Structure and basicity. Part VII. Cyclotriphosphazatrienes containing amino-, chloro-, and phenyl-groups. New data: substituent constants for amino-groups and their application to the ‘saturation effect’ and to structure determination; pK′a,2 and structure

Abstract

The basicities of certain cyclotriphosphazatrienes containing amino-, chloro-, and phenyl-groups in various combinations have been measured in nitrobenzene. The data are used in conjunction with those of earlier Parts of the Series to calculate substituent constants α_R and γ_R for the groups R = NH₂, NHMe, NHEt, NHPrⁿ, NHPrⁱ, NHBuⁿ, NHBu^t, NHC₆H₁₁, NHCH₂Ph, NHPh, NH·C₆H₄Me-p, NH·NHPh, NMe₂, NEt₂, and piperidino. Among aliphatic amino-groups, the primary are, on average, more base-strengthening, both in alpha and in gamma positions, than the secondary; reasons for this are discussed. The substituent constants are used to calculate ‘corrected’ pK′_a,1 values for the fully aminolysed derivatives N₃P₃R₆, where R is aliphatic. If R is secondary the ‘corrected’ values are close to those observed; but, if R is primary, the ‘corrected’ exceed the observed values by ca. 2 pK′ units, confirming that a ‘saturation effect’ limits the observed pK′_a,1 values in the latter case, as predicted earlier in the Series. The pK′_a,2 values of the amino- and aminophenyl-derivatives are shown to be structurally significant. The value of substituent constants in the determination of structure from basicity measurements is demonstrated.