A series of 30 phenyl substituted derivatives of 2-phenyl-l,l,3,3-tetramethylguanidine was synthesized and the pKa value of each was measured in acetonitrile using a conventional, general purpose glass electrode. The slope of the Hammett relationship, for the para and meta substituted compounds, p = 2.18 + 0.16, is significantly lower than p = 2.85 * 0.06 found for anilines in water, even though the phenyl ring is attached directly to the basic centre in both cases. The influence of electronic and steric effects on the basicity of the compounds is discussed. The solvent effect on the basicity is also discussed, by comparison of the pKa values in acetonitrile and water. The mean value of Ap~tN-H20 = 8.1 + 0.5 is slightly higher than the value previously observed for ammonia and aliphatic amines, A p~p -H 2 0 = 7.5 * 0. On a synthCtisC une skrie de 30 phtnyl-2 tttramCthyl-1,1,3,3 guanidines substitutes sur le noyau phCnyle et, utilisant une electrode de verre conventionnelle d'usage general, on a mesure les valeurs de leur pK, dans llacCtonitrile. Lapente de la relation de Harnmett, pour les composCs substituks en mPta et en para, p = 2,18 0,16, est beaucoup plus faible que la valeur de p = 2,85 2 0,06 qui a kt6 observee pour les anilines dans l'eau, m&me si le noyau phtnyle est attache directement au centre basique dans chacun des cas. On discute de I'influence des effets sttriques et electroniques sur la basicit6 des composCs. En se basant sur une comparaison des valeurs des pKa dans l'acttonitrile et dans l'eau, on discute aussi de I'effet du solvant sur la basicit&. La

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The second-order rate constants for the proton and deuteron transfer reactions between 4-nitrophenylnitromethane and the organic bases triethylamine and tri-n-butylaniine have been determined at eight different temperatures between 0 and 35"C, using the stopped-flow technique. The four reactions are characterized by low enthalpies of activation and large negative entropies of activation. The isotope effects give values for (AD: -AH$) of 1.02 and 0.74 kcal mol-' and (AS," -AS;) of 1.2 and 0.9 cal mol-l K-l for triethylamine and tri-n-butylamine respectively. The rate ratios are small for primary deuterium effects ; k ~/ k ~ for Et3N at 0°C is 3.56 and for n-Bu3N at 0°C is 2.41. These reaction constants are used to elucidate the type of energy barrier involved in the reaction and the nature of the transition state.The reaction of 4-nitrophenylnitromethane with the strong organic bases triethylamine and tri-n-butylamine in acetonitrile gives a reddish-brown solution. Proton magnetic resonance and visible-spectroscopic measurements indicate that this reaction is a reversible proton transfer whose product is a coloured ion-pair : 1-NO2C6H4CH,No2 + R3N=[NO2C6H4CHNO,]-HNR3.

The kinetic deuterium isotope effect for the proton transfer reaction between 4-nitrophenylphenylcyanomethane and tetramethylguanidine, kH/kD, at 25 °C, is 10.7 in acetonitrile solvent and 12.5 in benzonitrile. The enthalpy of activation difference, [Formula: see text], in both solvents is large, 2.2 kcal mol−1, and 2.4 kcal mol−1; respectively, indicating a significant contribution from proton tunnelling to the reaction. The equilibrium constants, K, measured spectrophotometrically are large, 6000 dm3 mol−1 in acetonitrile and 2000 dm3 mol−1 in benzonitrile and in good agreement with the values calculated from the kinetic measurements.

Equilibrium constants, rate constants, primary deuterium isotope effects, and activation parameters have been measured for the proton transfer reactions in acetonitrile solvent of 4-nitrophenylphenylcyanomethane and 2-methyl-4-nitrophenylphenyl-cyanomethane with tetramethylguanidine base and for the reactions of 2-methyl-4-nitrophenylphenylcyanomethane and 2,6-di-methyl-4-nitrophenylphenylcyanomethane with 1,5-diazabicyclo[5.4.0]undec-7-ene base. Introduction of the ortho-methyl groups in the substrate molecule caused significant reductions in the equilibrium and rate constants. The expected rise in the kinetic primary deuterium isotope effect was not observed when the first ortho-methyl group was introduced, but a 20% increase did accompany the introduction of the second ortho-methyl group. Enthalpy of activation measurements indicated that there was no increase in the proton tunnelling contribution to the isotope effect when the amount of steric hindrance is increased with ortho-methyl groups.