Sulfur-stabilized anions
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص660-661
2025-07-01
610
The stabilization of anions by sulfides, sulfoxides, and sulfones is a theme that runs right through this chapter. Sulfur has six electrons in its outer shell. As a sulfide, therefore, the sulfur atom carries two lone pairs. In a sulfoxide, one of these lone pairs is used in a bond to an oxygen atom—sulfoxides can be represented in at least two alternative but equivalent ways. The sulfur atom in a sulfone uses both of its lone pairs in bonding to oxygen, and is usually represented with two S=O double bonds.
Treatment of any of these compounds with strong base produces an anion on what was the methyl group. How does the sulfur stabilize the anion? This question has been the subject of many debates and we have not got space to go into the details of all of them. There are at least two factors involved, and the fi rst is evident from this chart of pKa values for protons next to sulfone, sulfoxide, and sulfide functional groups.
Clearly, the oxygen atoms are important—the best anion stabilizer is the sulfone, followed by the sulfoxide and then the sulfide. You could compare deprotonation of a sulfone with deprotonation of a ketone to give an enolate. Enolates have a planar carbon atom and the anion is mainly on the oxygen atom. Sulfone-stabilized carbanions have two oxygen atoms and the anionic carbon atom is probably planar, with the negative charge in a p orbital aligned midway between the S=O bonds.
Yet the attached oxygen atoms cannot be the sole reason for the stability of anions next to sulfur because even the sulfide functional group also acidifies an adjacent proton quite significantly. There is some controversy over exactly why this should be, but the usual explanation is that polarization of the sulfur’s 3s and 3p electrons (which are more diffuse, and therefore more polarizable, than the 2s and 2p electrons of oxygen) contributes to the stabilization.
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