minima. In such a diagram there
+
trophilic addition of H (from HBr) to propene (65), the is generally one transition state that is higher than the oth-
hydride transfer from borohydride ion to methanal (66), ers. That one is called THE transition state and its step is
and the cycloaddition of ozone to ethene (67). called the rate-limiting step. In the hydrolysis of phenyl