The reactions below are examples of substitution reactions of organic halides. In substitution reactions the parts of two reactants are exchanged.
These are examples of nucleophilic substitution reactions because both cyanide ion and hydroxide ion are nucleophiles.
Substitution at a saturated carbon by a nucleophile may occur if the carbon bears a good leaving group.
Experimental evidence has shown that the rate of the reaction depends on the structure of the halogenoalkane and the nature of the leaving group. These reactions can be used to best advantage only if the interplay between these factors is understood.
Halogenoalkanes react with hydroxide ion to give alcohols
OH and Br exchanged
Halogenoalkanes react with cyanide ion to give nitriles
I and CN exchanged
OH and Br exchanged
Halogenoalkanes react with cyanide ion to give nitriles
I and CN exchanged
These are examples of nucleophilic substitution reactions because both cyanide ion and hydroxide ion are nucleophiles.
Reactants that are nucleophiles have a non-bonding electron pair becomes bonding in the substitution product. Anions that have small negatively charged atoms are good nucleophiles (such as NC– with the negative C in and HO– with the negative O).
Substitution at a saturated carbon by a nucleophile may occur if the carbon bears a good leaving group.
The C-X bonding pair in the reactant becomes a non-bonding pair on the leaving group. The halide ions Cl–, Br– and I– are good leaving groups whereas OH– is a poor leaving group. The C-X bond is weaker for the halogen atoms because these atoms are bigger. This process also favours product ions where the negative charge is on a larger atom.
Experimental evidence has shown that the rate of the reaction depends on the structure of the halogenoalkane and the nature of the leaving group. These reactions can be used to best advantage only if the interplay between these factors is understood.