Leaving group

In this S_N2 reaction, bromide (Br^-) acts as the leaving group and hydroxide (^-OH) as the nucleophile.

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 or "tosylate" (TsO^-). Common neutral molecule leaving groups are water (H₂O), ammonia (NH₃), and alcohols (ROH)

The ability of a leaving group to depart is correlated with the pK_a of the conjugate acid, with lower pK_a 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 pK_a 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.

A list of leaving groups ordered approximately in decreasing ability to leave is as follows:^[1]

• R-N₂⁺ (Diazonium Salts)
• R-OR'₂⁺
• R-OSO₂C₄F₉ (Nonaflates)
• R-OSO₂CF₃ (Triflates)
• R-OSO₂F (Fluorosulfonates)
• R-OTs, R-OMs, etc. (Tosylates, Mesylates, and similar)
• R-I (Iodides)
• R-Br (Bromides)
• R-OH₂⁺ (Conjugate acid of an alcohol)
• R-Cl (Chlorides, and Acyl Chlorides 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-SR'₂⁺
• R-NR'₃⁺ (Tetraalkylammonium Salts)
• R-F (Fluorides)
• R-OCOR (Esters, and Acid anhydrides when attached to carbonyl carbon)
• R-NH₃⁺ (Ammonium Salts)
• R-OAr (Phenoxides)
• R-OH (Alcohols, and Carboxylic Acids when attached to carbonyl carbon)
• R-OR (Ethers, and Esters when attached to carbonyl carbon)

Uncommon Leaving Groups

• R-H
• R-NH₂ (Amides when attached to carbonyl carbon)
• R-Ar
• R-R'

See also

• Nucleophile
• Electrophile

References