Umpolung

Umpolung is a phenomenon in which the polarity of an atom (usually the carbon) is reversed. This opens up reactions on a functional group which is otherwise not possible. Umpolung is a German word meaning reversed polarity.

Two common reactions that showcase umpolung are benzoin condensation and reactions involving carbonyls and dithianes.

Benzoin Condensation

Benzoin Condensation is a reaction between two aromatic aldehydes catalysed by alcoholic KCN. The reaction begins with the nucleophilic attack of the cyanide ion on the carbonyl group which is followed by a rearrangement of the intermediate formed leading to reversal of polarity at the carbonyl carbon. The carbonyl carbon which is otherwise positive becomes negative after the rearrangement.

The carbon atom which is now negatively charged acts as a nucleophile and atatcks another molecule of the aromatic aldehyde leading to the formation of the product after a couple of steps including the one that eliminates the cyanide ion.

Dithiane Reactions

Ethylene diol is generally used to protect carbonyl compounds from undergoing any unwanted reaction in certain reactions. If the sulphur anolog (ethane-1,2-dithiol) of ethylene diol is used in protecting the carbonyl group, it opens up an opportunity of reversing the polarity of the carbonyl carbon which is otherwise not possible with ethylene diol.

The carbonyl carbon can carrying a negative charge is favoured as the negative charge can gain stbility by resonating in the vacant orbitals of sulphur atom which is not possible with oxygen atom in its place.