Which of the following statements best describes the relative rates of SN2 and E

Question

Which of the following statements best describes the relative rates of SN2 and E2 reactions?

A. The rates are equal because they are both bimolecular.

B. The relative rates of each depends only of the steric hinderance of the substrate and the nucleophile.

C. The relative rates of each depends on only the steric hinderance of the substrate and the nucleophile's ability to reach the antibonding orbital.

D. The relative rates are described by the probability of the nucleophle acting as a Lewis base relative to the probability of acting as a Bronsted-Lowry base.

E. No direct comparison can be made.

The answer key says D, but I don't understand why.

Explanation / Answer

Yes the answer is D. The relative rates are described by the probability of the nucleophle acting as a Lewis base relative to the probability of acting as a Bronsted-Lowry base.

We know rate of SN2 depends on both substrate and nucleophile. It is not always we use Bronsted-Lowry base such as OH- but we also use lewis bases like H2O, smaller electronegativity atoms. We know leaving group most of the time is a lewis acid (Cl-, Br-, CN- all are more electronegativity atoms, they pull bonding pair, accept pair of electrons are lewis acids) and nucleophile is electron rich (lewis base). As SN2 is bimolecular reaction LG & Nuc are in a pentacoordinate transition state. If attacking Nuc is stronger lewis base then the probability of rate of reaction rate will be higher.

The same is true for E2 reactions also as the strength of base (Nucleophile) is increased then rate of elimination also enhanced. here also the probability of attacking the lewis base (All bronsted-lowry bases are lewis bases but not vive versa) to abstract the proton adjacent to more electronegative group or atom decides rate of reaction.

Although bulkier bases will experience the steric hinderance, and leads to give less substituted alkene in case of E2 elimination reactions, but mainly strength of base is the key point to remove proton otherwise steric hinderance is meaningless. Suppose potassium tert-butoxide is bulkier yet it gives elimination product due to its strong lewis base characteristics.

Hope i have explained well to you, still doubts! please feel free to ask me by commenting below.

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