Question

Consider the following reaction and choose the correct statement(s). CH3CH 2Br + CH 30 Na* CH3CH2OCH 3 + NaBr 1. When the nucleophile is changed from CH30-to CH3CO2, the rate constant of the reaction does not change. II. When the leaving group is changed from Br to CI, the rate constant of the reaction decreases. III. When the solvent is changed from methanol to DMSO, the rate constant of the reaction decreases.

Answer 1

The given reaction is SN2 reaction, as primary carbon in ethylbromide is reacting to replace -Br group by -OCH3 group.

Note that SN2 reaction is concerted. This means it occurs in one step, and both the nucleophile (Nu) and substrate are involved in the rate determining step.

rate = k [substrate] [Nu] ; k is the proportionality constant

I) Stronger a nucleophile, greater is the rate of SN2 reaction.

CH3O- is methoxide ion , and CH3CO2- is acetate ion

Now, Because acetate ion has resonance (The negative charge on Oxygen is delocalized) and methoxide ion does not, acetate ion is a weaker base or nucleophile than methoxide ion.

Another way to understand is that stronger acids have weaker conjugate bases. And acetic acid is a stronger acid than methanol.

So, on replacing CH3O- by CH3CO2- would decrease the rate of reaction. So incorrect option.

II) Br- is a better leaving group than Cl-. Therefore on replacing Br by Cl would decrease the reaction rate. So correct option.

III) DMSO is a polar aprotic solvent, while methanol is a polar solvent. In DMSO, polar hydrogens are absent, so the nucleophile present does not participate in hydrogen bonding with the solvent and thus is more 'free' or become stronger. In other words its nucleophilicity increases.

Thus such polar aprotic solvents can be used to enhance the reactivity of the nucleophile and they increase the rate of SN2 reaction.

So, incorrect option.

Hence only option II is correct.

* Note that in the options rate constant (k) is the term used. We can interchangeably use rate (r) and rate constant for analyzing the problem.