SN2 reaction
Leaving group. The leaving group affects the rate of reaction. The more stable leaving group is, the more likely that it will take the two electrons of its carbon-leaving group bond with it when the nucleophile attacks the carbon. Therefore, the weaker the leaving group is as a conjugate base, the better the leaving group. Similarly,  the stronger its corresponding acid, the better the leaving group. Examples of good leaving groups are the halides (except fluoride) and tosylate. But HO- and H2N- are not good leaving groups.