The gas‐phase pyrolysis of alkyl nitrites. III. Isopropyl nitrite

Abstract

Abstract The rate of decomposition of isopropyl nitrite (IPN) has been studied in a static system over the temperature range of 130–160°C. For low concentrations of IPN (1–5 × 10 −5 M ), but with a high total pressure of CF 4 (∼0.9 atm) and small extents of reaction (∼1%), the first‐order rates of acetaldehyde (AcH) formation are a direct measure of reaction (1), since k 3 » k 2 (NO): \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$ {\rm IPN}\begin{array}{rcl} 1 \\ {\rightleftarrows} \\ 2 \\ \end{array}i - \Pr \mathop {\rm O}\limits^. + {\rm NO},i - \Pr \mathop {\rm O}\limits^. \stackrel{3}{\longrightarrow} {\rm AcH} + {\rm Me}. $\end{document} Addition of large amounts of NO (∼0.9 atm) in place of CF 4 almost completely suppressed AcH formation. Addition of large amounts of isobutane – t ‐BuH – (∼0.9 atm) in place of CF 4 at 160°C resulted in decreasing the AcH by 25%. Thus 25% of \documentclass{article}\pagestyle{empty}\begin{document}$ i - \Pr \mathop {\rm O}\limits^{\rm .} $\end{document} were trapped by the t ‐BuH (4): \documentclass{article}\pagestyle{empty}\begin{document}$ i - \Pr \mathop {\rm O}\limits^. + t - {\rm BuH} \stackrel{4}{\longrightarrow} i - \Pr {\rm OH} + (t - {\rm Bu}). $\end{document} The result of adding either NO or t ‐BuH shows that reaction (1) is the only route for the production of AcH. The rate constant for reaction (1) is given by k 1 = 10 16.2±0.4–41.0±0.8 /θ sec −1 . Since ( E 1 + RT ) and Δ H ° 1 are identical, within experimental error, both may be equated with D ( i ‐PrO‐NO) = 41.6 ± 0.8 kcal/mol and E 2 = 0 ± 0.8 kcal/mol. The thermochemistry leads to the result that \documentclass{article}\pagestyle{empty}\begin{document}$ \Delta H_f^\circ (i - {\rm Pr}\mathop {\rm O}\limits^{\rm .} ) = - 11.9 \pm 0.8{\rm kcal}/{\rm mol}. $\end{document} From Δ S ° 1 and A 1 , k 2 is calculated to be 10 10.5±0.4 M −1 ·sec −1 . From an independent observation that k 6 / k 2 = 0.19 ± 0.03 independent of temperature we find E 6 = 0 ± 1 kcal/mol and k 6 = 10 9.8+0.4 M −;1 ·sec −1 : \documentclass{article}\pagestyle{empty}\begin{document}$ i - \Pr \mathop {\rm O}\limits^. + {\rm NO} \stackrel{6}{\longrightarrow} {\rm M}_2 {\rm K} + {\rm HNO}. $\end{document} In addition to AcH, acetone (M 2 K) and isopropyl alcohol (IPA) are produced in approximately equal amounts. The rate of M 2 K formation is markedly affected by the ratio S / V of different reaction vessels. It is concluded that the M 2 K arises as the result of a heterogeneous elimination of HNO from IPN. In a spherical reaction vessel the first‐order rate of M 2 K formation is given by k 5 = 10 9.4–27.0 /θ sec −1 : \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm IPN} \stackrel{5}{\longrightarrow} {\rm M}_2 {\rm K} + {\rm HNO}. $\end{document} IPA is thought to arise via the hydrolysis of IPN, the water being formed from HNO. This elimination process explains previous erroneous results for IPN.