Leaving group

As the leaving group, Escaping group or breakaway. one in chemistry the split-off in a chemical reaction molecular region. This can both be individual atoms or ions, but most of all molecules and functional groups.

A nucleofuge is a leaving group, which drives the bonding electron pair, such as Cl -. A Elektrofug is a leaving group that does not drive the bonding electron pair, eg, H .

Quality of the leaving group

Whether a leaving group is cleaved or not depends largely on the stability of the cleaved molecule from ( nucleofugality ). Hydroiodic acid (HI), for example, a stronger acid than hydrochloric acid (HCl ); Therefore, the iodide ion will normally be replaced in a substitution reaction of chloride ions.

Such assessments must always also consider effects such as solvation into account. In the Finkelstein reaction, for example, the fact is exploited that sodium iodide ( NaI ) dissolved in acetone, sodium chloride ( NaCl) but is insoluble to answer the usual reactivities chloride to be replaced by iodide.

Example reactions

The term is used in particular substitution reactions or eliminations:

Substitutions

For substitutions, the leaving group is replaced by another functional group. An example is the Williamson ether synthesis:

Eg

The alcoholate is engaged with the positively polarized carbon of the methyl group, iodide exits as a leaving group from the molecule to form the salt is NaI.

Eliminations

In the elimination of a distinction between two cases:

Formation of an unsaturated compound

The following is a further elimination to form a multiple bond:

Eg

Under certain conditions, splits from bromoethane and hydrogen bromide is to ethene. To see elimination reaction mechanism.

Addition-elimination mechanism

The exit of the leaving group follows a previous addition; this reaction plays especially in the carbonyl an important role:

Eg

From carboxylic acid and alkoxide formed by elimination of hydroxide is an ester.