Substitution reactions, nucleophiles and leaving groups
The reactions below are examples of nucleophilic substitution reactions.
In substitution reactions the parts of two reagents are exchanged.
*This reaction requires a second mole of ammonia to react with the HCl released.
Substitution at a saturated carbon by a nucleophile may occur if the carbon bears a good leaving group.
Good leaving groups are stable anions or small molecules.
Better leaving groups are weaker bases (better able to bear negative charge).
The halide ions Cl–, Br– and I– are very weak bases due to being the conjugate of the strong acids HCl, HBr and HI.
The relative rates of reaction of the halides (Cl– < Br– < I– ) is due to the C-X bond strength being highest for chlorides.
The high C-F bond strength is the reason that fluoride is NOT a good leaving group.
Nucleophiles that react by substitution have an atom with non-bonding electron pairs. One of these is bonding in the substitution product.
In substitution reactions the parts of two reagents are exchanged.
| hydroxide anion |  + CH3–Br  HO–CH3(alcohol) +  |
| ammonia* | 2H3N: + CH3–Cl H2N–CH3(amine) +  + NH4+ |
| cyanide anion | :N≡C:– + CH3–I :N≡C–CH3(nitrile) +  |
Substitution at a saturated carbon by a nucleophile may occur if the carbon bears a good leaving group.
Good leaving groups are stable anions or small molecules.
Better leaving groups are weaker bases (better able to bear negative charge).
The halide ions Cl–, Br– and I– are very weak bases due to being the conjugate of the strong acids HCl, HBr and HI.
The relative rates of reaction of the halides (Cl– < Br– < I– ) is due to the C-X bond strength being highest for chlorides.
The high C-F bond strength is the reason that fluoride is NOT a good leaving group.
Nucleophiles that react by substitution have an atom with non-bonding electron pairs. One of these is bonding in the substitution product.