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Kinetic and equilibria studies in highly basic systems

University of British Columbia

Using α-cyanostilbenes as Lewis acids, Lewis acidity scales have been established in the systems DMSO-ethanol and DMSO-methanol containing the appropriate 0.01M sodium alkoxide and in the system sodium methoxide-methanol. This scale, designated HR-, describes the ability of the solvent to add an alkoxide ion to an alkene. The HR- values range from 11.73 in methanol to 21.74 in 93.27 mole % DMSO in ethanol. The most acidic indicator used to establish the scale was α-cyano-2,4-dinitrostilbene with a pK of 12.73 in DMSO-methanol. The least acidic was α-cyano-3-trifluoromethylstilbene with a pK of 21.98 in DMSO-ethanol. The effects of substituents in the two aromatic rings of α-cyanostilbenes on the Lewis acidity of these compounds has been measured. Using Ϭ values, an average ρ of 2.2 was obtained for the β -phenyl ring in the various solvent systems studied. Using Ϭˉ values, the ρ for α-cyanostilbenes with substituents in the α-ring is 4.77 in DMSO-ethanol and 4.24 in DMSO-methanol. In the α-phenyl ring the 4-nitro group exhibits a greatly exalted sigma value (1.75). In order to compare the HR- and H_ scales in one system, the H_ scale has been established in sodium methoxide-methanol using various carbon acids as indicators. The two scales are nearly parallel. The rates of the base catalyzed isomerization of α-eyano-cis-stilbenes were found to correlate with the HR- function. Plots of the logarithms of the apparent first order rate constants (k₁) for the isomerization against the HR- values gave excellent straight lines with slopes ranging from 0.426 to 0.665. The slopes of these lines depend on the substituent present in the α-phenyl ring and on the solvent system. The slopes were shown to be a measure of how closely the transition state for the isomerization reaction resembles the carbanion formed by the equilibrium addition of alkoxide ion to an alkene. The effect of substituents on the rate of isomerization of α-cyano-cis-stilbenes has been determined. Using Ϭ values the ρ for substituents in the α-phenyl ring is 3.07 in DMSO-ethanol and 2.82 in DMSO-methanol. The activation parameters for the base catalyzed isomerization reaction have been determined in DMSO-methanol. The enthalpies of activation range from 14.3 to 16.7 kcal. mole⁻¹ and the entropies of activation range from -9.3 to -13.7 e.u. depending on the substituent. The base catalyzed isomerization of α-cyano-cis-stilbenes is first order in base and first order in reactant. The mechanism of this reaction is discussed in terms of the kinetic-acidity function correlation, the substituent effects, and the activation parameters. The reactions of 1,1-bis-(4-nitrophenyl)ethene and 4,4'-dinitrobenzophenone with hydroxide or alkoxide ions in DMSO are described.

Text

2011-08-22

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http://hdl.handle.net/2429/36826

Chemistry

University of British Columbia

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