leavinggroup

A leaving group is an atom or group in a molecule that can depart with an electron pair during a chemical reaction, typically in substitution or elimination steps. Its departure allows the remaining part of the molecule to form a new bond or intermediate. In mechanism terms, the leaving group’s ability influences the rate and pathway of the reaction, such as SN1, SN2, E1, or E2 processes.

Good leaving groups are those that form stable, weakly basic anions upon departure. Common examples include halides such as iodide, bromide, and chloride; and neutral leaving groups formed by protonation or activation, such as water. Highly effective leaving groups include sulfonate esters like tosylate (OTs−), mesylate (OMs−), and triflate (OTf−), which are widely used to convert poor leaving groups (like alcohols) into excellent ones.

Poor leaving groups include hydroxide (OH−), alkoxide (RO−), and fluoride (F−) under typical conditions, because their conjugate bases are relatively basic and poorly stabilized. Leaving-group ability can often be enhanced by converting the substrate to a better leaving group (for example, turning an alcohol into a tosylate or mesylate) or by protonation to form water.

Mechanistic guidelines: SN1 and E1 reactions favor substrates and conditions that stabilize leaving-group departure, often with polar protic solvents and good leaving groups. SN2 reactions favor strong nucleophiles and substrates with excellent leaving groups in polar aprotic solvents, where concerted bond breaking and forming occur.

In practice, the choice or transformation of a leaving group is a key consideration in planning organic synthesis and reaction conditions.