Preparation of Amines through SN2 Reactions

Suppose you wanted to make the following primary amine It might be tempting to suggest the following synthesis  

This is just an Sn2 reaction, followed by a deprotonation. This approach does work, BUT it is difficult to get the reaction to stop after monoalkylation. The product is also a nucleophile and it competes with NH3 for the alkyl hahde. In fact, it is an even better nucleophile than NH3, because the alkyl group is electron donating. Therefore, it wiU attack again  

The final product has four alkyl groups (which is referred to as quaternary), and the nitrogen has a positive charge (called an ammonium ion). So, we call this a quaternary ammonium salt. This structure does not have a lone pair, so it is neither nucleophilic nor basic. 

If our intention is to make a quaternary ammonium salt, then the synthesis above is a good approach. But what if we want to make a primary amine We cannot simply alkylate ammonia  

To get around this problem, we nse a clever trick. We use a starting amine that already has two dummy groups on it  

---