Basicity of substituted 2-phenyl-1,1,3,3- tetramethylguanidines and other bases in acetonitrile solvent

Abstract

A series of 30 phenyl substituted derivatives of 2-phenyl-1,1,3,3-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, ρ = 2.18 ± 0.16, is significantly lower than ρ = 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 [Formula: see text] is slightly higher than the value previously observed for ammonia and aliphatic amines, [Formula: see text]. Additionally, the pKa values were measured for seven other cyclic bases, including 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN). Keywords: 2-phenyl-1,1,3,3-tetramethylguanidines, pKa measurements.