Question

Energy diagram of $S_{N}1$ and $S_{N}2$ reactions The order of hydrolysis of RX by $S{N}^1$ is $3^{\circ }>2^{\circ }>1^{\circ }$ RX and $S{N}^2$ path is $1^{\circ }>2^{\circ }>3^{\circ }$ RX.

Which of the following statements are correct for the above energy diagrams of $S_{N}1$ and $S_{N}2$ reactions?

• A. (I) in fig. (a) and (IV) in fig. (b) represent $S_{N}1$ reaction.
• B. Fig. (a) and fig. (b) are the energy diagrams for $3^{\circ }$ $R-X$ and $1^{\circ }$ $R-X$, respectively.
• C. Fig. (a) and fig. (b) are the energy diagrams for $1^{\circ }$ $R-X$ and $3^{\circ }$ $R-X$, respectively.
• D. (II) in fig. (a) and (III) in fig. (b) represent $S_{N}2$ reaction.

Answer 1

Answer: (A), (B), (C)

The correct options are
A

(I) in fig. (a) and (IV) in fig. (b) represent $S_{N}1$ reaction.

B

Fig. (a) and fig. (b) are the energy diagrams for $3^{\circ }$ $R-X$ and $1^{\circ }$ $R-X$, respectively.

C

Fig. (a) and fig. (b) are the energy diagrams for $1^{\circ }$ $R-X$ and $3^{\circ }$ $R-X$, respectively.

$S_{N}1$ reactions generally tend to happen in a two-step process. First, the ionization happens, leaving behind the carbocation, post which, the nucleophile establishes a bond with the carbocation. Hence there should be two peaks (energy barriers) in the P.E. graph. On the other hand $S_{N}2$ happens via a concerted mechanism, meaning there is just the one peak! So in Figs. (a) and (b), (II) and (III) represent $S_{N}2$ reactions, respectively. Obviously, the others are $S_{N}1$

The hydrolysis of $R-X$ by $S_{N}1$ path is $3^{\circ }>2^{\circ }>1^{\circ }$ and by $S_{N}21^{\circ }>2^{\circ }>3^{\circ }$.

Therefore, Fig. (a) represents hydrolysis of $1^{\circ }$ $R-X$, since $S_{N}2$ path is lower energy.

Figure (b) represents hydrolysis $3^{\circ }$ $R-X$, since $S_{N}1$ path is lower energy.

So statements (a), (b) and (c) are correct.