Radiolysis aliphatic carboxylic acids

ALIPHATIC CARBOXYLIC ACIDS The radiation chemistry of the simple aliphatic carboxylic acids has been widely investigated. The major products of gamma radiolysis of these compounds are typified by those found (1) for radiolysis of isobutyric acid at 273 K... 

For a wide variety of aliphatic carboxylic acids, including some with aromatic substituents, the product yields for CO and C02 are in an approximately constant ratio of 1 10 for radiolysis at 298 K. Values for some typical compounds are given below in Table II. 

The major radical intermediates formed following radiolysis of aliphatic carboxylic acids are also typified by those found following radiolysis of isobutyric acid (1). In Figure 1 are illustrated the ESR spectra found following radiolysis of this acid at 77 K and 195 K. 

The range of products formed on gamma radiolysis of N-acetylglycine was similar to that formed on radiolysis of the aliphatic carboxylic acids, but there are some noticeable differences in the yields of products. Carbon dioxide is by far the major volatile product of radiolysis and the corresponding product of the decarboxylation reaction, N-methyl acetamide, is also present in large yield, but the yield of this product was not quantitatively determined. By contrast, carbon monoxide is found in very small yield. The yield of acetamide, the product of N-Ca bond scission, is found in much greater yield. 

A similar behaviour has been found to occur with the other N-acetyl amino acids. In each case, the most stable radical observed at 303 K was the alpha carbon radical, as was also observed for the aliphatic carboxylic acids. In Table VI the radical yields observed following gamma radiolysis of a series of N-acetyl amino acids at 303 K are reported, together with the stable radical intermediates observed at this temperature (5). 

Again the close correspondence between the measured radical and carbon dioxide yields for 7-radiolysis of the N-acetyl amino acids in the solid state suggests that the mechanisms for radical production and carbon dioxide formation are closely related, as they were for the aliphatic carboxylic acids. The following mechanism has been proposed (5.) in order to account for the major degradation products and observed radical intermediates. 

As with the aliphatic carboxylic acid model compounds, the major volatile product observed on gamma radiolysis of the poly acids is carbon dioxide. However, the carbon dioxide yields are somewhat larger than those observed for the model compounds. 

Two stable radical intermediates are observed following gamma radiolysis at 303 K. The alpha carbon radical VIII and the side chain radical IX are formed in approximately equal yields, with the total G-value for radical production equal to 3.2. This value is similar to that observed for the poly acids. The observed radicals are those which would be expected on the basis of the aliphatic carboxylic acids and previous studies of the poly amino acids with aliphatic side chains. 

In the poly carboxylic acids, carbon dioxide is the major product of radiolysis, but the carbon monoxide yields are greater than they are for the aliphatic carboxylic acids. However, the radical yields are not greater than expected on the basis of the model compounds, which suggests that excited states play an important role in the degradation of these poly acids. 

The irradiation of substances that form crystals containing discrete molecules held together by dispersion forces results in radiolysis in the conventional sense. For example, the radiolysis of aliphatic carboxylic acids in the solid state yields hydrogen, carbon monoxide. 

In the radiolysis of aliphatic carboxylic acids, carboxylic add esters and amides preferred formation of a-carboxyalkyl type radicals (e.g., R CH COOR) are observed. 

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