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Alkylation of pyridinium‐ N ‐phenoxide betaine dyes with iodomethane: Substituent, solvent and temperature dependence
Author(s) -
Linert Wolfgang,
Strauss Bernhard,
Herlinger Erwin,
Reichardt Christian
Publication year - 1992
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610050509
Subject(s) - chemistry , substituent , reaction rate constant , arrhenius equation , betaine , solvent , photochemistry , alkylation , pyridinium , solvent effects , hammett equation , activation energy , medicinal chemistry , organic chemistry , kinetics , physics , quantum mechanics , catalysis
The rate constants of the pseudo‐first‐order methylation reaction of seven substituted pyridinium‐ N ‐phenoxide betaine dyes by iodomethane, were determined by UV–visible spectrophotometry in nine different solvents. The influence of substituents on the position of the long‐wavelength, intramolecular charge‐transfer absorption band of the unsubstituted betaine dye can be described by a modified Hammett equation. The solvent‐dependent absorption constant ρ A correlates satisfactorily with the acceptor properties of the nine solvents used. The substituent‐dependent alkylation rate constants (In k ) correlate linearly with the Hammett substituent constants, whereas the solventdependence of in k follows a more complicated pattern. Temperature‐dependent measurements of the rate of alkylation exhibit an isoenthalpic behaviour for all the betaines in all the solvents investigated, i.e. the Arrhenius activation energy is almost constant for all differently substituted betaines in a given solvent. Temperature‐dependent measurements of the alkylation rate of the unsubstituted betaine in different solvents led to an isosolvent relationship, i.e. the corresponding Arrhenius plots show a common point of intersection. According to the theory of isokinetic relationships, this isosolvent behaviour is interpreted in terms of a resonant energy exchange between the reacting species and the surrounding heat‐bath system, which in this particular case seems to be the betaine molecule itself.