Chapter 9, Problem 9.4P

Interpretation Introduction

(a)

Interpretation:

It is to be predicted which of the two nucleophiles in the given pair will react faster with ${\text{CH}}_{\text{3}}\text{I}$ in an ${\text{S}}_{\text{N}}\text{2}$ reaction.

Concept introduction:

The minimum amount of energy required by reacting molecules to reach the transition state is called the activation energy.

For nucleophilic substitution reactions, if the reactant is the same, the activation energy increases with increasing stability of the substitution product.

The reaction is faster when the energy barrier between the reactants and products is small. When energy difference between reactants and the transition state is smaller than the energy difference between products and transition state, the reactants are highly reactive and reaction is faster. Highly reactive reactant molecules serve as strong nucleophiles. The reactants are less reactive and the reaction slower, when the transition state is closer in energy to the products than the reactants.

Answer to Problem 9.4P

(i) Among the two nucleophiles, the nucleophile ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ will react faster than ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$ with ${\text{CH}}_{\text{3}}\text{I}$.

(ii) Among the two nucleophiles, the nucleophile ${\text{PH}}_{\text{3}}$ will react faster than ${\text{NH}}_{\text{3}}$ with ${\text{CH}}_{\text{3}}\text{I}$.

Explanation of Solution

(i) The first given pair of nucleophiles is ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ and ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$.

The reactions of each of these nucleophiles with ${\text{CH}}_{\text{3}}\text{I}$ are

There is a significant difference in charge stability on the reactant side but not on the product side because the negative charge in ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$ is resonance stabilized, and thus, more stable than ${\text{CH}}_{\text{3}}{\text{O}}^{-}$. Therefore, the energy barrier involving ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ is smaller; thus, the reaction of ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ with ${\text{CH}}_{\text{3}}\text{I}$ is faster than the reaction involving ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$ and ${\text{CH}}_{\text{3}}\text{I}$. This suggests that the nucleophile ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ is higher in energy than ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$ and, thus, is stronger. The rate of an ${\text{S}}_{\text{N}}\text{2}$ reaction increases with the strength of the nucleophile when the substrate is the same. Therefore, the reaction involving ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ will be faster.

(ii) The second given pair of nucleophiles is ${\text{NH}}_{\text{3}}{\text{ and PH}}_{\text{3}}$.

The reactions of each of these nucleophiles with ${\text{CH}}_{\text{3}}\text{I}$ are

There is a significant difference in charge stability on the product side but not on the reactant side. Because a positive charge is more stable on phosphorous atom than on nitrogen atom, the product of the nucleophilic substitution involving ${\text{PH}}_{\text{3}}$ is more stable.

Therefore, the energy barrier involving ${\text{PH}}_{\text{3}}$ is smaller; thus, the reaction of ${\text{PH}}_{\text{3}}$ with ${\text{CH}}_{\text{3}}\text{I}$ is faster than the reaction involving ${\text{NH}}_{\text{3}}$ and ${\text{CH}}_{\text{3}}\text{I}$. This suggests that the nucleophile ${\text{PH}}_{\text{3}}$ is higher in energy than ${\text{NH}}_{\text{3}}$ and, thus, is stronger. The rate of an ${\text{S}}_{\text{N}}\text{2}$ reaction increases with the strength of the nucleophile when the substrate is the same. Therefore, the reaction involving ${\text{PH}}_{\text{3}}$ will be faster.

Conclusion

As nucleophilicity of the attacking species increases in ${\text{S}}_{\text{N}}\text{2}$ reaction, the rate of the reaction increases.

Interpretation Introduction

(b)

Interpretation:

It is to be determined which of the given pairs is the stronger nucleophile.

Concept introduction:

The minimum amount of energy required by reacting molecules to reach the transition state is called the activation energy. For nucleophilic substitution reactions, if the reactant is the same, the activation energy increases with increasing stability of the substitution product. When energy difference between reactants and the transition state is smaller than the energy difference between products and transition state, the reactants are highly reactive and reaction is faster. Highly reactive reactant molecules serves as strong nucleophiles.

Answer to Problem 9.4P

(i) Among the pair of nucleophiles, ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ is stronger than ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$.

(ii) Among the two nucleophiles, ${\text{PH}}_{\text{3}}$ is stronger than ${\text{NH}}_{\text{3}}$.

Explanation of Solution

(i) The first given pair of nucleophiles is ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ and ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$.

In part (a), it is determined that the reaction of ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ with ${\text{CH}}_{\text{3}}\text{I}$ is faster than it is for ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$ with ${\text{CH}}_{\text{3}}\text{I}$. This suggests that the energy barrier for the reaction involving ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ is smaller. Thus, the reactant and a nucleophile, ${\text{CH}}_{\text{3}}{\text{O}}^{-}$, is stronger than ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$. Thus, among the two given nucleophiles, ${\text{CH}}_{\text{3}}{\text{O}}^{-}$ is stronger than ${\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{}^{-}$.

(ii) The first given pair of nucleophiles is ${\text{PH}}_{\text{3}}$ and ${\text{NH}}_{\text{3}}$.

In part (a), it is determined that the reaction of ${\text{PH}}_{\text{3}}$ with ${\text{CH}}_{\text{3}}\text{I}$ is faster than it is for ${\text{NH}}_{\text{3}}$ with ${\text{CH}}_{\text{3}}\text{I}$. This suggests that the energy barrier for the reaction involving ${\text{PH}}_{\text{3}}$ is smaller. Thus, the reactant and a nucleophile, ${\text{PH}}_{\text{3}}$, is stronger than ${\text{NH}}_{\text{3}}$. Thus, among the two given nucleophiles, ${\text{PH}}_{\text{3}}$ is stronger than ${\text{NH}}_{\text{3}}$.

Conclusion

Among the two given nucleophiles, the stronger nucleophile is the one that exhibits a faster reaction.