Treatment of thioacetal acid with a weak base generates the thioacetate ion -- a

Question

Treatment of thioacetal acid with a weak base generates the thioacetate ion -- a "sulfur analog" of a carboxylate. Just like carboxylates, thioacetate reacts with methyl halides irreversibly to give the corresponding thioesters. In principle, there are two possible products - alkylation could take place on oxygen or sulfur. Usually, alkylation is observed only on sulfur. Using a frontier molecular orbital argument, provide an explanation for the observation that thioacetate alkylates on sulfur rather than on oxygen.

Explanation / Answer

Alkylation takes place on Sulphur rather than on Oxygen because

1.The sulphur lone pairs are in higher energy level(3SP2 orbitals) than the oxygen lone pairs [energy level(2SP2 orbitals)and this increases the energy of the sulphur lone pairs compared to the oxygen lone pairs

The electronic configuration of Sulphur is 1S22S22P63S23P4

The electronic configuration of Oxygen is 1S22S22P4

2.Elctronegativity of oxygen(3.44) is more than sulphur(2.58) and this lowers the energy of the oxygen lone pairs compared to the sulfur lone pairs

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