Umpolung

Polarity reversal is called in organic chemistry modification of a functional group with the goal of reversing their polarity. This also reverses its reactivity: from a shortage of electrons an electron surplus situation can be generated; it opens up new possibilities reaction. The Germanism umpolung is also established in the English-speaking.

Many organic reactions proceed via polar donor and acceptor centers, which negatively polarized atoms in the molecule form compounds as nucleophiles with positively polarized atoms (or electrophilic centers ).

Heteroatoms, such as oxygen or nitrogen in a carbon skeleton cause by their higher electronegativity higher electron density at the hetero atom and the adjacent carbon atom a positive polarization. On a carbon skeleton caused by alternating donor and acceptor centers, which lead to a normal response pattern in polar reactions.

The method of reversal is often used to produce a CC linkage. The principle here is to create a negatively polarized carbon atom, which can then attack a positively polarized carbon atom nucleophile.

Dieter Seebach and EJ Corey introduced the concept. Classic examples of a reversal are the Grignard reaction and the benzoin condensation.

Carbonyl polarity reversal

A well-known example is the dithiane Corey -Seebach method or reaction., For example, a carbonyl group of an aldehyde is converted into a thioacetal, the carbonyl carbon is prior deprotonation with, for example n-butyl lithium to the nucleophile reacts with haloalkanes, carbonyl or oxiranes in a nucleophilic substitution. After hydrolysis of the thioacetal is obtained, for example, α - alkyl ketones.

Reversal by forming a Grignard reagent

In the synthesis of the Grignard compounds from haloalkanes a polarity reversal occurs.