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

Question

Treatment of thioacetic 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 occurs on sulphur because the S lone pairs are higher in energy than the O lone paire. In particlar the relevant lone pairs on O and S are the sp2 lone pairs and the other lone pair is tied up by resonance. The S lone pairs are higher in energy for two reasons:

1. Oxygen is far more electronegative than sulphur, and this lowers the energy of the oxygen lone pairs relative to the sulphur one pairs.

2. The oxyen lone pairs in 2s2 orbitals, where as the sulphur lone pairs are in 3 sp2 orbitals. In general 3sp2 orbitals are higher in energy than 2sp2 orbitals and this also increases the energy of the sulphur lone pairs as compared to the oxygen lone pairs.

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