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236 Physical Organic Chemistry

by the addition, in order to rule out direct reaction be- TABLE VIII Acidity Constants of Some
tween additive and substrate. For example, the hydrolysis Carbonyl, Nitro, Cyano, and Sulfone Com-
pounds in Dimethyl Sulfoxide
−
of HCCl 3 in base to HCO (plus CO) is thought to proceed
2
via CCl and the carbene CCl 2 . Indeed, if the hydrolysis Acid pK a
−
3
is performed in D 2 O and unreacted chloroform reisolated,
it is found to be largely DCCl 3 , from trapping of CCl . CH 3 COCH 2 COOC 2 H 5 14.2
−
3 16.4
Moreover, adding PhS does not increase the rate of disap- CH 2 (COOC 2 H 5 ) 2
−
pearance of HCCl 3 , but the product is HC(SPh) 3 instead, CH 3 NO 2 17.2
PhCOCH 2 Ph 17.65
from trapping of CCl 2 . Another example is the solvolysis
PhCH 2 CN 21.9
of Ph 2 CHCl or Ph 2 CHBr, both of which are thought to
CH 3 COCH 2 SO 2 Ph 22.1
+
proceed via Ph 2 CH as a common intermediate. Added
PhCH 2 COOC 2 H 5 22.6
sodium azide (NaN 3 ) does not increase the rate (except
(PhCH 2 ) 2 SO 2 23.9
through its effect on solvent polarity) but does change the
PhCOCH 3 24.7
product from Ph 2 CHOH to Ph 2 CHN 3 .
Cyclopentanone 25.8
A variant on trapping experiments is to test substances
CH 3 COCH 3 26.5
that would inhibit a free radical reaction. For example,
[(CH 3 ) 2 CH] 2 CO 28.2
certain phenols, like BHA and BHT in food products, can
scavenge free radicals. If the reaction does not take its nor-
mal course, then it is not necessary to detect a distinctive
product in order to conclude that free radical intermediates
A variant is independent synthesis of a presumed in-
are implicated.
termediate, which offers the further opportunity to test
A convincing procedure is to observe the intermedi-
whether it is converted to product (or, even better, to the
ate. This is particularly informative since it permits the
identical mixture of products) under the reaction con-
intermediate to be characterized spectroscopically and its
ditions. For example, it is possible to prepare salts of
properties elucidated. Usually the intermediate is too un- +
NO and to show that these react with aromatic hydro-
2
stable to be observed under the reaction conditions. One
carbons to form the same nitration products as with nitric
possibility is to modify the substrate so as to stabilize
acid.
the intermediate and increase its lifetime. Alternatively,
Another possibility is that the presumption of interme-
it may be possible to modify the conditions so that the
diacy must be rejected. For example, the Clemmensen re-
intermediate persists. For example, treatment of triphe-
duction of ketones R 2 C O to hydrocarbons R 2 CH 2 with
nylmethanol with sulfuric acid converts it to the carbo-
zinc metal had been thought to proceed via the alcohol
+
cation Ph 3 C , stabilized by resonance delocalization of
R 2 CHOH. However, this cannot be an intermediate be-
the positive charge into the three aromatic rings. Less
cause it is found to be inert to the reaction conditions.
stable carbocations can be prepared in “superacid” so-
lutions of HSbF 6 , where there is no nucleophile available
to react, and where they can be characterized by NMR V. ILLUSTRATIVE EXAMPLES
spectroscopy. Similarly, many intermediates can be pre-
served by eliminating other species that they might react
A. Computation and Structure
with. Thus carbanions can be observed under conditions
that scrupulously exclude water and other electrophiles. The location of the hydrogen in a hydrogen bond is an
For example, the basicity of enolates and related anions intriguing question of molecular structure that can now be
can be measured by titration in dimethyl sulfoxide, and addressed confidently by high-level computations. Usu-
Table VIII lists some acidity constants of their conjugate ally a hydrogen is bonded covalently to one oxygen and
acids. Preserving free radicals and carbenes (R 2 C) is more hydrogen-bonded to the other. If the two oxygens are
challenging since they react with themselves, but the rem- of equal or nearly equal basicity, the hydrogen may be
edy is to immobilize them in a solid matrix so that they bonded to either and jump quickly from one to the other
cannot encounter another reactive partner. Reactive inter- [Eq. (49)]. In contrast, for the monoanion of phthalic acid
mediates that rearrange by themselves, such as strained (124), recent quantum mechanical calculations indicate
structures (40 with n = 6–8, 41, 84) and some carbenes a symmetric structure, with the hydrogen centered be-
and carbocations, can often be generated at low temper- tween the hydrogens and bonded equally to both. This
ature, perhaps photochemically. Similarly, electronically conclusion is quite sensitive to the oxygen–oxygen dis-
excited states can be generated photochemically and their tance d O O , which must be short. With a highly accurate
behavior studied. method known as density functional theory d O O for the