Notes

The SN2 reactions are bimolecular nucleophilic substitution, involving one step (concerted mechanism). Rate determining step involves both the nucleophile and substrate; hence, the rate of this reaction depends on both the concentration of the nucleophile and substrate. One step of SN2 reaction consists of the nucleophile attacking the substrate as the leaving group leaves the substrate.
The SN1 reactions are unimolecular nucleophilic substitution, involving two steps. Step one consists of slow loss of leaving group which generates a carbocation intermediate. Step two consists of fast attack of a nucleophile on the electrophilic carbocation. The rate of Sn1 reaction depends on the concentration of substrate.
The SN2 reactions tend to proceed with strong nucleophiles, such as CH3O(-), N3(-), CN(-), and so on (usually negatively charged nucleophiles). The SN1 reactions tend to proceed with weak nucleophiles, such as CH3CH2OH, H2O, and so on (usually neutral compounds).
The SN2 reactions tend to proceed in polar aprotic solvents, such as acetone.
The SN1 reactions tend to proceed in polar protic solvents, such as water.
The SN2 reactions tend to produce a single stereisomer (inversion of configuration), since the SN2 reactions proceed with a nucleophilic backside attack. By contrast, SN1 reactions tend to produce mixtures of stereisomers (a mixture of retention and inversion of configuration), since the SN2 reactions proceed through either a nucleophile backside attack or a nucleophile frontside attack. Steric hindrance may prevent the SN2 reactions from happening, since the SN2 reaction will proceed only when the empty orbital is available. This is the reason for different alkyl halides having different SN2 reaction rates; 1°alkyl halides (fastest) > 2°alkyl halides secondary >> 3°alkyl halides (slowest). Carbocation stability may prevent the SN2 reactions from happening, since the SN1 reaction's first step is loss of a leaving group to generate carbocation. As the carbon's substitutions number increases, carbocation stability increases. Therefore, the SN1 reaction rate proceeds as 3°alkyl halides (fastest) > 2°alkyl halides >> 1°alkyl halides (slowest). (1) cite the source !!!! add the pictures for sn1 and sn2 reactions!