Leaving Group

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-N2+ diazonium salts
R-OR'2+ oxonium ions
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 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-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

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.

Other articles related to "leaving group, leaving groups, group":

More O'Ferrall-Jencks Plot - Applications - Elimination Reactions
... (C-H bond distance) and the vertical axes represent the extent of leaving group departure (C-LG distance) ... For example, the effects of introducing a better leaving group on a substrate that initially eliminates via an E2 mechanism are illustrated in Figure 2 ... A better leaving group increases the energy of the reactants and of the carbanion intermediate ...
Nucleophilic Acyl Substitution - Reactivity Trends
... towards nucleophilic substitution, since they possess no leaving group ... factor in determining the reactivity of acyl derivatives is leaving group ability, which is related to acidity ... Weak bases are better leaving groups than strong bases a species with a strong conjugate acid (e.g ...
Pyridines - Reactions - Nucleophilic Substitutions
... or sulfonic acid fragments which then become a leaving group ... So fluorine is the best leaving group for the substitution with organolithium compounds ... The hydride ion is generally a poor leaving group and occurs only in a few heterocyclic reactions ...
E1c B Elimination Reaction
... a sp3 - sp3 carbon to carbon covalent bond with an α-acidic hydrogen atom substituent and a β-leaving group ... This leaving group can be a halide or a sulfonic acid ester such as a tosyl group ... The electron pair then expels the leaving group and the double bond is formed ...

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