Rationalisations among Heterocyclic Partition Coefficients Part 1: The π‐Value of Phenyl

It is shown that πPh has a minimum theoretical value of ca. 1.7 in aromatic rings. This value is attained whenever phenyl is forced into a highly non‐planar conformation, as may happen through peri‐interactions or the heavy solvation of sterically hindering neighbouring groups. In the absence of such hindrance πPh will be greater, and can attain values as high as ca. 2.6 when there is no steric hindrance to coplanarity and two hetero‐atoms can be shielded. These and a number of intermediate situations are delineated; estimates of πPh are provided for each distinct case. The effect of phenyl substitution at sp3‐carbon is discussed in similar terms. It is shown that these variations make qualitative chemical sense and can be used for predictive purposes.