Sn1 is favoured when intemediate is a stable carbocation(tertiary),

Sn2 is favoured when reaction occurs through a transition state(no carbocation)

ther4 primary and secondary favours Sn2

 

 

generally, strong nucleophiles e.g OH- favors Sn2, and weak Nu e.g water  favours Sn1

one small correction when it is a stong base elimination takes place and there is less room for substitution

 

SN2 is preferred by substrates which are less sterically hindered in their transition state (preferred when B carbonyl group is there especially)

 

SN1 is preferred by those whose carbocations are highle stable!!!

thats all!!!

SN1 and SN2 are condition specific mechanisms. SN1 is a 2 step process. and SN2 is a one step process. the reason for this is, that the cpds. that undergo SN2 rxn have relatively unstable intermediates, moreover, there is no solvation of ions, hence to avoid the reverse rxn. the process is one stepped. SN1 rxn takes place when the solvent is polar-protic (i.e. it contains H+ ions and is capable of solvating ions and that for SN2 is polar- Aprotic. Also, there is a competition in Subst. and Elim. rxn. The former is favoured at lower temp. and the latter at higher temp.- Check the question for temp. conditions if given in the question)

Secondly the attacking species could be a strong/weak nucleophile. (note that when a base attacks a substrate, SN2 or E2 mech is followed) A weak base favours SN1 as its ions can easily be solvated in comparison to the stronger ones that MAY react with the solvent.

thirdly, the substrate matters. If the carbocation formed is relatively stable, then the mechanism followed is SN1, provided appropriate rxn conditions are maintained. If the cation formed is unstable, then SN2 or the respective elimination takes place.

The stability of the cation is scaled bt the nature of the grps attached to the cationic carbon. Ex: +I grps like alkyl grps will stabilise the carbocation and -I grps will destabilize it. Similarly check for Resonance factors, Hyperconjucation etc. in the correct priority order i.e. Resonance> Hyperconjugation> Inductive effect.

Hope this has helped you. Rate if satisfactory.