SURJJT S. NAGRA and DAVID A. ARMSTRONG. Can. J. Chem. 53,3305 (1975). The gas phase 60Co y-radiolysis of HBr in mixtures with C,F6 and C 0 2 has been studied at room temperature, using doserates determined from ionization chamber measurements. A new value of WCZF6 = 32.7 eV is reported, but Wco, was inagreement withearlier work. Ionization yields and hydrogen yields in the mixtures followed a linear energy partitioning equation.The increase in the fraction of scavengable electrons with increasing pressure of buffer gas can be satisfactorily explained by enhanced moderation of the energies of subexcitation electrons. A study of the competition for thermal electrons between SF6 and HBr indicates that the rate of electron capture by HBr maintains a second order dependence on HBr pressure over the 50 to 600 Torr range both in the presence and absence of buffer gas. There was no evidence for the collisionzl stabilization of HBr-* by the buffer gases and it is pointed out that thermal electron capture may involve (HBr), dimers.SURJIT S. NAGRA et DAVID AI ARMSTRONG. Can. J. Chem. 53,3305 (1975).On a Btudie, B la temperature de la pike, la radiolyse-y (60Co) en phase gazeuse du HBr en melange avec C,F6 et du CO,; pour ces etudes on a utilise des vitesses de dose determines i partir de mesures de chambre B ionisation. Une nouvelle valeur de W C 2~6 = 32.7 eV est obtenue mais la valeur de WCO2 est en accord avec cette determination lors de travaux antkrieurs. Des rendements en ionisation et des rendements en hydrogene dans les melanges suivent une equation de partitionnement d'energie qui est lineaire.Une augmentation dans la fraction d'6lectrons piegeables avec une augmentation de la pression du gaz tampon peut 2tre expliquk facilement par une moderation accrue des energies des electrons sous-excites. Une etude de la competition pour les electrons thermiques entre SF6 et HBr indique que la vitesse de capture des electrons par HBr maintient une dependance du second ordre sur la pression de HBr entre 50 et 600 T o n en presence et en absence de gaz tampon. I1 n'y a pas d'indication pour une stabilisation provenant de collision de HBr-* avec les gaz tampons et on indique que la capture d'electron thermique peut impliquer des dimeres de (HBr), .

SURJIT S. NAGRA and DAVID ANTHONY ARMSTRONG. Can. J. Chem. 54, 3580 (1976). New competition kinetic studies of electron reactions in HCl and HBr in the 10Is to 1020 molecule cm-3 concentration region are reported and shown to support the findings of earlier investigations. The capture of thermal electrons by (HX): dimers is examined from a kinetic and thernrodynamic point of view. The (HX)2-'"intermediate previously proposed by several groups is considered to be involved in both systems. Differences in the magnitudes and concentration dependences of the rates can be explained, if reaction 5b is fast (k56 = 1013 s+) in For personal use only.NAGRA AND ARMSTRONG

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The gas phase 60Co radiolysis of mixtures of HCI and HBr has been studied at room temperature. Using SF6 as a thermal electron scavenger, the rates of electron capture determined by competition kinetics were higher by a factor of 3 or more than the combined contributions of (HC1)2 and (HBr)2 dimers. The net increase in the rates is proportional to the product of the concentrations of HCI and HBr in the mixtures and can be explained by contributions of mixed (HCI, HBr) dimer species. The magnitude of the electron capture rate constant, 1.4 ± 0.2 X 10"28 cm6 molecule"2 s"1, of the mixed dimers is very similar to 1.0 ± 0.1 X 10"28 cm6 molecule"2 s"1 observed in pure HBr, and it is suggested that the main reaction of the electron-mixed dimer ions is dissociation to H + ClHBr".

The gas phase pyrolysis of N-isopropylacetamide (IPA) has been studied in a static system at temperatures between 430 and 480°C. The decomposition occurs by a unimolecular mechanism to yield propene and acetamide. This was accompanied by a bimolecular decomposition to give isopropylamine, propene, acetic acid and acetonitrile. The latter reaction was also cataIysed by the acetic acid formed. The rate constants for the unimolecular (kl), bimolecular (k2) and the catalysed reaction (k,) are defined respectively by the Arrhenius equations k , = 1012*07*0.33 exp [(-54 OOO+ 1100) cal mol-'/RT] s-I kz = 1013.75f0.85 exp [( -42 200 k 2900) cal mol-l IRT] cm3 mokl s-I k3 = 1013*66*0-30 exp [(-35 SOOi-990) cal mol-'/RT] cm3 mol-l s-l where R = 1.987 cal mol-I K-l, 1 caI = 4.187 J.The relative rates of unimoIecular elimination of acetamide in the N-substituted acetamides are compared with those for the elimination of carboxylic acid from the esters. The greater mesomeric effect in the amide than in the ester group is responsible for a relatively smaller effect of cc-methylsubstitution in the N-alkyl substituted acetamides. The acetic acid catalysed decomposition of Nalkyf substituted acetamides is similar to that of the catalysed decomposition of acetamide.A number of workers 1-3 have suggested that the pyrolysis of N-alkylamides is similar to the pyrolysis of esters, the product being an olefin and the parent amide. Hurd and Blunk ' 9 proposed it six-centred transition state for the pyrolysis of esters. Similarities between substitutent effects 6 v on the rate of thermal decomposition of alkyl halides and esters in the gas phase and certain S , l and El reactions in solution suggested that ionic structures, formulated as a separation of a carboxylate and a carbonium ion (in esters) make an appreciable contribution to the transitional state. The quasi-heterolytic hypothesis was proposed to explain the effect of substituents for molecular elimination reactions in the gas phase. It was hoped that the kinetic studies of the thermal decomposition of N-alkyl substituted acetamides might throw some more light on the polar mechanism proposed for other molecular elimination reactions. The homogeneous and unimolecular gas phase decomposition of N-tbutylacetamide l o between 385 and 460°C has already been described. To investigate the effect of a-methyl substitution on the rate of the elimination reaction, we have studied the thermal decomposition of N-isopropylacetamide (IPA). EXPERIMENTALThe gaseous materials used in this work were commercial samples of purity greater than 99 %. IPA was prepared by the reactions of acetyl chloride with isopropylamine below 0°C. The constant boiling middle fraction (b.p. 95°C at 13 mmHg) was retained for use.The purity of IPA was checked by g.1.c. and the mass spectrum confirmed its identity. Most of the analyses were performed by gas chromatography using a Perkin Elmer F11 instrument with a flame ionization detector. The gaseous hydrocarbons were quantitatively