KINETICS AND MECHANISM OF ADDITION OF ACIDS TO OLEFINS .3. ADDITION OF HYDROGEN CHLORIDE TO 2-METHYL-1-BUTENE, 2-METHYL-2-BUTENE, AND ISOPRENE IN NITROMETHANE

The kinetics of addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, and isoprene in nitromethane are first order in olefin but second order in acid. Added tetraethylammonium chloride suppresses the rate of addition by combining with free hydrogen chloride to produce hydrogen dichloride ions while tetraethylammonium perchlorate has no such effect. Parallel studies of conductance and of infrared and nuclear magnetic resonance spectroscopy of hydrogen and deuterium chloride solutions in nitromethane provide a control on the interpretation of kinetic results. The data enable the conclusion that the un-ionized hydrogen chloride molecule acts as the dominant proton donor in these media and that the proton transfer from acid to olefin is assisted by a second molecule of acid which hydrogen bonds to the developing chloride ion. During addition there is no detectable interconversion of 2-methyl-1butene and 2-methyl-2-butene (kinetic control). The two isomers are, however, interconverted u der conditions leading to thermodynamic control (2-methyl-2-butene: 2-methyl-1butene = 85:15). Similarly, no incorporation of deuterium into unreacted olefin could be detected during the addition of deuterium chloride to either alkene. These results are consistent with a rate-determining proton transfer from hydrogen chloride to olefin leading to the formation of a carbonium hydrogen dichloride ion pair. Under strict kinetic control, the addition of hydrogen chloride to isoprene is shown to yield predominantly (>96%) 1,2-addition product. © 1969, American Chemical Society. All rights reserved.