In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as para-toluenesulfonate ("tosylate", TsO−). Common neutral molecule leaving groups are water (H2O), and ammonia.
The ability of a leaving group to depart is correlated with the pKa of the conjugate acid, with lower pKa being associated with better leaving group ability. The correlation is not perfect because leaving group ability is a kinetic phenomenon, relating to a reaction's rate, whereas pKa is a thermodynamic phenomenon, describing the position of an equilibrium. Nevertheless, it is a general rule that more highly stabilized anions act as better leaving groups. Consistent with this rule, strong bases such as alkoxide (RO−), hydroxide (HO−), and amide (R2N−) are poor leaving groups.
|Leaving groups ordered approximately in decreasing ability to leave |
|R-OTs, R-OMs, etc.||tosylates, mesylates, and similar|
|R-OH2+||(Conjugate acid of an alcohol)|
|R-Cl||chlorides, and acyl chloride when attached to carbonyl carbon|
|R-OHR'+||Conjugate acid of an ether|
|R-ONO2, R-OPO(OH)2||nitrates, phosphates, and other inorganic esters|
|R-OCOR||esters, and acid anhydrides when attached to carbonyl carbon|
|R-OH||alcohols, and carboxylic acids when attached to carbonyl carbon|
|R-OR||ethers, and esters when attached to carbonyl carbon|
It is uncommon for groups such as H- (hydrides), R3C- (alkyl anions, R=alkyl or H), or R2N- (amides, R=alkyl or H) to depart with a pair of electrons because of the instability of these bases. However, the requirement for a good leaving group is relaxed in the case of E1cb mechanisms, such as the elimination step in the addition-elimination mechanism of nucleophilic acyl substitutions. Here, alkoxides and even amides can act as leaving groups due to the entropic favorability of having one molecule split into two.
See also 
- Smith, March. Advanced Organic Chemistry 6th ed. (501-502)