Leaving groups and nucleophiles
The reactions below are examples of substitution reactions of organic halides.
In substitution reactions the parts of two reactants are exchanged.
alkoxide anions
+ CH3–Br 
CH3O–CH3(ether) + Br–
alkynide anions
HC≡C:–+CH3–Cl HC≡C–CH3(alkyne) + Cl–
cyanide ion:N≡C:– + CH3–I :N≡C–CH3(nitrile) + I–
Substitution at a saturated carbon by a nucleophile may occur if the carbon bears a good leaving group.
Nucleophiles that react by substitution have an atom with non-bonding electron pairs. One of these becomes bonding in the substitution product.
Experimental evidence has shown that both the mechanism for and the product of reaction of a nucleophile with a saturated halide depends on the nature of the nucleophile, the structure of the halide and the reaction conditions. The preparative chemist can use these reactions to best advantage only if the interplay between these factors is understood.
In substitution reactions the parts of two reactants are exchanged.
alkoxide anions
+ CH3–Br CH3O–CH3(ether) + Br–alkynide anions
HC≡C–CH3(alkyne) + Cl–cyanide ion
:N≡C–CH3(nitrile) + I–Substitution at a saturated carbon by a nucleophile may occur if the carbon bears a good leaving group.
The halide ions Cl–, Br– and I– are good leaving groups as these ions are not basic, due to being the conjugate base of the strong acids HCl and HBr.
Nucleophiles that react by substitution have an atom with non-bonding electron pairs. One of these becomes bonding in the substitution product.
Recall that basic nucleophiles can also react with halides to cause elimination.
Experimental evidence has shown that both the mechanism for and the product of reaction of a nucleophile with a saturated halide depends on the nature of the nucleophile, the structure of the halide and the reaction conditions. The preparative chemist can use these reactions to best advantage only if the interplay between these factors is understood.