458
AL-AWADI ET AL.
products of the elimination reaction of the present sub-
strates and related compounds involves a semicon-
certed 6-membered transition state (Scheme I). It is to
be noted that ketene and acetone, acetic/thioacetic
acid, or acetamide are the only products of fragmen-
tation in these reactions.
given in Table I, and to facilitate comparisons and cor-
relation of reactivity to structure the rate coefficients
at 600 K are recorded in Scheme III. The reactions of
these substrates were confirmed to be homogeneous
and unimolecular [1]. Rates were measured over a
temperature range of Ͼ50 K, and the reported first-
order rate constants represent average values of at least
three kinetic runs which are in agreement to within
Ϯ2%. The Arrhenius plots were strictly linear, and the
O
X
CH2
a
c
heat
1
magnitude of the log A (sϪ ) values is as expected for
X
H
CH39C
ϩ O"C"CH2
such thermal gas-phase elimination processes [1,5,6].
A typical plot is shown in Figure 1 for 1-cyano-1-
phenylhydrazonopropanone(1). The products of reac-
tion were analyzed and identified using FT–IR and
NMR techniques [1].
O
OH
b
CH3
Scheme I Generic elimination pathway: X ϭ CH2, O, S,
and NH
The overall reactivities are the result of electronic
synergism involving bonds a, b, and c and the stabi-
lizing influences associated with the hydrazono sub-
stituents and the electronic and steric effects of groups
Y and Z (Scheme II); Y would, of course, be absent
when bond (b) is that of a C#N moiety.
The rates of reaction at 600 K of pentane-2,4-dione
(8), acetic anhydride, diacetylsulphide, and diacetam-
ide are in the ratio of 1 : 3 ϫ 102 : 6 ϫ 103 : 5 ϫ 104,
respectively. This increase in relative reactivity is the
result of an increase in the polarity of the carbonyl
C9X bond (a), from the methylene (CH2) unit in the
dione (8) to O of the anhydride, to S of the sulphide,
and to the NH unit of the diacetamide molecule
(Scheme I). We have attempted to enhance the rela-
tively low reactivity of the dione system by replacing
the methyl moiety of the acetyl group with the elec-
tron-withdrawing CH3O and CH3CH2O substituents,
in order to increase the polarity of bond (a) and, hence,
rates of reaction. However, this structural modification
resulted in a change of mechanism of reaction to a
pathway involving intermolecular elimination leading
to cyclization and loss of alcohol [1]. Besides, our in-
vestigations have shown that it was possible to restore
the elimination pathway of the dione reaction to the
semiconcerted 6-membered transition state when an
arylhydrazono (ArNHN") group was incorporated
into the dione frame in lieu of the methylene hydrogen
atoms of the dione. We have accordingly prepared the
series of compounds (1–5) examined here, in addition
to compound 6 reported elsewhere [1]. It is instructive
to note that all these substrates contain the hydrazono
substituent.
O
Z
(i) (ii)
c
a
G
NH9N
H
O
b
Y
Scheme II Effective bond polarization and competitive
(opposing) delocalization of lone pair of electrons on NH
moiety
The present and reported kinetic data and relative rates
(Table I and Scheme III) are discussed on the basis of
the elimination pathway suggested for the reactions
(Schemes I and II) as follows.
The reactivity of compound (7) exceeds that of (8)
by a factor of 257. This rate enhancement is attributed
to the influence which the phenylhydrazono (PhNHN)
substituent has on the polarity of bond (a), and to the
protophilicity [4] of bond (b) through an extended
conjugation involving the lone pair of electrons on the
NH moiety of the phenylhydrazono group. It has al-
ready been noted above that when this group is incor-
porated into the molecular framework of a suitable
substrate, the mechanism of the elimination process is
restored to a semiconcerted six-membered transition
state, thus, providing a uniform mechanistic base for
comparison purposes [1]. The effect which the hydra-
zono substituent is exerting is further revealed when
the rate of reaction of compound (1) is compared with
that of substrates (2) and (3): the rate factors involved
are, respectively, 17 and 0.79. Reactivity is increased
by the electron-withdrawing p-NO2 group in 2 and
slightly retarded by the mesomerically electron-do-
Further, we have measured the rates of reaction of
these compounds and examined the influence of bonds
a, b, and c, as well as the effects of groups G, Y, and
Z on the reactivity of these substrates (Schemes I, II,
and III).
RESULTS AND DISCUSSION
1
The kinetic data and Arrhenius log A (sϪ ) and Ea (kJ
1
molϪ ) for the present series of compounds (1–6) are