Subject: Hammett ortho substituent constants?

From: "Hr. Dr. S. Shapiro" <[email-masked]>
Date: Thu, 30 Jan 1997 13:40:21 +0100

Dear Colleagues;

        It is my understanding that Hammett sigma constants are restricted
to substituents at meta and para positions because steric influences on the
functionality at C-1 (e.g., COOH, OH) cannot be clearly distinguished from
electronic influences, whereas steric influences on the functionality at C-1
by substituents in the meta or para positions is negligible.

        Has anyone ever undertaken (and pubished) a detailed analysis of the
influence of ortho substituents in an effort to factor the influence of
ortho substituents in to electronic and steric components?  I note that in
the 1982 book "pKa Prediction for Organic Acids and Bases" by D. D. Perrin,
B. Bempsey, and E. P. Serjeant Appendix A.5 is a small list of apparent
sigma ortho values.  Does anyone know of a more recent and/or extensive
lists, particularly ones containing values factored into steric and
electronic components for benzoic acids and phenols?

Thanks in advance to all responders,

S. Shapiro
[email-masked]