Question

503 50 Sp> arb) sps se 9. Explain why phenol is substantially more acidic than methanol, but benzoic acid is not much more acidic than acetic acid. Phenol is more acidic than methanol " Mars methanol benzolo OH

Answer 1

1) ОН -H Conjugate base, Phenoxide ion Stabilized by +R effect, 5 resonating structure H+ CH2OH- CH, 8 Conjugate base, not stabilized by resonance effect, rather distabilized by +l effect of Me group Acidity depends on H+ dissociation and the stability of conjugate base. So, Phenoxide ion is more stabilized than -CH2O ion through resonance. Thus, phenol is more acidic than methanol. HO_0 000 000 0000 0000 0000 000 Y -H+ Resonance structure of conjugate base of benzoic acid, unfavourable for dianionic charge но обно оө но о ноо CE More favourable resonance structure than its conjugate base part Но, о . Sp2: HO H-C-0-OH но H-C-C-OH HO H-C-C-00+ HOO H-C-C=0 Sp3 Conjugate base of acetic acid, destabilized by +l effect of methyl group. 1) Benzoic acid has an Sp2 hybridized carbon compared to sp3 hybridized carbon of the methyl group of acetic acid. The more s-character of the hybrid orbital, more the electronegetivity, which increases more acidicty of benzoic acid than acetic acid. 2) Acetic acid has electron donating(+1 effect) methyl group, which distabilize conjugat base. 3) In benzoic acid benzene can give electron through +R effect (shown with resonance structure). Which can give electron on acid counter part as well as in conjugate base. In conjugate base it will form dianionic species which is unfavourable. But in acid part the resonance structure is favourable and thus decreases the amount of conjugate base. Due to this kind of distability of conjugate base, acidity decreases Now, acoording to point 1 benzoic acid has more acidity than acetic acid, but due to point 3 it decreases the acidity of benzoic acid in some extent. So benzoic acid is not much more acidic than acetic acid.