Addition of sodium bisulfite to aldehydes and ketones mechanism
Answers
Bisulfite addition products are formed from aldehydes but reaction with ketone is limited to methyl ketones and cyclic ketones upon treatment with sodium bisulfite
The reaction of carbonyl compounds with NaHSO3 proceeds in aqueous solution which involves initial attack on the carbonyl carbon by the nucleophilic bisulfite ion.
Even though the oxygens are negatively charged and are stronger bases than sulfur, the sulfur is a better nucleophile when behaving as a nucleophile.
The sulfur atom of bisulfite has a localized and “less solvated” unshared pair of electrons.
[Remember that in sulfite ion, the negative charge of oxygen is resonance-delocalized and also more hydrated due to small charge of oxygen atom.]
The following mechanism has been proposed for the addition reaction of sodium bisulfite to an aldehyde or ketone.
Bisulfite addition products are formed from aldehydes but reaction with ketone is limited to methyl ketones and cyclic ketones upon treatment with sodium bisulfite.
*All aldehydes form the hydroxy sulfonates, but not all ketones react. The reaction is sensitive to steric effects.
NOTE-The sulfite ion (SO32-) demonstrates the differences between bases and nucleophiles. The sulfite ion has a central sulfur with three oxygen atoms bonded to it. Each oxygen atom bears a single formal negative charge, and the sulfur bears a formal positive charge.
The sulfur is more nucleophilic than the oxygens thus sulfite ion reacts with MeI to produce methyl sulfonate, MeSO3Na.
