- Determination and interpretation of second order rate constants for the addition of hydrogen halides to alkenes
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An extensive range of second order rate constants for the addition of hydrogen halides to alkenes in 98% v/v ethanoic acid (acetic acid)-water have been obtained by conductivity measurements. The rate constants are in the expected order of HF HCl HBr HI. The rates with different alkenes cannot be rationalised solely by a consideration of carbocation stability and it has been necessary to consider steric effects in order to explain the observed order.
- Borgeaud, Robert,Newman, Henry,Schelpe, Arabella,Vasco, Veronica,Peter Hughes
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p. 810 - 813
(2007/10/03)
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- Electrophilic Cleavage of Cyclopropanes. Acetolysis of Alkylcyclopropanes
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The solvent kinetic hydrogen isotope effect showed that proton transfer is at least partially rate determining for the acetolysis of cyclopropanes which span a range of 1010 in reactivity.The energies and structures of protonated cyclobutanes were calculated and provide an explanation for the large difference in reactivity between cyclopropanes and cyclobutanes despite their similarity in enthalpies of reaction.The rates and products of acetolysis of a series of alkyl-substituted cyclopropanes were examined.The data, along with the results of ab initio calculations, indicate that for alkyl-substituted cyclopropanes, the protonated species is highly unsymmetrical.Cleavage of the cyclopropane ring always occurs so that the nucleophile becomes attached to the most substituted carbon, but the proton may attack either of the remaining carbons.Proton attack may lead to either retention or inversion of configuration depending on the orientation of the attacking proton with respect to the ring.
- Wiberg, Kenneth B.,Kass, Steven R.
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p. 988 - 995
(2007/10/02)
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- Kinetics and Mechanisms of Nucleophilic Displacement with Heterocycles as Leaving Groups. 17. Solvolysis of 14-(Primary alkyl)-5,6,8,9-tetrahydro-7-phenyldibenzoacridiniums: Rates, Identification of Products, Activation Parameters, and a General Discussion of Mechanism
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Solvolysis rate are reported for the Me, Et, n-Pr, n-Pent, n-Oct, i-Bu, neo-Pent, PhCH2CH2, and MeOCH2CH2 title compounds in MeOH, EtPH, PentOH, CH3CO2H, and CF3CO2H.Rate variations with alkyl group structure are far less than the corresponding rate variations for the tosylate solvolysis, and afford no evidence for rate-enhancing participation by β-phenyl or β-methoxy groups in the acridinium solvolyses.The n-propyl, n-pentyl, and n-octyl title compounds solvolyze in CH3OD and CH3CO2D to give mixtures of normal and rearranged products, none of which contain deuterium and which are therefore not formed via olefin intermediates.Methanolysis of the isobutyl title compounds occurs via olefin, but the acetolysis also involves an important nonolefinic pathway yielding isobutyl and sec-butyl acetates.Methanolysis products from the neopentyl derivative are heavily deuterated, but acetolysis yields undeuterated neopentyl acetate as well as deuterated tert-pentyl acetate.Product proportions calculated using GC/MS were used to deduce the fractions of reactions by various mechanistic pathways.Individual rates are calculated for solvolysis to the various unrearranged and rearranged products.They indicate that normal substitution in MeOH occurs by a classical SN2 reaction, but that such substitution in AcOH involves ion-pair intermediates.It is concluded that such ion pairs under go Me and H migration after the rate-determining stage, in competition with substitution.Activation parameters provide further evidence for the mechanistic paths proposed which are discussed in relation to literature data available for the corresponding tosylate.
- Katritzky, Alan R.,Dega-Szafran, Zofia,Lopez-Rodriguez, Maria L.,King, Roy W.
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p. 5577 - 5585
(2007/10/02)
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