The gas‐phase pyrolysis of alkyl nitrites. II. s‐butyl nitrite

Abstract

AbstractThe rate of decomposition of s‐butyl nitrite (SBN) has been studied in the absence (130–160°C) and presence (160–200°C) of NO. Under the former conditions, for low concentrations of SBN (6 × 10−5 − 10−4M) and small extents of reaction (∼1.5%), the first‐order homogeneous rates of acetaldehyde (AcH) formation are a direct measure of reaction (1) since k3c » k2(NO): . Unlike t‐butyl nitrite (TBN), d(AcH)/dt is independent of added CF4 (∼0.9 atm). Thus k3c is always » k2 (NO) over this pressure range. Large amounts of NO (∼0.9 atm) (130–160°C) completely suppress AcH formation. k1 = 1016.2–40.9/θ sec−1. Since (E1 + RT) and ΔH°1 are identical, within experimental error, both may be equated with D(s‐BuO‐NO) = 41.5 ± 0.8 kcal/mol and E2 = 0 ± 0.8 kcal/mol. The thermochemistry leads to the result ΔH°f (s‐\documentclass{article}\pagestyle{empty}\begin{document}${\rm Bu}\mathop {\rm O}\limits^{\rm .}$\end{document}) = − 16.6 ± 0.8 kcal/mol. From ΔS°1 and A1, k2 is calculated to be 1010.4 M−1 · sec−1, identical to that for TBN. From an independent observation that k6/k2 = 0.26 ± 0.01 independent of temperature, \documentclass{article}\pagestyle{empty}\begin{document}${\rm s - Bu}\mathop {\rm O}\limits^{\rm .} + {\rm NO}\mathop \to \limits^{\rm 6} {\rm MEK} + {\rm HNO}$\end{document}, we find E6 = 0 ± 1 kcal/mol and k6 = 109.8M−1 · sec−1. Under the conditions first cited, methyl ethyl ketone (MEK) is also a product of the reaction, the rate of which becomes measurable at extents of conversion >2%. However, this rate is ∼0.1 that of AcH formation. Although MEK formation is affected by the ratio S/V for different reaction vessels, in a spherical reaction vessel, this MEK arises as the result of an essentially homogeneous first‐order 4‐centre elimination of HNO. \documentclass{article}\pagestyle{empty}\begin{document}${\rm SBN}\mathop \to \limits^{\rm 5} {\rm MEK} + {\rm HNO}$\end{document}; k5 = 1012.8–35.8/θ sec−1. Sec‐butyl alcohol (SBA), formed at a rate comparable to MEK, is thought to arise via the hydrolysis of SBN, the water being formed from HNO. The rate of disappearance of SBN, that is, d(MEK + SBA + AcH)/dt, is given by kglobal = 1015.7–39.6/θ sec−1. In NO (∼1 atm) the rate of formation of MEK was about twice that in the absence of NO, whereas the SBA was greatly reduced. This reaction was also affected by the ratio S/V of different reaction vessels. It was again concluded that in a spherical reaction vessel, the rate of MEK formation was essentially homogeneous and first order. This rate is given by kobs = 1012.9–35.4/θ sec−1, very similar to k5. However, although it is clear that the rate of formation of MEK is doubled in the presence of NO, the value for kobs makes it difficult to associate this extra MEK with the disproportionation of s‐\documentclass{article}\pagestyle{empty}\begin{document}${\rm Bu}\mathop {\rm O}\limits^{\rm .}$\end{document} and NO: s‐\documentclass{article}\pagestyle{empty}\begin{document}$s{\rm - Bu}\mathop {\rm O}\limits^{\rm .} + {\rm NO}\mathop \to \limits^{\rm 6} {\rm MEK} + {\rm HNO}$\end{document}. NO at temperatures of 130–160°C completely suppresses AcH formation. AcH reappears at higher temperatures (165–200°C), enabling k3c to be determined. Ignoring reaction (6), d(AcH)/dt = k1k3 (SBN)/[k3c + k2(NO)]; k3c = 1014.8–15.3/θ sec−1. Inclusion of reaction (6) into the mechanism makes very little difference to the result. Reaction (3c) is expected to be a pressure‐dependent process.