R3.3 Electron sharing reactions

Comparing experiments 1 and 2: doubling $[{\text{CH}}_3({\text{CH}}_2{)}_3\text{Br}]$ while keeping $[{\text{OH}}^{-}]$ constant doubles the rate. So the reaction is first-order in 1-bromobutane.

Comparing experiments 1 and 3: doubling $[{\text{OH}}^{-}]$ while keeping $[{\text{CH}}_3({\text{CH}}_2{)}_3\text{Br}]$ constant also doubles the rate. So the reaction is first-order in ${\text{OH}}^{-}$.

Overall order = 1 + 1 = second-order. Since the rate depends on both the substrate and the nucleophile, the rate-determining step involves both species colliding simultaneously — this is characteristic of an ${\text{S}}_{\text{N}}2$ mechanism.

The statement that the rate depends on both reactant concentrations and points to a bimolecular rate-determining step (${\text{S}}_{\text{N}}2$) is correct.

The question asks which conclusion is supported by the kinetic data — specifically what the rate dependence tells us about the reaction mechanism.

The option stating '${\text{S}}_{\text{N}}1$ because first-order overall' is wrong — the reaction is second-order overall (first-order in each reactant), not first-order overall.

The option suggesting the rate is independent of nucleophile concentration describes ${\text{S}}_{\text{N}}1$ kinetics, but the data shows the rate does depend on $[{\text{OH}}^{-}]$ — eliminate this option.

The option claiming zero-order in 1-bromobutane contradicts the data directly — doubling $[{\text{CH}}_3({\text{CH}}_2{)}_3\text{Br}]$ doubles the rate, showing clear first-order dependence. Eliminate this.

The remaining option — that the rate depends on both reactant concentrations indicating a bimolecular (${\text{S}}_{\text{N}}2$) mechanism — is consistent with the data.

A nucleophile is a species that attacks electron-deficient centres. By the Lewis base definition, it is an electron pair donor — it uses a lone pair to form a new bond with an electron-poor atom.

A Lewis base donates an electron pair. A nucleophile behaves as a Lewis base by donating its lone pair to form a coordinate (dative) bond with a substrate, typically at a carbon atom bearing a leaving group.

The option 'a species that donates an electron pair to form a new covalent bond' directly matches the Lewis base / nucleophile definition.

The question asks for the correct Lewis base definition of a nucleophile — focus on the electron-donating nature of nucleophiles.

'A species that accepts an electron pair' describes a Lewis acid (or electrophile), not a nucleophile. Eliminate this.

A positively charged species attracted to electron density would be an electrophile, not a nucleophile. Nucleophiles are typically negatively charged or neutral with lone pairs. Eliminate this.

Having a large dipole moment alone does not define a nucleophile. A molecule could be polar without having available lone pairs to donate. This option is too vague and incorrect. Eliminate it.

'A species that donates an electron pair to form a new covalent bond' is the correct Lewis base definition of a nucleophile.