Kinetics of Hydrolysis of Substituted β-Nitrostyrenes. Transition-State Imbalances and Intrinsic Rate Constants for Three Different Types of Nitronate Ion Forming Processes. Relevance to the Nitroalkane Anomaly

Article abstract of DOI:10.1021/ja00197a059

A kinetic study of the hydrolysis of β-nitrostyrene and of the 4-chloro and 3-nitro derivatives in 50percent Me2SO-50percent water (v/v) at 20 deg C is reported.The mechanism involves four steps: nucleophilic addition of OH(-) or water to form ArCH(OH)CHNO2(-) (T^-_OH); carbon protonation of T^-_OH by water, H(+), and buffer acids to form ArCH(OH)CH2NO2 (T⁰_OH); deprotonation of the OH group in T⁰_OH to form ArCH(O(-))CH2NO2 (T^-_O); collapse of T^-_O into ArCHO and CH2NO2(-).In strongly acidic solution the aci form of T⁰_OH, PhCH(OH)CH=NO2H (T⁰_OH,aci), which is generated by protonation of T^-_OH on the nitro group, could also be detected.All steps are reversible, and the rate and equilibrium constats of most of them could be determined by a combination of kinetic experiments starting with the substrate, the products, the independently synthesized T⁰_OH, and T^-_OH (generated at high pH and then subjected to a pH jump) and by measuring product ratios spectrophotometrically or by HPLC analysis.The intrinsic rate constants of the proton transfer, T^-_OH <-/-> T⁰_OH, are close to those for the deprotonation of nitromethane and 2-nitroethanol, and the Br<*>nsted coefficients show a transition-state imbalance (α_CH >> β_B(-)) that is typical for the deprotonation of nitroalkanes.The other two processes that lead to the formation of a nitronate ion, i.e., nucleophilic addition to the olefin to form T^-_OH and collapse of T^-_O to form CH2NO2(-), show similar structure-reactivity behavior as the proton transfer, such as transition-state imbalances as manifested by αⁿ_nuc > βⁿ_nuc for nucleophilic addition and depressed intrinsic rate constants.However, these imbalances are smaller compared to the one in the proton transfers, especially so for the nucleophilic addition to the olefin.It is suggested that there is an inherent tendency for reactions leading to resonance-stabilized ions to have imbalanced transition states, but in proton transfers there are two imbalance enhancing factors, namely sp³ hybridization of the carbon in the protonated form and hydrogen bonding in the transition state, while in the collapse of T^-_O there is one such factor (sp³ hybridization of the carbon in T^-_O).