The Chemical Bond
Functional Groups in Organic Chemistry
Acids and Bases
Nucleophiles and Electrophiles
Reactions of Alkyl Halides
|Why are alkyl halides ideal substrates for substitution or elimination? Fluorine, chlorine, and bromine are more electronegative than carbon (iodine only slightly so), so the carbon halogen bond of an alkyl halide is polarized with the electron pair more in the orbit of the halogen, so the alkyl halide is primed for heterolytic cleavage. Cleavage can occur in either under direct nucleophilic attack, a bimolecular process, or by means of a unimolecular process in which the leaving group departs spontaneously.|
However, even though the carbon-fluorine bond is very polar, fluoride ion is a poor leaving group. Do you know why? Fluoride ion is a poor leaving group for the same reason that hydrofluoric acid is only a weak acid. The fluoride ion is too small to stably support a full negative charge. The best halide leaving groups are the conjugate bases of the strongest acids. Chloride is a much better leaving group than fluoride.
Although halide is the quintessential leaving group in substitution or elimination reactions, other possible leaving groups are the amines of quaternary ammonium ions, the protonated hydroxyl of alkyloxonium ions, and the alkyl sulfonate anions of sulfonate esters.