Substituent effects: resonance and induction COOH COOH OH ?? ?? pKa 4.20 pa4.09
ID: 1037912 • Letter: S
Question
Substituent effects: resonance and induction COOH COOH OH ?? ?? pKa 4.20 pa4.09 pka 4.47 pK 10.00 p 9.28 pKa 9.81 (a) (b) When the methoxy group is in the para position in C, is the acid stronger or weaker than in A? Is the methoxy group (c) When the methoxy group is in the meta position in B, is the acid stronger or weaker than in A? Is the methoxy group in the meta position electron-donating or withdrawing? (d) (e) () in the para position electron-donating or withdrawing? How can this apparent contradiction be explained? Which effect is stronger for methoxy? When the fluoro group is in the meta position in E, is the acid stronger or weaker than in D? Is the fluoro group in the meta position electron-donating or withdrawing? When the fluoro group is in the para position in F, is the acid stronger or weaker than in D? Is the fluoro group in the para position electron-donating or withdrawing? Compare the results of the fluoro group with those of the above methoxy group. What must be different about the relative strength of the resonance and inductive effects for fluoro compared with methoxy?Explanation / Answer
A higher pKa value indicates lower dissociation constant or ionization constant thus implying a weaker acid. In benzoic acids, the effects of the substituents are such that if electron-withdrawing groups are present in the meta-position they stabilize the carboxylate ion by making the benzene ring electron-deficient while their presence in ortho or para positions relative to the acid group does not withdraw electron density thus not stabilizing the anion. Conversely, electron-donating groups in ortho and para positions pump electron density into the ring thus destabilizing the carboxylate and thus decreasing the acidity but these groups have less influence when in meta positions. In general comparison, an EWG increases acidity while and EDG lowers it. A similar effect of substituents is also observed in case of phenols where EWGs stabilize phenoxide anion.
a) B has lower pKa than A so it is a stronger acid. Due to resonance, the involvement of the methoxy oxygen's lone pairs is absent when in meta positions. Thus inductive effect of O plays and it appears electron-withdrawing though by resonance it is an EDG.
b) C has higher pKa than A and so is a weaker acid. Methoxy is an EDG in para position and so destabilizes the carboxylate anion thus lowering the acidity.
c) The apparent contradiction is explained through resonance structures of benzoic acids in meta and para substituted with alkoxy groups. In para positions, the group gives a partial positive charge on the carboxylic acid group thus destabilizing the formation of a carboxylate anion. However in meta position, this charge induction is absent. Instead, the electronegativity of the O atom w.r.t. C inductively withdraws electron density from the ring.
d) E is more acidic than D. F is a highly electronegative atom which in meta position similar to alkoxy groups withdraw electron density towards itself rather than contribute to the delocalization of a negative charge on the ring thus stabilizing a phenoxide. Instead, this inductive effect results in a positive character on the oxygen atom thus decreasing the acidity.
e) F is a stronger acid than D. In the para position also, F is electron-withdrawing. However, due to greater contribution of resonance effects, the parital positive charge on the oxygen atom in the phenol is absent. Thus F is relatively less acidic compared to E but still more acidic than D.
f) F is a weak EWG while methoxy is a strong EDG. So in F the resonance effect is weaker than induction as it is a highly electronegative atom while in methoxy, it is the opposite due to the invovlement of the oxygen's non-bonding electron-density in resonance thus making this effect stronger than the inductive effect relative to those of F.