Electronic Effects of Substituents in Polar Reactions

Solvent effects are useful in differentiating polar from non-polar reactions, but give no direct information about the location or polarity 

A convenient system for studying substituent effects is the equilibrium between meta- and / ara-substituted benzoic acids and their corresponding anions (reaction 3.5). The acids are straightforward to synthesize, and the acidities in water at 25 °C are readily determined the pH of a solution containing equal molar quantities of the acid and its fully ionized sodium salt will be equal to — log A, where KA is the dissociation constant of the acid. Ortho substituents are not considered because of complications caused by steric effects. 

We define a substituent constant a (Greek sigma = s for substituent) for any particular meta or para substituent as log (Ka/Kh), where KA is the dissociation constant for the substituted benzoic acid and KH is the dissociation constant for benzoic acid itself (equation 3.7). r values for some common substituents are given in Table 3.2. 

Resonance effects are frequently opposed to the inductive effect. The methoxy group is a striking example. The oxygen atom will withdraw electrons inductively but release them from its lone pair by resonance. The inductive effect predominates at the meta position, but the resonance effect predominates at the para position. Why is this  

Inductive effects arise from the differing electronegativities of different atoms in the molecule. They operate through bonds or space and fail off rapidly with distance. Resonance effects, arising from interactions between non-bonding orbitals or ir-bonds with a n-system. can be transmitted over long distances. 

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