Kinetics and mechanisms of the gas-phase elimination of 2-substituted primary, secondary and tertiary hydroxy groups in nitroalkanes

The kinetics of the gas-phase elimination of several 2-substituted primary, secondary and tertiary hydroxy groups in nitroalkanes were determined in a static reaction system over the temperature range 220-400 degreesC and pressure range of 29-235 Torr. The reactions, in seasoned vessels, are homogeneous and unimolecular and obey a first-order rate law. The presence of secondary and tertiary hydroxy substituent at the 2-position of the nitro group in nitroalkanes leads to a retro-aldol type of decomposition. The mechanism may be rationalized in terms of a six-membered cyclic transition state to give the corresponding aldehyde or ketone and the nitroalkane, respectively. However, some of the primary 2-hydroxy groups in nitroalkanes undergo a dehydration process with very limited isomerization to the corresponding alkyl nitrate. The mechanism of dehydration is believed to proceed through a six-membered rather than the already reported four-membered cyclic transition state to give the nitroalkene and water. In the case of the primary hydroxy substituent in 2-methyl-2-nitro-1-pentanol, the products of elimination are HNO2 gas and 3-hydroxy-2-methyl-1-propene. This reaction is rationalized in terms of a four-membered cyclic transition state type of mechanism. The kinetic and thermodynamic parameters of the hydroxynitroalkane substrates are presented and discussed. Copyright (C) 2004 John Wiley Sons, Ltd.