Norrish type 1 photo-cleavage

The end result of these studies showed very clearly that two major processes were important, i.e. photolysis and photo-oxidation. Photolysis reactions were posited to be the result of the well-known Norrish Type 1 and Norrish Type 2 cleavage reactions. As we shall see, the Type 1 cleavage followed by several subsequent reactions can account for many of the observed degradation products. 

Further detailed information on the reactive excited states of p. -unsaturated ketones has been acquired with (3a-c). Inefficient intersystem crossing from the singlet to the triplet state precedes population of Tz(.n,Tt ) from which the 1,3-acyl shift, induced by Norrish type I cleavage, occurs more readily than internal conversion to Ti Ti has been estimated to lie within 8 W mol of the acetone triplet (335 kJ mol ). That the 1,3-acyl shift is coupled not only with singlet-state but also Tz-state reactivity has been unequivocally shown by two methods. Thermal decomposition of a dioxetane, a derivative of (3a), generates predominantly the T2 state from which the 1,3-acyl shift product is formed concurrently with the ODPM products (4a) and (5a) via A photo-CIDNP study has independently demon-... 

Norrish II type hydrogen abstraction and Norrish I photo cleavage were shown to take place in the triplet state of diaUcoxy acetophenones ... 

Norrish type II cleavage Photoenolization (s. a. 3-Elimination, photo-enolization-induced) review 31,128 Photoflnorination 32, 484 Photo-Friedel-Crafts synthesis, intramolecnlar 32, 975 mem-Photo-Fries rearrangement, skeletal 31,685 Photoisomerization... 

Several of the more common commodity polymers like the polyolefins are susceptible to photo-oxidation. For a polymer like polyethylene, photo-oxidation leads to increasing amounts of carbonyl compounds. In-chain ketone groups act as sensitisers by UV light absorption. Through the well-known Norrish type I and II degradations radicals, end-vinyl and ketone groups are formed. Other products often observed in photo-oxidised low-density polyethylene (LDPE) are esters [5]. Scheme 1 shows one mechanism for abiotic ester formation. By Norrish type I cleavage the radical formed can react with an alkoxyl radical on the polyethylene (PE) chain. 

There are four distinct processes initiated by y-hydrogen abstraction in excited carbonyl compounds Norrish type II photoelimination, Yang photo-cyclization (cyclobutanol formation), Yang photoenolization (o-xylylenol formation), and (3-cleavage of radicals from carbons adjacent to the radical sites of the 1,4-biradicals. Some of these require unique structures and generate distinct products. 

A considerable number of examples described below in which nucleic acid radicals are independently generated take advantage of the a-photo-cleavage (Norrish type I) of ketones (Scheme Most of the examples that wiU be cited involve photolysis of i-butyl or benzyl ketones (R ). This is consistent with the general quantum yield efficiency and rate constant for a-cleavage of the triplet excited state ketone. Consideration of the rate constant for decarbonylation of the ground state acyl radical is also relevant, with the efficiency of decarbonylation correlating with radical stability." ... 

The classic photoreaction within photo-oxidative degradation is the photolysis of hydrogen peroxides. Also known in organic photochemistry are photo-induced reactions such as Norrish type I and Norrish type II reactions that result in cleavage of the carbon structure and thus to a loss in molecular weight, Figure 5.12 [381. 

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