Author:
Badger GM,Drewer RJ,Lewis GE
Abstract
A satisfactory method has been developed
for the determination of quantum yields for the photochemical cyclization
reactions of aromatic azo compounds in strongly acidic solutions. With the
three azobenzenes selected for study there was no direct correlation between
the quantum yields of the cyclization reaction and Hammett substituent
constants. A general decrease in quantum yield and increase in yield fraction
of cyclized product were observed as the acidity of the reaction medium was
progressively raised. At 436 mμ and 25� �(azobenzene)
= 0.016-0.014 (14-18N H2SO4), �(4-chloroazobenzene)
= 0.006-0.005 (14-20N H2SO4), and �(4-methylazobenzene)
= 0.005-0.004 (12-18N H2SO4). Variations in quantum yield
with change in temperature to 15�, and with change in wavelength of the
exciting radiation to 405 mμ, were investigated
with azobenzene and were found to be quite small. A mechanism for the
cyclization reaction is now proposed in which a short-lived photoexcited
state of the cis-conjugate acid of
the azo compound is envisaged as the species involved in the ring-closure step.
The variations in quantum yields with changes of substituents and acidity are
discussed in relation to this mechanism, and extensions of the mechanism to
accommodate the observed ejection and migration of substituent groups from
positions of cyclization are considered.
Cited by
43 articles.
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