Both SN1 and SN2 are nucleophilic substitution reactions. However, there are two different types of nucleophilic substation reactions in organic chemistry.
The SN1 reaction is a stepwise process. SN2 is a concerted reaction. In SN1 reaction, the group that is being substituted leaves and in its place we have carbocation, which is then attacked by a nucleophile. In SN2, the substrate and the nucleophile are affected simultaneously. This is the rate determining step. The rate depends on the concentration of the nucleophile and the substrate.
The most striking difference between SN1 and SN2 is the big barrier. Understanding this key element will help you to have a better idea of the difference. In case of SN2, steric hindrance is the big barrier. The SN2 reaction takes a backside approach. The attack will only happen if the nucleophile can access the empty orbital. If there are many groups or substrates in the vicinity and they are not readily leaving thus making way for the nucleophile, then the reaction will be slower. The rate of the reaction will be very slow with too many groups and will be the fastest if there is one group or just an empty orbital.
SN1 and SN2 Reactions
In case of SN1, carbocation stability is the big barrier. SN1 starts when a group leaves and hence facilitates carbocation. The stability of this carbocation will determine the rate of the SN1 reaction. The more stable the carbocation, the faster the reaction. Carbocation of course becomes more stable as there is increasing or faster substitution of carbon.
SN1 reactions are slower to begin with and then become faster as carbocation stability increases. SN2 reactions start at a fast rate and as steric hindrance increases the reaction slows down. Hence, the primary, secondary tertiary state of SN1 reactions are lowest, slow and fastest and in case of SN2 reactions are fastest, slow and slowest.
Both SN1 and SN2 occur in alkyl halides, tosylate and mesylates among other related compounds. The leaving group is typically detached from an alkyne or alkene. SN1 happens in compounds that have relatively weaker nucleophiles, like H2O, CH3CH2OH and CH3OH. SN2 happens in compounds that have relatively stronger nucleophiles, like CH3O(-), N3(-), CN(-), HO(-) and RS(-) among others. SN1 requires polar protic solvents like water, carboxylic acids and alcohols. SN2 requires polar aprotic solvents. In other words, SN2 works better with solvents like acetone, acetonitrile and DMSO. SN1 has a mix of retention and inversion while SN2 has an inversion of stereochemistry.