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PROBLEM 7.78

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Organic Chemistry
Found in: Page 295

Short Answer

Question: Explain why quinuclidine is a much more reactive nucleophile than triethylamine, even though both compounds have N atoms surrounded by three R groups.

Answer

The trimethylamine has more steric crowding. Therefore, it is less reactive.

The quinuclidine has a cyclic surrounding which stabilizes the electrons. Therefore, it is more reactive.

See the step by step solution

Step by Step Solution

Nucleophilicity

A nucleophilicity is the tendency of a Lewis base to donate the lone pair of electrons.

The stronger the Lewis base, the greater the nucleophilicity will be.

Factors affecting the nucleophilicity

The factors affecting the nucleophilicity are as follows:

  • Electronegativity: The electronegative atom is not more able to donate electron pairs due to the more attraction of electrons towards the nucleus.

  • Steric crowding: The steric crowding around the atom containing lone pair of electrons does not get the electron pairs free for donation.

Explanation

In the trimethylamine, the three ethyl groups repel the electron pair on the nitrogen atom. Therefore, trimethylamine is less available for the donation of electrons. Hence, it is slow reactive.

The quinuclidine has free lone pair of electrons due to the cyclic alkyl group. The cyclic group stabilizes the electrons at N, and, therefore, more effectively, it can give the electrons to the other electrophilic group. Consequently, it is more reactive.

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