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S_N1

The Substrate: Substrates which can form relatively stable carbocation intermediates favour S_N1 reactions. The order of stability of carbocations is: 3^o > 2^o > benzyl > allyl > 1^o.

The Nucleophile: The nucleophile is not involved in the rate-determining step in an S_N1 reaction but the S_N1 pathway is more likely to be followed if the nucleophile is poor, e.g. H₂O.

The Leaving Group: The leaving group is also involved in the rate-determining step for an S_N1 reaction, so the same reactivity order as for S_N2 is followed.

The Solvent: The solvent can have an effect on the rate of the S_N1 reaction, but for different reasons. Solvent effects arise from stabilisation of the transition state and not the reactants themselves. The rate of S_N1 reaction is increased in a polar solvent such as water or aqueous ethanol

S_N2

Steric Effects: The transition state in the S_N2 reaction involves partial bonding between the nucleophile and the substrate. The bulkier the substrate, the more difficult it is for the transition state to be reached. The reactivity order is 1^o> 2^o> 3^o.

The Nucleophile: By definition, a nucleophile must have an unshared pair of electrons, whether it is charged or neutral. Nucleophilicity follows approximately basicity, so pKa values can be used. Nucleophilicity usually increases going down a group in the periodic table. The reactivity order of the more common nucleophiles is: CN^-> I^-> MeO^-> HO^-> Cl^-> H₂O.

The Leaving Group: The leaving group is normally ejected with a negative charge. Therefore the best leaving groups are those which can best stabilise a negative charge. Weak bases (TsO^-, I^-, Br^-) are generally good leaving groups, whereas strong bases (F^-, HO^-, RO^-) are generally poor leaving groups.

The Solvent: Polar aprotic solvents are best for S_N2 reactions. These include acetonitrile (CH₃CN), dimethyl sulphoxide (Me₂SO) and N,N-dimethylformamide (Me₂NCHO). Protic solvents tend to form a 'cage' around the nucleophile, decreasing its reactivity.

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