- the mechanism in Sn2 is concerted
it’s mean that all happen at the same time this called a single step (one step)
• back side attack (substrate effects)
CH312
methly halides are the most reactive and fast
becose the bulkiness is small and the Nuc can squeeze in to do the back side attack
 - back side attack (nucleophile effects)
we need here a strong nucleophile that have negative charge ( anions )
example : CN - Cl - I
 - back side attack (solvents effects)
the difference between protic / aprotic
Aprotic solvents are solvents that cannot display (hydrogen bonding ) 
to run protic solvents we prefer (SN1-E1)