Question

Difference between primary and secondary photochemical reactions

Answer 1

Photochemical cyclization and halogen abstraction reactions of secondary alkylcobaloximes 1b and 1d compared to their primary counterparts 1a and 1c show an enhanced reactivity of secondary alkylcobaloximes: Thus, cyclohexylcobaloxime 1d reacts 13 times faster with CCl4 than n1-hexylcobaloxime 1c, 6-hepten-2-yl-cobaloxime 1b rearranges 58 times faster to its cyclopentylmethyl isomer 4b than 5-hexenylcobaloxime 1a. Although free alkyl radicals are reactive intermediates in photolytic conversions of alkylcobaloximes 1 in organic solvents, the presence of BrCCl3 or bromobenzene in photoreactions of primary alkylcobaloximes 1a and 1c seems to cause a more efficient homolytic cleavage of the Co – C bond.

Answer 2

Answer: Primary reactions occur due to absorption of light while secondary reactions are those thermal reactions that occur subsequent to primary reactions.

Explanation: Primary reactions obey the law of photochemical equivalence strictly while secondary reactions do not have any concern with the law of photochemical equivalence.

Example

HBr + hv  → H + Br  (primary reaction)

H + HBr  →H2 +Br  ( Secondary reaction)

Br + Br → Br2  (secondary reaction)