Norrish solvents

Majeti11 has studied the photochemistry of simple /I-ketosulfoxides, PhCOCH2SOCH3, and found cleavage of the sulfur-carbon bond, especially in polar solvents, and the Norrish Type II process to be the predominant pathways, leading to both 1,2-dibenzoylethane and methyl methanethiolsulfonate by radical dimerization, as well as acetophenone (equation 3). Nozaki and coworkers12 independently revealed similar results and reported in addition a pH-dependent distribution of products. Miyamoto and Nozaki13 have shown the incorporation of protic solvents into methyl styryl sulfoxide, by a polar addition mechanism. 

A superficially related dependence of on the medium has been observed by Norrish and Smith working with methyl methacrylate, and by Burnett and Melville with vinyl acetate. Rates in poor solvents are high, and determination of by the rotating sector method reveals what appears to be a decrease in kt in the poor solvents. This apparent decrease in kt accounts for the increased rate of polymerization. Actually, precipitation of the polymer seems to be responsible for the effect. The growing radicals become imbedded in precipitated droplets, presumably of very small size. The termination reaction is suppressed owing to isolation of the chain radical in one droplet from that in another. This gel effect is fairly common in systems yield-... 

The values of kjkp derived from the plots are shown in Table 4. At least at -78° they are almost independent of the nature of the solvent and thus the difference between these values and that derived from Norrish and Russell s results (a factor of about 30) is probably due to the difference in the co-catalyst, and therefore in the structure of the anion. It is worth noting that with TiCl4 (in contrast to SnCl4) a change of co-catalyst from HzO to CC13C02H does not produce a significant change in kjkp (see Table 5). 

Water has also been shown to be essential for the liquid phase polymerization of isobutylene with stannic chloride as catalyst (Norrish and Russell, 87). The rates of reaction were measured by a dilatometric method using ethyl chloride as common solvent at —78.5°. With a mixture consisting of 1.15% stannic chloride, 20 % isobutylene, and 78.8% ethyl chloride, the rate of polymerization was directly proportional to the amount of added water (up to 0.43% of which was added). A rapid increase in the rate of polymerization occurred as the stannic chloride concentration was increased from 0.1 to 1.25% with higher concentrations the rate increased only gradually. It was concluded that a soluble hydrate is formed and functions as the active catalyst. The minimum concentration of stannic chloride below which no polymerization occurred was somewhat less than half the percentage of added water. When the concentration of the metal chloride was less than about one-fifth that of the added water, a light solid precipitated formation of this insoluble hydrate which had no catalytic activity probably explains the minimum catalyst concentration. The addition of 0.3% each of ethyl alcohol, butyl alcohol, diethyl ether, or acetone in the presence of 0.18% water reduced the rate to less than one-fifth of its normal value. On the other hand, no polymerization occurred on the addition of 0.3 % of these substances in the absence of added water. The water-promoted reaction was halved when 1- and 2-butene were present in concentrations of 2 and 6%, respectively. 

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

Norrish and Smith f have studied two reactions, namely, the interactions of trimethylamine with m- and with p-nitrobenzyl chloride in non-polar solvents, with a view to correlating the absolute rates of change with the values for the energy of activation as calculated from the temperature coefficients. They find here also that there is a marked deactivating effect of the solvent, which, as they point out, is not surprising, since in solution the mean free path is of the same order as the molecular diameter, and nearly every collision between potentially reactant solute molecules must therefore of necessity partake of the nature of a ternary collision at least, in which the third body is a solvent molecule . 

The influence of these various effects may be manifested in measurable parameters of the reaction like the overall quantum yields (On) and the photoproduct ratios for fragmentation to cyclization (E/C) and for trans to cis cyclobutanol formation (t/c) as shown in Scheme 41. The values of these quantities and their variations as the media are changed can provide comparative information concerning the relative importance of solvent anisotropy on Norrish II reactions, also. Specifically, they reveal characteristics of the activity of the walls and the size, shape, and rigidity of the reaction cavities occupied by electronically excited ketones and their BR intermediates. 

As shown in Figure 42 for the Norrish II reactions of a simple ketone, 2-nonanone, not only do the shapes of the products differ from those of the reactant, but so do their molecular volumes [265]. Interestingly, the volume of the fragmentation products, 1-hexene and 2-hydroxypropene (which ketonizes to acetone), are closer in volume to 2-nonanone than is either of the cyclization products. They are also capable of occupying more efficiently the shape allocated by a stiff solvent matrix to a molecule of 2-nonanone in its extended conformation the cross-sectional diameter of either of the cyclobutanols is much larger than that of extended 2-nonanone or the fragmentation products when spaced end-on. Both of these considerations should favor fragmentation processes if isomorphous substitution for the precursor ketone in the reaction cavity is an important requirement for efficient conversion to photoproducts. 

Most aldehydes and ketones in inert solvents or in the gas phase undergo one or two photoreactions, called Norrish Type I and Norrish Type II processes.81 The Norrish Type I reaction, shown in Equation 13.62, may originate... 

From a synthetic point of view, the extension of the concept, described in Sec. 3.4.2.1 (type Ih), on y-ketoamides, does not only provide an attractive method but also a molecular system that is very valuable for the improvement of the mechanistic understanding of the Norrish-Yang reaction. Thus, on irradiation y-ketoamides 50 cyclize to 5-lactams 51 in good yields [48]. It should be noted that the reaction outcome strongly depends on the solvent used. Whereas the irradiation in dichloromethane gives the 5-lactams 51, the only products formed in diethylether were pinacols [49]. The formation of cyclopropanes from 50 [50] described earlier is probably based on a misinterpretation of the spectral data [49]. 

Norrish and Smith [29] and later Tromsdorff et al. [30] described a polymerization of methyl methacrylate, the rate of which increased from a certain conversion. The number of monomers of similar behaviour was extended by methyl acrylate [31 ], butyl acrylate [32] and other acrylates [33] and methacrylates [34], and vinyl acetate. The effect was explained by the reduction of the termination rate caused by hindered macroradical mobil-ity in viscous medium it was called the gel effect, or the Norrish-Tromsdorff effect. The gel effect is clearly manifested in radical polymerizations of weakly transferring monomers in bulk. It is significant also in the presence of a good solvent. The gel effect is suppressed by the presence of poor solvents++ and by... 

The TREPR experiments and simulations described here have provided an enormous amount of structural and dynamic information about a class of free radicals that were not reported in the hterature prior to our first paper on this topic in 2000. Magnetic parameters for many main-chain acrylic radicals have been established, and interesting solvent effects have been observed such as spin relaxation rates and the novel pH dependence of the polyacid radical spectra. It is fair to conclude from these studies that the photodegradation mechanism of acrylic polymers is general, proceeding through Norrish 1 a-cleavage of the ester (or acid) side chain. Recently, model systems have... 

As a consequence of the alignment of solute molecules in liquid-crystalline solvents, the ratio of products formed in competitive reaction pathways can be different from that observed in isotropic liquids. This is illustrated by the Norrish type II photolysis of alkyl phenyl ketones with varying alkyl chain length in the isotropic, smectic, and... 

The direct irradiation (A = 300 nm) of the ketone (5) in Bu OH yields (6) and (7), the two possible Norrish Type I fragmentation products. a-Cleavage does not occur on irradiation in Pr OH but rather reductive loss of fluorine yields cyclohexanone. The cyclohexenone (8) shows an analogous solvent-dependent selectivity. Sonawane et al. have shown that the photochemical... 

The solvent dependency of the photochemistry of the dione (268) has been studied.The diketones (269a-f) undergo Norrish Type II processes to yield... 

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