Ketones Norrish type

Keywords czv-9-dccalyl aryl ketone, Norrish type II photochemistry, cyclopropanol, cyclobutanol, cyclopentanol... 

Keywords civ-4-/er/-butylcyclohexyl ketone, Norrish type II photoreaction... 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Attaching the ketone groups to the polymer backbone is more efficient on a chain scission/ketone basis because some of the light energy that the pendent ketone absorbs leads direcdy to chain scission via the Norrish type II mechanism, as well as photooxidation via the Norrish type I mechanism (see... 

As a side reaction, the Norrish type I reaction is often observed. The stability of the radical species formed by a-cleavage determines the Norrish type 1/Norrish type II ratio. For example aliphatic methyl ketones 10 react by a Norrish type II-mechanism, while aliphatic tcrt-butyl ketones 11 react preferentially by a Norrish type I-mechanism. 

Typical chemical reactions of photoexcited aldehydes and ketones are cleavage reactions, usually designated as Norrish Type I [equation (54)], II [equation (55)] and III [equation (56)], hydrogen abstraction [equation (57)] and cycloadditions, such as the Paterno-Buchi reaction [equation (58)]. Of these, Norrish Type II cleavage and the related... 

Norrish Type I cleavage of cyclic ketones necessarily yields biradicals, and in certain cases (e.g., cycloheptanone, camphor) strong emissions due to T i S mixing have been reported (Gloss and Doubleday, 1972). 

When applied to ketones, this is called Norrish Type / cleavage or often just Type I cleavage. In a secondary process, the acyl radical R —CO can then lose CO to give R radicals. Another example of a category 1 process is cleavage of CI2 to give two Cl atoms. Other bonds that are easily cleaved by photolysis are the 0—0 bonds of peroxy compounds and the C—N bonds of aliphatic azo compounds R—N=N—R. The latter is an important source of radicals R , since the other product is the very stable N2. 

Two examples from ketone photochemistry that has been recently analyzed within the context of solid-to-solid transformations are the Norrish type and Nor-rish-Yang type Ip44,i45 tactions. In general terms, the type I reaction consists of a homolytic cleavage of bond a-to the carbonyl to generate an acyl-alkyl radical pair (RP-A) or an acyl-alkyl biradical (BR-A) when the ketone is cyclic (Scheme 7.15). 

Frequently B will also undergo a back hydrogen transfer which regenerates the parent ketone, as well as cyclization (in most cases a minor reaction) as a result of this competition the quantum yields of fragmentation are typically in the 0.1-0.5 range in non-polar media. When the Norrish Type II process takes place in a polymer it can result in the cleavage of the polymer backbone. Poly(phenyl vinyl ketone) has frequently been used as a model polymer in which this reaction is resonsible for its photodegradation, reaction 2. 

First the interaction of selected tetramethylpiperidine (TMP) derivatives with radicals arising from Norrish-type I cleavage of diisopropyl ketone under oxygen was studied. These species are most probably the isopropyl peroxy and isobutyryl peroxy radicals immediately formed after a-splitting of diisopropyl ketone and subsequent addition of O2 to the initially generated radicals. Product analysis and kinetic studies showed that the investigated TMP derivatives exercise a marked controlling influence over the nature of the products formed in the photooxidative process. The results obtained point to an interaction between TMP derivatives and especially the isobutyryl peroxy radical. 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

Das zur Diskussion stehende 7t, n-angeregte Keton geht eine a-Spal-tung (Norrish Type I Process n>) zum entsprechenden 1,5-, 1,6- oder 1,7-Alkyl/Acyl-Biradikal ein, das... 

In some photochemical reactions both Si and T take part in the reaction. For example, both the Si and Tt states of aliphatic ketones, such as hexan-2-one, take part in Norrish type 2 reactions ... 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

In solution, the Norrish type 1 reaction of ketones results in the non-selective free-radical combination reactions to give products AA, AB and BB in the ratio of 1 2 1, whereas photolysis of ketones in zeolites produces ... 

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