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Can. J. Chem. Vol. 81, 2003
Table 1. Buffer base rate constants (kB) for the reaction of 6-
methylene-2,4-cyclohexadienylideneketene in aqueous solution at
25°C.a
(10c, 19). In the present case, protonation is more likely to
occur on the exo-carbon atom of the methylene group, be-
cause that will regenerate a benzene ring and benefit from a
gain of benzene resonance energy (eq. [7]). In any event,
there will be a hydron in flight in
Base
pKa (BH)
kB (106 M–1 s–1)
CH3CO−2
4.76
7.20
8.07
9.25
1.23
1.01
0.283
1.72
HPO24−
(CH2OH)3CNH2
NH3
aIonic strength = 0.10 M (NaClO4).
the rate-determining step of this reaction and the isotope ef-
fect will consequently have a primary component. It will,
however, have an inverse secondary component as well, pro-
duced by tightening of the hydrogen–oxygen bonds of the
transition state moiety on its way to becoming a water mole-
cule. The isotope effect observed (kH+ /kD+ = 2.98) is in fact a
typical value for such a process (20).
Uncatalyzed hydration of ketenes, on the other hand, oc-
curs through nucleophilic attack of a water molecule on the
carbon atom of the ketene carbonyl group, generating an
enol intermediate, which then ketonizes in a fast subsequent
step (10a, 15) (eq. [8]). Formation of the
Table 2. Comparison of rates of hydration of 6-methylene-2,4-
cyclohexadienylideneketene with those of an analogous ketene
not having a cyclohexadienyl structure.a
Substrate
kHO− (M–1 s–1) kuc (s–1)
kH+ (M–1 s–1)
1.44 × 105
9.26 × 106
1.35 × 105
6.24 × 103
2.30 × 101
2.78 × 103
aAqueous solution, 25°C; data for pentamethyleneketene from ref. (15).
base reaction. Using this reasonable assumption, the buffer-
dependent rate constants for the biphosphate and tris-
(hydroxymethyl)methylammonium buffers (kbuff) were trans-
formed into buffer base rate constants. The results, together
with the buffer base rate constants obtained from acetic acid
and ammonium ion buffers, are listed in Table 1.
Inspection of Table 1 shows that the reactivity of the buffer
bases does not increase regularily with buffer base strength, as
it would if the buffer bases were acting as proton transfer
agents. Tris-(hydroxymethyl)methylamine, for example, is a
stronger base than either acetate or hydrogen phosphate ions,
and yet its rate of reaction is only a quarter of that of the ions.
The reactivity order shown by Table 1, on the other hand, is
what might be expected if the buffer bases were reacting as
nucleophiles: tris-(hydroxymethly)methylamine, with its large
bulk, should be a poorer nucleophile than the other smaller
bases of this table. Bases, of course, react with ketenes as
nucleophiles and not as proton transfer agents (11), and the
data in Table 1, therefore, provide still more evidence that the
substance produced by photolysis of benzocyclobutenone is
6-methylene-2,4-cyclohexadienylideneketene.
enol could occur either through a zwitterion, or the
zwitterion could be avoided by simultaneous proton shuf-
fling. In either case, the solvent isotope effect would be
small because there would be no primary component; in
zwitterion formation because no hydron transfer takes place,
and in simultaneous proton shuffling because the hydron
transfer here is between oxygen atoms with the hydron lying
in a stable potential well at the transition state and conse-
quently not being in flight (21). The isotope effect observed
(kH/kD = 1.46) is a typical value for such a process (15).
Reaction in buffers
The rate measurements in acetic acid and ammonium ion
buffers were each made at four different buffer ratios, which
allowed separation of the buffer-dependent rate constants of
eq. [4] (kbuff) into their buffer base (kB) and buffer acid (kHA
)
components. This was done through the use of eq. [9], in
which fA is the fraction of buffer present in the acid form.
[9]
kbuff = kB + (kHA – kB)fA
Least-squares fitting of this equation gave kB = (1.23 ±
0.07) × 106 M–1 s–1 and kHA = –(8.84 ± 5.26) × 104 M–1 s–1 for
the acetic acid buffers, and kB = (1.72 ± 0.11) × 106 M–1 s–1
and kHA = (1.76 ± 8.99) × 104 M–1 s–1 for the ammonium ion
buffers. In both cases, therefore, the buffer-dependent reaction
was wholly of the basic type.
A similar analysis could not be carried out for the
biphosphate and tris-(hydroxymethyl)methylammonium ion
buffers because measurements here were made at only one
buffer ratio. It seems fair to assume, however, that the
buffer-dependent reaction here was also wholly of the basic
type, inasmuch as the acid and base strengths of the compo-
nents of these buffers lie between those (acetic acid and am-
monium ion) for which analysis using eq. [9] showed only a
Reactivity
Rates of hydration of the presently studied ketene are con-
siderably faster than those of other ketenes whose reactions
do not convert a cyclohexadienyl structure into an aromatic
benzene ring. The data assembled in Table 2 provide a com-
parison with pentamethyleneketene (15), a substrate also
possessing a six-membered carbocyclic ring. It may be seen
that the present ketene is the much more reactive substance,
by a factor of 1500, corresponding to a free energy of activa-
tion difference of δG≠ = 4.3 kcal mol–1, for the hydroxide-
ion-catalyzed process and by a factor of 5900 or δ∆G≠ =
5.1 kcal mol–1 for the uncatalyzed reaction. The greater dif-
ference for the slower uncatalyzed hydration is of course
© 2003 NRC Canada