Gemeinsame Ursache enthalpischer und entropischer Substituenteneffekte bei Reaktionen von Benzhydryl‐Kationen mit Nucleophilen

2010

Macromolecules

Self Cite

The cumyl cation was generated by laser flash photolysis of cumyl tris(4-chlorophenyl)phosphonium tetrafluoroborate in CH2Cl2 and identified by its UV spectrum. From the decay of its absorbance at λ = 335 nm in the presence of variable concentrations of several nucleophiles with CC double bonds, rate constants for the reactions of the cumyl cation with these π-nucleophiles were determined. The linear free energy relationship log k 20°C = s(N + E) (eq ) was used to calculate the electrophilicity parameter E = 5.74 of the cumyl cation from the rate constants determined in this work and the previously reported N and s parameters of the nucleophilic reaction partners. Substitution of E of the cumyl cation and of the previously reported N and s parameters of α-methylstyrene into eq predicts the temperature-independent rate constant of the addition of the cumyl cation to α-methylstyrene (1.2 × 108 M−1 s−1), which is relevant for the cationic polymerization of α-methylstyrene.

Section: Resultsmentioning

confidence: 99%

Electrophilic Reactivity of the α,α-Dimethylbenzyl (Cumyl) Cation

Ammer

2010

Macromolecules

Self Cite

“…Because 2 + (or 3 + ) represents a model for the active chain end in carbocationic isobutylene polymerization, k c given in Scheme corresponds to the propagation rate constant of this oligomerization. Since fast bimolecular reactions do not have an enthalpic barrier, this rate constant can be expected to be indepenent of temperature. Comparison with literature data shows a good agreement between k c and the propagation rate constant determined by Taylor for the bulk polymerization of isobutylene initiated by ionizing irradiation .…”

Section: Discussionmentioning

confidence: 99%

A Novel Method for the Determination of Propagation Rate Constants: Carbocationic Oligomerization of Isobutylene

Roth

1996

Macromolecules

Self Cite

The tertiary carbenium ions 2 + and 3 +, models of the propagating species in carbocationic isobutylene polymerization, are generated from the tertiary chlorides 2c and 3c with a Lewis acid in the presence of isobutylene (1c) and one of the allylsilanes 1a or 1b, which act as terminators. Since the rate of the termination reaction is known, the ratio of the telomerization products can be evaluated to give a propagation rate constant of (6 ± 2) × 108 L mol-1 s-1 (independent of temperature) for the carbocationic oligomerization of isobutylene in dichloromethane.

“…Reactions that have rate constants between 10 −6 and 10 10 L mol −1 s −1 are found in the green and yellow zones. When moving from left to right or from top to bottom in the green zone, the increase in reactivity is predominately a result of a decrease in Δ H ≠ ,15b which becomes zero in the yellow strip. The yellow domain encompasses reactions that are not diffusion controlled with Δ H ≠ =0 (entropy‐controlled reactions) 15…”

Section: Substitution Pattern and Electrophilicity Parameters Of The mentioning

confidence: 99%

Kinetic Studies of Carbocation-Carbanion Combinations: Key to a General Concept of Polar Organic Reactivity

Lucius

Loos

2002

Angew. Chem. Int. Ed.

Self Cite

220

117