Consider the reaction sequence given below:

Which of the following statements is true:

The two elementary steps hidden in the statement are of different molecularities.
Let us first recall the general rate‐laws.

Case 1: $$S_N1/E1$$ mechanism (unimolecular)
Rate-determining step is the slow, first-order ionisation of the alkyl halide to give a carbocation.
Hence $$\text{Rate} = k\,[\text{substrate}]$$ and is independent of the concentration or basic strength of the nucleophile / base present in solution.

Case 2: $$S_N2/E2$$ mechanism (bimolecular)
Here the nucleophile / base attacks in the rate-determining transition state.
Hence $$\text{Rate} = k\,[\text{substrate}]\,[\text{base}]$$ and it does depend on both the substrate and the base. A stronger base such as $$^{\ominus}OR$$ will generally accelerate the step compared with $$OH^{\ominus}$$.

Looking at the options we infer:

• Statement A — says changing $$OH^{\ominus}$$ to $$^{\ominus}OR$$ “will have no effect on reaction (2)”. If reaction (2) is bimolecular, its rate will certainly change with the strength of the base, so A is false.

• Statement B — says changing the concentration of base will have no effect on reaction (1). If reaction (1) proceeds by an $$S_N1/E1$$ path, the rate is independent of base concentration, so B is true.

• Statement C — says doubling base concentration doubles the rate of both reactions. This is wrong because reaction (1) is zero-order in base.

• Statement D — says changing base concentration has no effect on reaction (2). Because reaction (2) is bimolecular, this is again false.

Therefore the only correct statement is:

Option B which is: Changing the concentration of base will have no effect on reaction (1).