Question

For the given reaction, which of the following compounds is the best nucleophile?

• A. I,III
• B. Only II
• C. I and II
• D. Only I

Answer 1

The correct option is D Only I

For the given reaction, we will have to select the best nucleophile from the given compounds. Nucleophilicity is a kinetic phenomenon; so a better nucleophile will invariably yield better reaction rates. You are not expected to remember the data from this table but it helps compare the differences in basicities and nucleophilicities:

Between triethylamine and quinuclidine, the latter is 63 times faster! Both seem to be similar and even have nearly the same basicities (a nucleophile acts as a base when it removes or abstracts a proton). But why is there such a huge difference in their reaction rates? Quinuclidine is a better nucleophile because of steric factors. Owing to the bicycle (and in general a cyclic) structure, the alkyl groups are pulled back into the ring whereas the alkyl groups of triethylamine are more spread out. The pulled back nature of the quinuclidine makes the lone pair on the lone nitrogen atom more approachable/available than in the counterpart of triethylamine.
The other bicyclo nucleophile is better than triethylamine for the same reason – despite the fact that there is another electron withdrawing nitrogen in the same ring.