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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 25, Number 9
BKCSDE 25(9)
September 20, 2004 

 
Title
Stoichiometric Solvent Effect on SN1 Solvolytic Reactivity Accounting for Phenomenon of Maximum Rates in Methanol-Nitromethane Mixtures
Author
Zoon Ha Ryu*, Su-Han Choi, Gui Taek Lim, Dea Dong Sung, T. William Bentley
Keywords
Solvolysis, Third order kinetic model, Isodielectric mixture, Solvent effect
Abstract
Pseudo-first order rate constants (kobs) are reported for the following solvolyses in approximately isodielectric mixtures: 3- and 4-methoxybenzyl chloride, bromo- and chlorodiphenylmethane, and 4-chloro-, 4,4'-dichloroand 4-methyl-chlorodiphenylmethane in 0-80% v/v nitromethane-methanol mixtures; and bromo- and chlorodiphenylmethane and 4-methyl-chlorodiphenylmethane in various acetonitrile-methanol mixtures (in the range 0-50% v/v) at 25 ºC. These data, and literature data for t-butyl halides (Cl, Br, and I), and for p-methoxybenzoyl chloride, show rate maxima in solvent compositions of ca. 30% aprotic cosolvent, explained by a stoichiometric solvent effect on electrophilic solvation. Linear relationships are observed between (kobs)/[MeOH]2 and [AP]/[MeOH], where [AP] refers to the molar concentration of aprotic cosolvent. The results are consistent with competing third order contributions to kobs, kMM[MeOH]2 with hydrogen-bonded methanol as electrophile, and kMAP[MeOH][AP] with hydrogen-bonding disrupted by the aprotic solvent.
Page
1346 - 1350
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