The near ultra-violet photolysis of alkyl nitrites
The alkyl nitrites (1 torr), t-butyl nitrite, t-amyl nitrite and s-butyl nitrite are photolysed in the presence of carbon tetrafluoride (50-900 torr) and low concentrations of nitric oxide over the temperature range 303-393K. Alkyl nitrites (RONO) were found to be good photolytic sources of alkoxy radicals (RO): ( la ) Me3CONO + hv Me 3 CO + NO ( lb ) Me 2 (Et) CONO + hv Me 2 ( Et ) CO + NO ( lc ) Et ( Me ) ( H ) CONO + hv Et( Me ) ( H ) CO + NO By carrying out the photolysis in the presence of nitric oxide, the rate of decomposition of the alkoxy radicals can be calculated relative to the rate of reaction (2), the recombination of nitric oxide and alkoxy radical. ( 2 ) RO + NO RONO ( 3 ) Me3 CO + M Me2 CO + Me ( -3 ) Me2 CO + Me Me3 CO The main objective of this work was to determine the pressure dependent characteristics of the decomposition reactions of the alkoxy radicals and derive values for the Arrhenius parameters. The decomposition of the t-butoxy radical to form acetone and a methyl radical was found to be markedly pressure dependent, the high pressure result for reaction (3) being: logk3(oo)/s-1 =14.02+0 .72 - 14.99 2. 24/2.303RT This result agrees well with previous studies carried out in this laboratory on the thermal decomposition of di-t-butyl peroxide and with the RRKM modelling results on the thermal decomposition of the t-butoxy radical in carbon tetrafluoride; the Arrhenius parameters derived from the RRKM modelling are logA3 (oo)/s-1= 14.2 and E3(oo) = 14.8 kcal.mol-1. A value for the rate constant for the reverse reaction ( -3 ), the addition of methyl radicals to acetone, was determined to be: logk3/M-1.s-1 = 6.8 0.5 - 10.16 1/2.303RT The result for k3 is in good agreement with Arrhenius parameters obtained for similar reactions involving the addition of methyl radicals to ketones which therefore adds support to the results obtained for the decomposition of the t-butoxy radical. In the photolysis of t-amyl nitrite, acetone was the only major product, the acetone being produced via reaction ( 4a ): (4a) Me2 ( Et ) CO + M Me2 CO + Et ( 4b ) Me 2 ( Et ) CO + M MeCOEt + Me Reaction ( 4b ) did not occur to any appreciable extent under the present experimental conditions, the ratio k4a/k4b being 33 at 393.2K. The decomposition of the t-amyloxy radical via reaction ( 4a ) was found to be pressure independent, the high pressure rate constant for this reaction being: logk4a(oo)/s-1 = 14.3 7 0 .53 - 11.78+0 .84/2.303RT A RRKM calculation on this reaction predicted some pressure dependence, the reaction reaching the high pressure region at 150 torr. The decomposition of the s-butoxy radical was found to proceed predominantly via reaction (5a)/ the reaction being pressure dependent at temperatures greater than 350K but pressure independent below this temperature. logk5a(oo)/s-1 = 14.2+0.85 - 12.71 l.33/2.303RT (5a) Me ( Et ) ( H ) CO + M MeCHO + Et (5b) EtCHO + Me (5c) MeCOEt + H No propionaldehyde was detected and the small quantities of methyl ethyl ketone could have been formed via the disproportionation reaction between the alkoxy radical and nitric oxide, reaction (6) and/or via the elimination of nitroxyl from the nitrite, reaction (7). (6) Me ( Et ) ( H ) GO + NO HNO + MeCOEt ( 7 ) Me ( Et ) ( H ) CONO HNO + MeCOEt.