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Substituent Effects on the Acidity of Weak Acids. 1. Bicyclo[2.2.2]octane-1-carboxylic Acids and Bicyclo[1.1.1]pentane-1-carboxylic Acids
journal contribution
posted on 2002-02-05, 00:00 authored by Kenneth B. WibergThe acidities of 3- and 4-substituted bicyclooctane-1-carboxylic acids and 3-substituted bicyclo[1.1.1]pentane-1-carboxylic acids have been calculated at the MP2/6-311++G** theoretical level.
There is good agreement between the calculated and observed gas-phase acidities. The acidities of
the 4-substituted bicyclooctane acids were found to be linearly dependent on the C−X bond dipoles,
as expected from a field effect. The substituents had a negligible effect on the electron density at
C1. The difference in acidity between 4-chlorobicyclo[2.2.2]octane-1-carboxylic acid and the parent
acid (6.2 kcal/mol) is reproduced by the Kirkwood−Westheimer treatment of substituent effects on
acidity, but only if the bicyclooctane ring is given an effective dielectric constant of unity. The
acidities of the 3-substituted bicyclooctane acids are linearly related to the corresponding
4-substituted acids with a slope of 0.9. The acidities of the 3-substituted bicyclo[1.1.1]pentane-1-carboxylic acids are linearly related to the C−X bond dipoles for this ring system (which are different
than those for the bicyclooctanes), and they are also linearly related to the acidity of the 4-substituted
bicyclo[2.2.2]octanecarboxylic acids with a slope of 1.34. The larger slope is due to the smaller
bridgehead−bridgehead distance in the bicyclopentane ring than in bicyclo[2.2.2]octane.