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Substitution reaction is a nucleophilic reaction where the Nucleophilic attack and bond breaking between carbon and electronegative group happens either in a step manner or simultaneously.

Example: SN¹ and SN² reaction.

Mechanism of SN² reaction is given above.

Elimination reaction is basically a reaction between alkyl bromide and a base in a step-by-step manner or in a concerted mechanism of bond breaking and making.

Example: E1, E2 E1cB mechanism.

Mechanism of E1 reaction is given above.

In substitution reaction, nucleophile attack the alkyl bromide but in case of elimination reaction, a base is used to undergo the reaction as it is better to abstract proton than nucleophile. So, the nucleophilic and basic character determines the reaction path.

Between CH₃O^- and CH₃S^-, CH₃O^-is more basic than CH₃S^- as O is more electronegative than S and as a matter of fact, in presence of CH₃O^-, elimination reaction is more favorable than substitution. Whereas in case of CH₃S^-, as Sulphur is more polarizable, it is more nucleophilic and favors the substitution reaction more. Thus, ratio of substitution to elimination reaction increases in presence of CH₃S^-.