NDLI: A Marcus Treatment of Rate Constants for Protonation of Ring-Substituted α-Methoxystyrenes

Content Provider	PubMed Central
Author	Richard, John P. Williams, Kathleen B.
Abstract	Rate and equilibrium constants were determined for protonation of ring-substituted α-methoxystyrenes by hydronium ion and by carboxylic acids to form the corresponding ring-substituted α-methyl α-methoxybenzyl carbocations at 25 ° C and I = 1.0 (KCl). The thermodynamic barrier to carbocation formation increases by14.5 kcal/mol as the phenyl ring substituent(s) is changed from 4-MeO- to 3,5-di-NO2-, and as the carboxylic acid is changed from dichloroacetic to acetic acid. The Brønsted coefficient α for protonation by carboxylic acids increases from 0.67 to 0.77 over this range of phenyl ring substituents and the Brønsted coefficient β for proton transfer increases from 0.63 to 0.69 as the carboxylic acid is changed from dichloroacetic to acetic acid. The change in these Brønsted coefficients with changing reaction driving force, ∂ α ∕ ∂ Δ G av o = ∂ β ∕ ∂ Δ G av o = 1 ∕ 8 Λ = 0.011 is used to calculate a Marcus intrinsic reaction barrier of Λ = 11 kcal/mol which is close to the barrier of 13 kcal/mol for thermoneutral proton transfer between this series of acids and bases. The value of α = 0.66 for thermoneutral proton transfer is greater than α = 0.50 required by a reaction that follows the Marcus equation. This elevated value of β may be due to an asymmetry in the reaction coordinate that arises from the difference in the intrinsic barriers for proton transfer at the oxygen acid reactant and resonance stabilized carbon acid product.
Related Links	http://dx.doi.org/10.1021/ja071007k
Ending Page	6961
Page Count	10
Starting Page	6952
File Format	PDF
ISSN	00027863
e-ISSN	15205126
Journal	Journal of the American Chemical Society
Issue Number	21
Volume Number	129
Language	English
Publisher Date	2007-05-01
Access Restriction	Open
Subject Keyword	Colloid and Surface Chemistry Biochemistry Chemistry(all) Catalysis Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Chemistry Colloid and Surface Chemistry Biochemistry Catalysis

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