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Encyclopedia of Physical Science and Technology EN012G-576 July 28, 2001 12:44

Physical Organic Chemistry 231

that the atoms of one molecule of 2-bromopropane (three v = k obs [glucose]
carbons, seven hydrogens, and a bromine) are present in
+
= {k H [H ] + k HA [HA]}[glucose]. (39)
the transition state. The dependence on [OH ] means that
−
there are two pathways occurring together, one with an
oxygen, an additional hydrogen, and an electron contained
in the transition state, and the other without these.
One fundamental limitation of this method is that it is
impossible to determine the kinetic order for the solvent Such behavior is called general acid catalysis since each
molecules. It is not possible to vary the concentration of acid is capable of serving as a catalyst. There must be one
+
solvent without also introducing solvent effects of the sort molecule of acid, either H or buffer HA, present in the
arising via Eqs. (27) and (28). Therefore there is no way transition state, along with a molecule of the substrate. The
to know how many solvent molecules are present in the currentinterpretationisthatthetransitionstateforreaction
+
transition state, and the composition of the transition state of 94 with HA has structure 96, with H detached from
−
is always subject to an uncertainty of an arbitrary num- A . This example shows how the kinetic order tells us
ber of solvent molecules. For example, the k 1 pathway in how many of each kind of atom are in the transition state
Eq. (34), which involves no hydroxide, might have a water but it does not itself tell us how those atoms are arranged.
molecule (two hydrogens and an oxygen) present in the
transition state or it might not.
The first example of mechanistic inference from kinet-
ics came from Lapworth’s 1904 observation that the rate
of bromination of acetone to form bromoacetone is first
order in acetone and first order in acid but zero order in B. Solvent Effects
bromine:
Although it is not possible to determine the kinetic order in
d[CH 3 COCH 3 ] d[CH 3 COCH 2 Br]
v = − = solvent, the fact that polar solvents can have large effects
dt dt on reaction rates means that solvent effects can be used to
+
= k[CH 3 COCH 3 ][H ]. (38) diagnose whether a reaction creates or destroys ions. For
example, reactions of trialkylsulfonium ions RS(CH 3 ) +
2
Even though bromine is a reactant, the rate of reaction is with hydroxide show different solvent effects, depend-
4
independent of its concentration. These results mean that ing on R. For R = CH 3 the rate decreases 2 × 10 -fold on
the transition state is composed of three carbons, seven changing from ethanol to water, whereas for R = (CH 3 ) 3 C
hydrogens, an oxygen, and a positive charge (and an un- the decrease is only 3-fold. The large effect in the former
known number of the atoms that constitute water) but no case is consistent with a transition state [HO δ− ··· CH 3 ···
δ+ ‡
bromines. Therefore the rate-limiting step occurs before S(CH 3 ) ] , where ions are being destroyed. According to
2
the bromine enters the reaction. The current interpretation Fig. 12, this is a case where the more polar solvent, water,
is that the rate-limiting step is proton removal by a wa- stabilizes the reactants and renders them less reactive. The
ter molecule from the conjugate acid of acetone to form small effect in the latter case is consistent with a transi-
δ+ ‡
the enol, CH 3 C(OH) CH 2 , as an intermediate that subse- tion state [(CH 3 ) 3 C δ+ ··· S(CH 3 ) ] that is still an ion
2
quently reacts rapidly with bromine. because hydroxide is not yet involved.
Another classic example is the nitration of benzene and There are empirical measures more suitable than dielec-
other reactive aromatic hydrocarbons, where the rate de- tric constant for assessing quantitatively the polarity of a
pends on the concentration of nitric acid but is indepen- solvent. Even these do not always account for the specific
dent of the concentration of the aromatic. The current in- interactions whereby a solvent stabilizes ions or dipoles.
terpretation is that the rate-limiting step is formation of Table VI lists some relative rate constants for nucleophilic
+
−
nitronium ion, NO , which then reacts rapidly with the substitution of Cl on CH 3 I (by the D N A N mechanism).
2
aromatic. There is no correlation with dielectric constant. The lower
The mutarotation of glucose is the interconversion of reactivity in the first three solvents is because they are
its α (93) and β (94) anomers, whose rate can be followed protic, with OH δ+ or NH δ+ groups that approach close to
by the change of optical activity. The open-chain aldehyde chloride anion and stabilize it greatly (“hydrogen bond-
(95) is an intermediate. The reaction is acid-catalyzed, but ing”). To make the chloride react, it must be stripped of
the rate is also found to increase with increasing buffer its solvation. In contrast, the last two solvents are apro-
concentration, even at constant pH. Therefore the rate has tic, with their δ+ buried in the center of the molecule,
the following form, where HA is the buffer acid: where it cannot stabilize anions as well. Consequently the