Posted by: Mark Foreman | February 2, 2013

Reversable attack on a carbonyl

Dear Reader,

Like many things in life carbonyl chemistry keeps on going, it is the gift which keeps on giving ! One bit of carbonyl chemistry I want to share with you is the chemistry of sodium hydrogen sulfite, this is a reagent which is very much maligned and misunderstood. If this reagent was a person I would say that he/she is something of a genius.

Now here is the reagent it is a simple anion made up of sulfur, oxygen and hydrogen atoms.

The hydrogen sulfite anion

The hydrogen sulfite anion

Now we have two places on the ion where it could act as a nucleophile, we have the oxygen atoms and the sulfur atom. For a set of electronic reasons (summed up in the HSAB theory) it tends to react using the lone pair on the sulfur. A classic use of this reagent is the purification of ketones and aldehydes, the hydrogen sulfite anion can react with ketones and aldehydes to form a hydroxy sulfonic acid salt. These hydroxy sulfonic acid salts have properties which are very different to ketones and aldehydes, they also tend to be crystalline and do not to dissolve in organic solvents. Below is shown the reaction, it is up to you to draw the curved arrows for the reaction.

Reaction of sodium hydrogen sulfite with cyclohexanone

Reaction of sodium hydrogen sulfite with cyclohexanone

The key thing about this reaction is that by treatment of the product with either acid or alkali it is possible to reverse the reaction to reform the carbonyl compound and form either sulfurous acid or sulfite dianions. I will let you work out the mechanisms for the backwards reactions yourselves.

The reaction can be extended to alpha,beta unsaturated carbonyl compounds such as ethyl 2-cyano-2-cyclohexylideneacetate, diethyl 2-propylidenemalonate and 2-(2-chlorobenzylidene)malononitrile (CS gas). All these compounds can react with sodium hydrogen sulfite to form addition compounds which on treatment with either acid or alkali reform the original compounds. Here is the random selection of alpha,beta unsaturated compounds which I choose, I have worked with one of the three (I will not say which) and I can tell you form personal experience that the reaction works very nicely as a means of purifiying the compound.

A selection of three randomly chosen alpha beta unsaturated compounds

A selection of three randomly chosen alpha beta unsaturated compounds

The reaction of the first with the sodium hydrogen sulfite forms an addition compound according to the following two stage mechanism. I have not drawn in the curved arrows, I will let you do that at home.

Reaction of sodium hydrogen sulfite with an alpha/beta unsaturated cyanoester

Reaction of sodium hydrogen sulfite with an alpha/beta unsaturated cyanoester

 

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