The Transmission of Polar Effects Polar Effects. Part II

The pK a* values for nine acyclic, mono‐ and bicyclic α‐, β‐ and γ‐(trimethyl)ammonio acids, 1(α‐γ), 2(α‐γ) and 3(α‐γ), respectively, have been measured in 50% aqueous ethanol. The comparison of these values with those for the nine homomorphous α‐, β‐ and γ‐acids, 4(α‐γ), 5(α‐γ) and 6(α‐γ), respectively, permits an evaluation of the steric effect of the (trimethyl)ammonio groups which is super‐imposed on their polar effect in the former acids. The pK a* values, corrected for the steric effect, correlate very well with the reciprocal distance between the positive nitrogen atom and the dissociable proton. This confirms our earlier conclusion that the strength of these acids is determined by the field effect of the (trimethyl)ammonio groups only, the inductive model failing to account for the results. The distinction between two different transmission mechanisms for the polar effect therefore becomes unnecessary, the inductive model appearing as an atomistic description of the field effect.