Question: Which of the following nucleophilic substitution reactions will take place?
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 pKa values.
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 pKa of 35. So, this reaction is not feasible easily.
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 pKa value of -9. So, this reaction is feasible easily.
Thus, the answer is option (b).
Question: Consider the following SN2 reaction.
Question: You will often return to nucleophilic substitution, in particular the SN2 reaction, in subsequent chapters and concentrate on the nucleophile rather than the alkyl halide. By using different nucleophiles, nucleophilic substitution allows the synthesis of a wide variety of organic compounds with many different functional groups. With this in mind, draw the products of each two-step sequence. (Hint: Step  in each part involves an acid-base reaction that removes the most acidic hydrogen from the starting material.)
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