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Weak bases and weak nucleophiles such as methanol promote a mixture of both ${\mathbf{\text{S}}}_{\mathbf{\text{N}}}\mathbf{\text{1}}$ and E1 reactions as observed in reaction a.

Strong bases and strong nucleophiles such as hydroxides and promote a mixture of both ${\mathbf{\text{S}}}_{\mathbf{\text{N}}}\mathbf{\text{2}}$and E2 reactions as observed in reaction b.

Tertiary halides undergo a ${\text{S}}_{\text{N}}\text{1}$ and E1 reaction respectively when a weak nucleophile and a weak base are used, such as methanol.

Secondary halides undergo a ${\text{S}}_{\text{N}}\text{1}$ and E1 reaction respectively when a weak nucleophile and a weak base are used. They undergo a ${\text{S}}_{\text{N}}\text{2}$ and E2 reaction, respectively, when a strong nucleophile and a strong base are used.

In both (a) and (b), the substrate used is a tertiary halide. The mode of the reactions changes due to the behavior of the base/nucleophiles.

The mechanism for the formation of the substitution and elimination products are shown below:

Representation of detailed reaction scheme of reaction a

Representation of detailed reaction scheme of reaction b