how to understand a mechanism of a reaction??

i know all the basic concepts but i dont understand that which reaction is SN1,SN2,     where there occurs a homolytic fission and where heterolytic one.

how to understand that where an electrophile or a nucleophile will attack.

 

i have read many books but just explain the definitions or the characteristics,some reactiond      but i am not getting how to write the mechanism on our own. 

SN1 reaction is a single step reaction which involves the formation of an unstable transition state which leads to the formation of something known as the intermediate. In SN2 the transition stste is itself stable and no intermediate is formed. You can tell whether it will undergo Sn1 or Sn2. The simplest way is to first assume that Sn2 will take place. Now if you recall the mechanism of this reaction you will see that the nucleophile attack from the side which is away from the halogen. If the attack is looking feasible then it is Sn2 else it is Sn1.

For Sn2 you must see that the attack of site is not hindered. If it is hindered then Sn1 would be followed. Also if there is stablalisation using certain factors such as resonance then also it is Sn2.