[Babcock_Hall]

I have been looking into Hammett correlations of both reactions between two small molecules and enzymes with electrophilic inactivators.  Some involve inactivators bearing aromatic rings.  The values for σ_meta for the phenyl, 2-pyridyl, and 2-pyrimidinyl are 0.06, 0.33, and 0.23, respectively.  For additional comparison the values for 3-pyridyl and 4-pyridyl are 0.23 and 0.27, respectively.  These values are from a 1991 review article.

My question is why does 2-pyridyl have a more positive value of σ_meta than does the 2-pyrimidinyl group?  Relative to the phenyl group, if one nitrogen is electron-withdrawing, it seems as if 2-nitrogens should be more electron-withdrawing.

[pgk]

The electron-withdrawing effects of 1- and 3- nitrogen atoms of are competitive, as concluded by drawing the free electron pairs and arrows accompanied, conjugation forms of pyrimidine.