Effect of desolvation of the reactants during change in the acidity of the medium on the electrophilic hydroxylation and nitration of alkanes

Abstract

The effect of the acidity of the medium on the hydroxylation and nitration of alkanes (RH) in 90-98% H 2 SO 4 at 25°C is described quantitatively by a model taking account of the thermodynamic activity of the RH, H 2 O, and HSO 4 -particles. It was concluded that in the transition states the reagents H O HSO 3 2 4 + -and NO HSO 2 4 + -are present as HO + and NO 2 + ions without the bases H 2 O and HSO 4 -, the alkanes are present without hydrophobic shells, and the initial reaction products are ROH 2 + and RNO 2 H + . of water and bisulfate ions, electrophilic insertion.Olah and co-workers [1] observed the similarity of the products from the reactions of alkanes (RH) with hydrogen peroxide in the superacids HSO 3 F and HSO 3 F-SbF 5 -SO 2 ClF, in media HF and HF-BF 3 not forming peroxo acids, and in 96% H 2 SO 4 and concluded [1] that the reagent in these systems is the H O 3 2 + ion, which attacks the electrons of the C-H bond in the form of the generated OH + cation:The transition state (TS) contains a three-center two-electron bond (3c-2e).Olah substitution model (3c-2e), which was also proposed for nitration with NO 2 + ions [2] and other reactions of alkanes with electrophiles, was subject to criticism from Schreiner and co-workers [3]. According to their quantum calculations the scheme (1) is not general and only applies to the reactions of alkanes with protons (H/D exchange) and carbocations; in the transition state of the reaction of CH 4 + NO + in the gas the electrophilic center (N ä+ ) of the reagent interacts with the C atom but not with the C-H bond while the vacant electron pair of the oxygen atom accepts a proton. The NO + is inserted into the C-H bond with the formation of O-protonated nitrosomethane H 3 CNOH + [the first product, which then changes to the O-protonated formamide HC(NH 2 )OH + ]. [In [3] this reaction was called substitution, although in actual fact insertion occurs, while Olah process with the 3c-2e transition state or intermediate is called insertion.] Olah and co-workers [4] supported a special 20 0040-5760/10/4601-0020