Q6(c). Photochemistry of butadiene (S1, T1) and of pentan-2-one (Norrish I, II)

Introduction. Excited states react differently: the short-lived singlet (S1) of a diene gives concerted, intramolecular products; the triplet (T1), reached by sensitisation, lives long enough to meet another molecule and reacts through biradicals. Ketones react from the n,π* state by α-cleavage (Norrish I) or γ-H abstraction (Norrish II).

(i) Buta-1,3-diene

Scheme: Direct irradiation (S1) and triplet-sensitised dimerisation (T1).

1. S1 (direct irradiation, dilute): s-cis butadiene closes disrotatorily to cyclobutene; an internal crossed [π2ₛ+π2ₛ] gives bicyclo[1.1.0]butane.
2. T1 (sensitiser such as benzophenone or acetophenone): the triplet diene adds to a ground-state diene, giving an allylic 1,4-biradical that closes to dimers: trans-1,2-divinylcyclobutane (major), cis-1,2-divinylcyclobutane and 4-vinylcyclohexene.

(ii) Pentan-2-one

Scheme: γ-H transfer through a six-membered TS, then cleavage or ring closure.

1. Norrish type I (α-cleavage): CH₃COCH₂CH₂CH₃ →CH₃CO + CH₂CH₂CH₃ (the bond to the propyl group breaks, giving the more stable alkyl radical). The acyl radical loses CO: CH₃CO → CH₃ + CO. The radicals recombine or disproportionate: CO, ethane (CH₃• + CH₃•), butane, hexane, and propane + propene.
2. Norrish type II: the excited C=O oxygen abstracts a γ-hydrogen (on C-5) through a six-membered cyclic TS, giving a 1,4-biradical. It cleaves at the β C–C bond to ethene and the enol of acetone, which tautomerises to acetone; some biradicals close to 1-methylcyclobutan-1-ol (Yang cyclisation).

Conclusion. (i) S1: cyclobutene + bicyclo[1.1.0]butane; T1: divinylcyclobutanes + 4-vinylcyclohexene. (ii) Norrish I: CH₃CO• + propyl radical → CO + alkanes; Norrish II: acetone + ethene (and 1-methylcyclobutanol).

Application. Norrish type I and II reactions cause the photodegradation of ketone-containing polymers (ethylene–CO copolymers are designed to break down in sunlight this way).