PROBLEM 7.49 - Organic Chemistry

Question

Question: Which of the following nucleophilic substitution reactions will take place?

a.

b.

Step-by-Step Solution

Answer

ANSWER

a.

b.

Leaving group ability

The leaving group is the one that readily takes up the electrons when a bond is broken. So, it is a weak Lewis base and should have a low pK_a values.

Explanation of (a)

a. In this reaction, the leaving group in the substrate molecule is NH₂^- . The nucleophile is an iodide ion. But NH₂^- is a poor leaving group and its conjugate acid NH₃ has a value pK_a of 35. So, this reaction is not feasible easily.

Explanation of (b)

b. In this reaction, the leaving group in the substrate molecule is I^-. The nucleophile is a methoxide ion. The iodide ion is one of the best leaving groups, and its conjugate acid has pK_a value of -9. So, this reaction is feasible easily.

Thus, the answer is option (b).

Nucleophilic substitution reaction