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Physical Organic Chemistry 229

one of the two faces of the carbonyl. Much current re- It is cumbersome to consider transition states, with all
search is devoted to understand the steric interactions that their resonance forms and partial bonds. Instead it is eas-
govern these preferences, with the aim of finding con- ier to focus on products and recognize their similarity
ditions to select one diastereomeric transition state over to the transition states. Then one can say that the more
another. stable product is formed faster because it is more sta-
Even if the reactant is (±)-3-methyl-2-pentanone, the ble. This cannot be strictly true since rates depend on
products are equal amounts of (2R,3R)- and (2S,3S)- transition-state stabilities. However, if some feature sta-
3-methyl-2-pentanol (a racemic mixture), plus equal bilizes both transition state and product, then this state-
amounts of (2R,3S)- and (2S,3R)-3-methyl-2-pentanol ment is a convenient simplification. Thus the reactivity
(another racemic mixture). However, the amount of the order of hydrocarbons toward bromine atoms may be
first pair is not equal to the amount of the second be- attributed to the fact that radical stabilities increase in
cause the transition states are diastereomeric. The two the order CH 3 · < CH 3 CH 2 · < (CH 3 ) 2 CH· < (CH 3 ) 3 C· <
transition states for formation of the first pair are enan- CH 2 CHCH 2 ·. Another example is the borohydride re-
tiomeric, hence the racemic mixture. It is a general rule duction of cyclopentanone (87, n = 5) and cyclohexanone
that only achiral products or racemic mixtures can be ob- (88, n = 6), where it is justifiable to say that the latter
tained from achiral or racemic reactants. It is not possible is faster because cyclohexanol is the more stable product
to obtain optically active product from optically inactive owing to the absence of torsional strain.
starting materials. The mechanism for the origin of the In contrast, kinetic control is the case where one prod-
single enantiomers of chiral natural substances is not yet uct is formed faster than another because of rate constants
understood. unrelated to equilibrium constants for the overall reaction.
Such a case is the faster solvolysis of tert-butyl chloride
[Eqs. (29)–(30)] compared to isopropyl chloride [Eq. (31)]
K. Relation between Kinetics even though the equilibrium constants for the two reac-
and Thermodynamics tions are nearly the same. Therefore this is not product-
development control. Yet it is possible to understand the
There is no universal relation between kinetics and ther-
greater reactivity of tert-butyl chloride because its transi-
modynamics. Just because a reaction is themodynamically
tion state has nine additional hyperconjugative resonance
very favorable does not mean that it must be fast. There
forms. It is cumbersome to consider the transition states.
are many reactions that are very exothermic but proceed
Instead it is easier to focus on the carbocation interme-
extremely slowly. However, there is enough of a relation
diate, which is more stable in the tert-butyl case. Thus
between kinetics and thermodynamics to make it useful
through an understanding of the mechanism, it is possi-
for understanding.
Very often it is observed that in a pair of related reac- ble to understand this case of kinetic control in terms of
the stability of intermediates that resemble the transition
tions the one that is more favorable thermodynamically
states.
is the faster one. When a more stable product is formed
because the transition state leading to it partakes of the
stability of the product, this is said to be a case of product-
development control. For example, the rates of reaction IV. METHODOLOGY OF MECHANISTIC
of bromine atoms with the hydrocarbons of Table IV fol- STUDIES
low the series CH 3 H < CH 3 CH 2 H < (CH 3 ) 2 CH H <
In the study of chemical kinetics one of the key pieces
(CH 3 ) 3 C H < CH 2 CHCH 2 H. This order parallels the
of information is how the reaction rate depends on reac-
bond-dissociation energies, in that the weaker the bond is,
tion conditions. Among the variables are solvent, temper-
the faster it cleaves. The transition state for any of these
ature, and the concentrations of the various reactants and
reactions can be written as a resonance hybrid of 91 and
catalysts and possibly other chemical species. Also, care-
92, adapted from the generic 60 and 61. However, there
ful attention to product structures can provide additional
are additional resonance forms. Because 92 is a contribut-
information. However, it must be recognized that physical
ing resonance form, so are forms like 76 or like 78, 78 ,
organic chemistry is an inductive science. It is never pos-

and 78 , which provide additional delocalization and ad-
sible to prove a mechanism. At best it may be possible to
ditional stabilization. Consequently the transition states
obtain experimental results that are inconsistent with all
become increasingly stabilized and the reaction becomes
conceivable mechanisms save one. Even then, there may
faster.
be an “inconceivable” mechanism that was overlooked.
Besides, for reasons of brevity experimental results are
usually presented only as supporting a mechanism without