In esterification RCOOH reacts with ROH in the presence of an acid giving H+. Why does H+ react with RCOOH when it is a better acid than ROH? Shouldnt H+ react with the better base??

Answer 1

That RCOOH is a stronger acid than ROH means that RO⁻, not ROH, is a stronger base than RCOO⁻,not RCOOH, (by a factor of about 10¹¹ based on acid pKa values).

However, it is true that ROH is a stronger base than RCOOH (by a factor of about 10⁴ based on the pka values of [ROH₂]⁺ and [RCOOH₂]⁺).

So the concentration of [ROH₂]⁺ will be greater than [RCOOH₂]⁺ by a factor of about 10⁴.

However [RCOOH₂]⁺ is more than a billion times more reactive (towards nucleophilic addition of ROH) than [ROH₂]⁺ (towards s_N2 reaction with RCOOH).

Since rate is proportional to concentration × reactivity, protonation of RCOOH leads to a much faster rate (by a factor of greater than 100,000) than protonation of ROH.

So the reason esterification of RCOOH proceeds through protonation of RCOOH (instead of ROH) is not because of basicity, but because of much greater reactivity of the electrophile formed by protonation of RCOOH.

Hope this helps.