Chemistry Alkyl Halides SZABMU 2024
PMDC Verified Question 5 of 13
Unimolecular nucleophilic substitution reaction involves ____.
A
1st order kinetics
B
3rd order kinetics
C
2nd order kinetics
D
zero order kinetics
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: 1st order kinetics
Concept:

The molecularity of a reaction corresponds to the number of species involved in the rate-determining step.

Solution:

  • "Unimolecular nucleophilic substitution" is the formal name for the \( S_N1 \) mechanism.
  • In this mechanism, the rate-determining (slowest) step is the spontaneous ionization of the alkyl halide to form a carbocation.
  • The nucleophile is not involved in this slow step.
  • Because the rate depends strictly on the concentration of only one reactant (the alkyl halide), the reaction follows 1st order kinetics.

    $$ \text{Rate} = k [\text{Alkyl Halide}]^1 $$


Why other options are incorrect:

  • Option C: 2nd order kinetics apply to bimolecular reactions (\( S_N2 \) and \( E_2 \)), where both the substrate and nucleophile/base are in the rate equation.

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