ChemTalk

What is a Leaving Group?

br leaving group

Core Concepts

In this tutorial, you will learn what a leaving group is, as well as how to identify a good one. In addition, you will learn a list of examples of leaving groups you may see in different organic chemistry reactions.

Topics Covered in Other Articles

Vocabulary

  • Anion- a negatively charged ion
  • Basicity- how a molecule acts as a base
  • Cation- a positively charged ion
  • Conjugate base- molecule formed when an acid loses its acidic hydrogen
  • Resonance- electrons moving to adjacent places in a molecule to create different charge locations

Leaving Group Definition

A leaving group is an atom or group of atoms that are able to break away from a molecule with a lone pair, breaking the bond between it and the molecule. It acts as the opposite of a nucleophile; instead of donating electrons to form a covalent bond, it breaks that bond and takes the electrons as it leaves. Because of this, the main molecule is usually left missing electrons, becoming a cation.

Leaving groups are typically anions or neutral, but they can be cations too.

What makes a good leaving group?

The more stable an atom or group of atoms can be on its own, the better of a leaving group it is.

Weak Base

A good leaving group is the conjugate base of a strong acid, a weak base. The more stable the conjugate base with the lone pair is, the weaker the base will be. pKa can be used as a measurement of stable a lone pair of electrons can be. The lower the pKa, the stronger the acid and the weaker the conjugate base. Read more here.

leaving group example

Size vs. Basicity

As we move down and to the right of the periodic table, size increases. Size and basicity have an inverse relationship, meaning as size increases, basicity decreases. Therefore, the bigger the size of the leaving group, the weaker it is as a base and the more likely it is to break away from the molecule.

Resonance

Resonance plays a great role in the stability of a molecule; it is when charges can be relocated or neutralized by the movement of electrons within the molecule. Weak bases usually have resonance structures that are stable enough to stand alone. So, if a leaving group can be resonance stabilized, it is more likely to easily break away from the main molecule.

Leaving Group Examples

  • Halides- Br–, I–, Cl–
  • Sulfonates- OTs, OMs
  • Ammonia- NH3
  • Water- H2O

Further Reading