Positional Selectivity of Polychlorobenzenes towards Photodechlorination in both Neutral and Alkaline 2-Propanol

Abstract

Polychlorobenzenes were photolyzed in both alkaline and neutral 2-propanol in order to investigate the positional selectivity towards dechlorination. The selectivity under neutral conditions was considerably different from that in the presence of alkali (Table 2, 3 and 4), which was elucidated on the basis of the steric and the electronic effects.
Chlorine atoms were, classified into the following three types; 1) one having two adjacent chlorine atoms on benzene ring, 2) one having one adjacent chlorine atom, and 3) one having no adjacent chlorine atom. Chlorine atom of type 1 was most reactive. The reactivities of chlorine atoms in type 2 and 3 decreased greatly in that order (Table 5).
The positional selectivity under neutral conditions was considered to be determined predominantly by the steric effect, caused by the adjacent chlorine atom. The electronic effect on the reactivities in the presence of alkali was estimated by the electron densities on carbon and chlorine atom of corresponding radical anion, which were calculated by use of Hiickel MO method. The reactivities in the presence of alkali were successfully explained by the electronic effect (Table 6 and 7), rather than by the steric effect.
The reactivities of polychlorobiphenyls were similarly discussed from the viewpoints of the electronic and steric effects (Table 6 and 7).