178
M. J. D’SOUZA, M. E. BOGGS AND D. N. KEVILL
the naphthoyl chloride solvolyses is 0.65 ꢂ 0.31, neither
YBnCl nor YxBnCl will be appropriate similarity model
scales, but the YBnCl scale associated with an h/m ratio of
1.799,12 will be very slightly preferable to the YxBnCl scale
with an h/m ratio of 1.97.
viously.3 The multiple regression analyses were performed
using commercially available statistical packages.
Acknowledgement
This work was completed as a Directed Study Project and
was supported by the NIH NCRR INBRE, grant No. 2P20
RR016472. D.N.K. thanks Professor H. Mayr (Universi-
CONCLUSION
¨
tat Munchen) for hospitality during the time this paper
was being prepared.
¨
Replacing the sulfur of 2-thiophenecarbonyl chloride
(1a) by the oxygen of 2-furancarbonyl chloride (1b) has
a profound effect on the mechanism of solvolysis. In the
majority of solvents, the sensitivity parameters (l ¼ 0.65;
m ¼ 0.86) obtained for solvolyses of 1a17 are consistent
with an ionization mechanism with assistance from
nucleophilic solvation and, in the majority of solvents,
the sensitivity parameters obtained in this study for
solvolyses of 1b (l ¼ 1.73; m ¼ 0.60) are consistent with
an addition–elimination (association–dissociation) me-
chanism, with the addition step being rate-determining.
For both solvolyses, it is possible by appropriate choice
of solvents to switch over to the alternative mechanism.
Correlations of the solvolyses of naphthoyl chlorides
have been proposed previously to be perturbed by the
presence of the two aromatic rings, with incorporation of
an hI term13 or use of the YBnCl or YxBnCl scale14 being a
feature of the analyses. A more thorough treatment has
been made possible by combining the specific rate data
from the two publications. We find for 2-naphthoyl
chloride (3) and its 6-methoxy derivative (5) that there
is negligible sensitivity towards changes in the value of
the I parameter (h values of 0.00 ꢂ 0.23 and 0.11 ꢂ 0.20,
respectively) and only for 1-naphthoyl chloride (2) is the
incorporation of the hI term not statistically insignificant
(h value of 0.54 ꢂ 0.23, with a probability of 0.037 that
the hI term is statistically insignificant). For 2, there is
a degree of steric hindrance due to the peri-hydrogen on
C-8 and it may be, as with 2,6-dimethylbenzoic acid (7),
that hindrance to nucleophilic solvation at the reaction
site leads to an increased sensitivity toward changes in
the solvation effects at the aromatic rings.
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EXPERIMENTAL
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Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 173–178