Bulletin of the Korean Chemical Society

  • 제25권9호
  • /
  • Pages.1346-1350
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  • 2004
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  • 0253-2964(pISSN)
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  • 1229-5949(eISSN)
대한화학회 (Korean Chemical Society)
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Stoichiometric Solvent Effect on SN1 Solvolytic Reactivity Accounting for Phenomenon of Maximum Rates in Methanol-Nitromethane Mixtures

  • 발행 : 2004.09.20
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초록

Pseudo-first order rate constants $(k_{obs})$ are reported for the following solvolyses in approximately isodielectric mixtures: 3- and 4-methoxybenzyl chloride, bromo- and chlorodiphenylmethane, and 4-chloro-, 4,4'-dichloro and 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$25^{\circ}C.$ 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 $(k_{obs})/[MeOH]^2$ and [AP]/[MeOH], where [AP] refers to the molar concentration of aprotic cosolvent. The results are consistent with competing third order contributions to $k_{obs}$, $k_{MM}[MeOH]^2$ with hydrogen-bonded methanol as electrophile, and $k_{MAP}[MeOH][AP]$ with hydrogen-bonding disrupted by the aprotic solvent.

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