Premium
Kinetic and thermodynamic studies of carbenium ion additions towards alkenes
Author(s) -
Mayr Herbert,
Schneider Reinhard,
Schade Christian
Publication year - 1988
Publication title -
makromolekulare chemie. macromolecular symposia
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.257
H-Index - 76
eISSN - 1521-3900
pISSN - 0258-0322
DOI - 10.1002/masy.19880130106
Subject(s) - chemistry , standard enthalpy of formation , enthalpy , equilibrium constant , reaction rate constant , standard enthalpy of reaction , kinetic energy , ion , kinetics , organic chemistry , thermodynamics , physics , quantum mechanics
The ionisation (Ar 2 CHCl + BCl 3 ⇌ Ar 2 CH + BCl − 4 ) and dissociation (Ar 2 CH + BCl − 4 ⇌ Ar 2 CH + + BCl − 4 ) equilibria of diarylmethyl chlorides in boron trichloride/dichloromethane solution have been studied by conductimetry, photometry and 1 H NMR spectroscopy. Small differences in the UV‐vis spectra of diaryl‐carbenium tetrachloroborates, which have been observed in solutions of low and high tetrachloroborate concentration, can be attributed to the formation of 1:1 ion‐pairs in the more concentrated solutions. Low temperature calorimetry was used to determine the heats of addition of diarylcarbenium tetrachloroborates to 2‐methyl‐1‐pentene (Ar 2 CH + BCl − 4 + H 2 C‐CRR' + Ar 2 CH‐CH 2 ‐CRR'Cl + BCl 3 ), and it is estimated that the standard free enthalpy of this reaction is greater than 0 for systems with pK R+ > −2.6. Kinetic studies have shown that paired and unpaired diarylcarbenium tetrachloroborates exhibit identical reactivity towards alkenes. A rationalisation for the different situation in carbocationic and carbanionic polymerisation is presented. The rate constants for the initiation of isobutene, styrene and isoprene polymerisation by diarylcarbenium ions have been determined, and it is shown, how propagation rate constants can be estimated from linear free energy relationships.