Question

Rank the relative nucleophilicity of the indicated species in water.

CH3COO-, CH3COOH, CH3O-, CH3OH, CH3S-

Answer 1

First consider the nucleophilicity of these species without water as solvent :

* conjugate bases are more nucleophilic (more basic) than their equivalent neutral species.

therefore, CH₃COO^- > CH₃COOH and CH₃O^- > CH₃OH

* methoxide is stronger nucleophile than carboxylate because carboxylate is resonance stabilized therefore the lone pair of oxygen is less available for donation (attack on electrophilic carbon center).

CH₃O^- > CH₃COO^-

And as you move down along a group in periodic table, nucleophilicity increases because the atom size increases and therefore more polarizable

CH₃S^- > CH₃O^-

so overall the nucleophilicity order is as follows :

CH₃COOH < CH₃OH < CH₃COO^- < CH₃CO^- < CH₃S^-

Now water is polar protic solvent and can form hydrogen bond with nucleophiles and alter the nucleophilicity strength dramatically.

Methanol and acetic acid forms hydrogen bonds with water and completely miscible in water.

water solvates the CH₃CO^- by forming hydrogen bonds and now the nucleophile is less available for nucleophilic attack. Oxygen forms stronger H-bonds than Sulfar.

therefore, in these case the nucleophilicity order doesn't change and is as follows :

CH₃COOH < CH₃OH < CH₃COO^- < CH₃CO^- < CH₃S^-