To determine if a reaction is sn 1 or sn2 ,these simple rules will help you:
1. Primary substrates undergo SN2 reactions because
- they form unstable primary carbocations and
- are unhindered to attack by the nucleophile
2. Tertiary substrates undergo SN1 reactions because they
- are too hindered to undergo backside attack by the nucleophile and
- can form stable tertiary carbocations.
3. Secondary substrates can undergo substitution by either SN1 or SN2mechanisms.
The mechanism will be
- the nucleophile has a negative charge (e.g. RO-,HO-,NH-2,SH-,CN-,N-3,RCOO-,X-) or
- the solvent is a polar aprotic solvent (DMF , DMSO, acetone) or
- the leaving group is the conjugate base of a weak acid (pKa<0). Examples are F- and RCOO-.