Leaving group

In chemistry, a leaving group is an atom or molecule that leaves another molecule during a reaction. This reaction can be for example a nucleophilic substitution. In this case the leaving group gets a pair of electrons. Some leaving groups are better than others. This is because some atoms or molecules like getting extra electrons. The halides are a good example of good leaving groups.

File:BromoethaneSN2reaction-small.png

In this S_N2 reaction, bromide (Br⁻) is the leaving group and hydroxide (OH⁻) is the nucleophile.

The type of leaving group can change the type of reaction that happens. For example, bad leaving groups make an SN1 reaction more difficult, while they make an SN2 reaction easier.

Leaving Group Media

• Leaving groups-3-types-of-reactions.png

  Common mechanistic contexts that involve the departure of a nucleofugal leaving group. The leaving group (LG) is shown in red. Top: SN2 reaction; middle/left: first step of SN1 and E1 reactions; middle/right: second step of E1cb, AAC2, and BAC2 reactions; bottom: E2 reaction.

• Ionization reaction wide.png

  In an ionization reaction, as in all reactions that involve leaving group departure, the leaving group bears a larger negative charge in the transition state and products than it does in the starting materials

• Propyl halide toluene thiolate reaction.png

  Propyl halide toluene thiolate reaction

• Ethyl halide ethoxide reaction.png

  Ethyl halide ethoxide reaction

• E1cB reaction.png

  Mechanism of a general E1cB reaction

• Elimination continuum.png

  Continuum from E1cB to E2 eliminations

• Leaving group activation by lewis acid complexation png.png

  Leaving group activation by lewis acid complexation png

• Leaving group reductive elimination png.png

  The ability of hyper leaving groups is enhanced by entropic factors