Azides (N3‾) Definition:
Alkyl halides (or tosylates) will react with azide ions (such as NaN3 or KN3) in SN2 reaction to give alkyl azides.
Azides (N3‾) Explained:
Azide ion (N3‾) is the conjugate base of hydrazoic acid (HN3). It is a linear ion which can be described by several resonance structures.
The principal source of the azide moiety is sodium azide which can displace an appropriate leaving group (e.g. chloride, bromide) to give the azido compound in SN2 reaction. The azide anion performs the nucleophilic attack on the sp³ hybridized carbon of methyl chloride from the backside. In the transition state, azide ion and chloride are on the opposite side and they are partially bonded to the carbon which now is sp² hybridized. After the loss of the leaving group, carbon is again sp³ hybridized.
This reaction is best performed with primary alkyl halides in a polar aprotic solvent, and possibly with secondary but under certain conditions.