Dominating electronic effects Hammett equation

roberts , moreland studied reactivities of 4-substituted bicyclo[2.2.2]octane-1-carboxylic acids , esters. in such molecule, transmission of electrical effects of substituents through ring resonance not possible. hence, hints on role of π-electrons in transmission of substituent effects through aromatic systems.

reactivity of 4-substituted bicyclo[2.2.2]octane-1-carboxylic acids , esters measured in 3 different processes, each of had been used benzoic acid derivatives. plot of log(k) against log(ka) showed linear relationship. such linear relationships correspond linear free energy relationships, imply effect of substituents exerted through changes of potential energy , steric , entropy terms remain constant through series. linear relationship fit in hammett equation. 4-substituted bicyclo[2.2.2.]octane-1-carboxylic acid derivatives, substituent , reaction constants designated σ’ , ρ’.

comparison of ρ , ρ’

reactivity data indicate effects of substituent groups in determining reactivities of substituted benzoic , bicyclo[2.2.2.]-octane-1-carboxylic acids comparable. implies aromatic π-electrons not play dominant role in transmission of electrical effects of dipolar groups ionizable carboxyl group difference between ρ , ρ’ reactions of acids diphenylazomethane due inverse relation solvent dielectric constant d

comparison of σ , σ’

for meta-directing groups (electron withdrawing group or ewg), σmeta , σpara more positive σ’. (the superscript, c, in table denotes data hammett, 1940.) ortho-para directing groups (electron donating group or edg), σ’ more positive σmeta , σpara. difference between σpara , σ’ (σpara – σ’) greater between σmeta , σ’(σmeta − σ’). expected electron resonance effects felt more @ p-positions. (σ – σ’) values can taken reasonable measurement of resonance effects.