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Thermodynamic dissociation constants are reported for 4-substituted bicyclo[2.2.2]oct-2-ene-l -carboxylic acids, 4-substituted dibenzobicyclo[2.2.2]octa-2,5-diene-l-carboxylic acids, and cubanedicarboxylic acid in 50% (weight) ethanol-water at 25°. The experimental results indicate that variations in the hybridization of the carbon-carbon bonds between the substituent and the reaction site and a variation in the number of paths between the substituent and the reaction site have a negligible influence on the efficiency of propagation of the polar effect. These observations are compatible with the field effect model and incompatible with the inductive model for the propagation mechanism. The Kirkwood-Westheimer model yields reasonable estimates of the substituent effects. In addition, the origin of the polar effects of the methyl and trifluoromethyl groups are discussed.Two distinct models, the field effect and the inductive effect, have been considered in recent discussions of the mechanism of propagation of the polar effect.2-7 In brief, the inductive model presumes that substituent effects are propagated by the successive polarization of the bonds between the substituent and the reaction site. The net polar effect, according to this model, depends on the number and nature of the paths between the substituent and the reaction site. Substituent effects are transmitted through both the -bond network ( -inductive effect) and, when present, the -bond network ( -inductive effect).3•7 There is no general agreement concerning either the absolute or relative importance of these contributions to the observed polar effect. However, theoretical considerations require that substituent effects be transmitted more effectively through the more polarizable bonds than through bonds. The transmission efficiency is measured empirically by e, the transmission coefficient, and many values have been proposed for this parameter.3-8 Bowden's suggestions,4 e (sp3 carbon) = 0.48, e (sp2 carbon) = 0.67, e (aryl carbon) = 0.60, reflect the viewpoint that a large value is necessary for the accommodation of the experimental facts3-5 and that polar effects are propagated more effectively through bonds.3-5•7 The field effect model presumes that the polar effect originates in bond dipole moments and that the effect is propagated according to the classical laws of electrostatics.3-7 In the Kirkwood-Westheimer formulation of this theory,9-11 the molecule is treated as a cavity(1) Chemistry of the Bicyclo[2.2.2]octanes. V. This research was supported by a grant, GP4398, from the National Science Foundation.(2) The term, polar effect, is used to characterize the influence of unconjugated, sterically remote substituents on equilibrium or rate processes. This designation permits the use of the terms, inductive effect and field effect, for the description of the transmission mechanism.(3) M.

p-nitrobenzoyl chloride,29 2,3-di-O-methyl-D-mannose 1,4,6tri-p-nitrobenzoate was obtained: m.p. 192-194°, [ ]22 +66°( c 0.3, chloroform); lit.29 m.p. 194°, [a]n +65°(chloroform).(29) See ref. 12, p. 539.

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In vitro fermentation studies demonstrated that diaryliodonium chemicals were inhibitors of amino acid utilization. Only small differences were found in comparative effectiveness of different analogs and salt forms of chemicals tested. Protection against degradation and(or) uptake was greatest for valine, methionine, isoleucine, leucine and phenylalanine. Although changes were not as large, animals fed 4,4'-dimethyldiphenyliodonium chloride for 112 d had higher ruminal concentrations of amino acids and lower concentrations of ammonia. Decreased production of acetate in conjunction with increased production of propionate and butyrate resulted in improved fermentation efficiencies. Fermentative shifts in vivo were not as large as those observed in vitro and additional investigations are needed to determine whether feeding diaryliodonium chemicals increases the transfer of energy in digested carbohydrates to volatile fatty acids.