Chapter 20, Problem 45P

Suggest a reasonable explanation for each of the following observations:

The second-order rate constant $k$ for saponification (basic hydrolysis) of ethyl trifluoroacetate is over $1$ million times greater than that for ethyl acetate $\left(25°\text{C}\right)$.

The second-order rate constant for saponification of ethyl $2,2$-dimethylpropanoate, ${\left({\text{CH}}_{\text{3}}\right)}_{\text{3}}{\text{CCO}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{3}}$, is almost $100$ times smaller than that for ethyl acetate $\left(30°\text{C}\right)$.

The second-order rate constant $k$ for saponification of methyl acetate is $100$ times greater than that for tert-butyl acetate $\left(25°\text{C}\right)$.

The second-order rate constant $k$ for saponification of methyl m-nitrobenzoate is $40$ times greater than that for methyl benzoate $\left(25°\text{C}\right)$.

The second-order rate constant $k$ for saponification of $5$-pentanolide is over $20$ times greater than that for $4$-butanolide $\left(25°\text{C}\right)$.

The second-order rate constant $k$ for saponification of ethyl trans- $4$-tert-butylcyclohexane-carboxylate is $20$ times greater than that for its cis diastereomer $\left(25°\text{C}\right)$.

Interpretation Introduction

Interpretation:

The reasonable explanation for given observations is to be stated.

Concept introduction:

The replacement or substitution of one functional group with another different functional group in any chemical reaction is termed as substitution reaction.

In nucleophilic substitution reaction, nucleophile takes the position of leaving group by attacking on the electron deficient carbon atom.

Answer to Problem 45P

Solution:

a) The rate constant for saponification of ethyl trifluoroacetate is more than ethyl acetate because the anionic intermediate is stabilized in ethyl trifluoroacetate.

b) The rate constant for saponification of ethyl acetate is more than ethyl $2,2$-dimethyl propanoate because the anionic intermediate is stabilized in ethyl acetate.

c) The rate constant for saponification of methyl acetate is more than tert-butyl acetate because the incoming nucleophile is less sterically hindered in methyl acetate.

d) The rate constant for saponification of methyl m-nitrobenzoate is more than methyl benzoate because the anionic intermediate is stabilized in methyl m-nitrobenzoate.

e) The rate constant for saponification of $5-\text{pentanolide}$ is more than $4-\text{pentanolide}$ because the anionic intermediate is stabilized in $5-\text{pentanolide}$.

f) The rate constant for saponification of ethyl $\text{trans}-4-\text{tert}-\text{butylcyclohexane}-\text{carboxylate}$ is more than ethyl $\text{cis}-4-\text{tert}-\text{butylcyclohexane}-\text{carboxylate}$ because the incoming nucleophile is less sterically hindered in ethyl $\text{trans}-4-\text{tert}-\text{butylcyclohexane}-\text{carboxylate}$.

Explanation of Solution

a) The saponification of esters involves cleavage of $-\text{C}\left(\text{O}\right)-\text{O}-$ bond. The products obtained from the reaction are carboxylic acid and alcohol. It can take place under acidic and basic condition. In the first step of saponification, the incoming nucleophile attack on the carbonyl carbon and generate anionic intermediate. The stability of anionic intermediate is determined by the alkyl group attached to carbonyl carbon.

In ethyl trifluoroacetate, ${\text{CF}}_{\text{3}}$ group withdraws the electron density and stabilizes the intermediate. However, in ethyl acetate electron donating ${\text{CH}}_{\text{3}}$ group destabilizes anionic intermediate. Therefore, the second order rate constant for saponification of ethyl trifluoroacetate is much higher over one million times than ethyl acetate. The structure of ethyl trifluoroacetate and ethyl acetate is shown below.

b) The saponification of esters involves cleavage of $-\text{C}\left(\text{O}\right)-\text{O}-$ bond. The products obtained from the reaction are carboxylic acid and alcohol. It can take place under acidic and basic condition. In the first step of saponification, the incoming nucleophile attack on the carbonyl carbon and generate anionic intermediate. The stability of anionic intermediate is determined by the alkyl group attached to carbonyl carbon.

In ethyl acetate, ${\text{CH}}_{\text{3}}$ group donates the electron density to carbonyl carbon and destabilizes the anion intermediate. However, in ethyl $2,2$-dimethyl propanoate, $\text{C}{\left({\text{CH}}_{\text{3}}\right)}_{\text{3}}$ group donates electron density to carbonyl carbon even more than ${\text{CH}}_{\text{3}}$ group. Therefore, the second order rate constant for saponification of ethyl acetate is much higher than ethyl $2,2$-dimethyl propanoate. The structure of ethyl acetate and ethyl $2,2$-dimethyl propanoate is shown below.

c) The saponification of esters involves cleavage of $-\text{C}\left(\text{O}\right)-\text{O}-$ bond. The products obtained from the reaction are carboxylic acid and alcohol. It can take place under acidic and basic condition. The mechanism followed by the saponification reaction is ${\text{S}}_{\text{N}}\text{2}$.

In the first step of saponification reaction, the incoming nucleophile attack on the carbonyl carbon and generate anionic intermediate. In tert-butyl acetate, the attack of incoming nucleophile is sterically hindered by the presence of three methyl groups. However, in methyl acetate only one methyl group is present. Therefore, the second order rate constant for saponification of methyl acetate is much higher than tert-butyl acetate. The structure of methyl acetate and tert-butyl acetate is shown below.

d) The saponification of esters involves cleavage of $-\text{C}\left(\text{O}\right)-\text{O}-$ bond. The products obtained from the reaction are carboxylic acid and alcohol. It can take place under acidic and basic condition. In the first step of saponification, the incoming nucleophile attack on the carbonyl carbon and generate anionic intermediate. The stability of anionic intermediate is determined by the alkyl group attached to carbonyl carbon.

In methyl m-nitrobenzoate, nitro group is attached to benzene ring which withdraws the electron density and stabilizes the intermediate. However, in methyl benzoate electron withdrawing group is not attached to benzene ring. It stabilizes anionic intermediate less than the methyl m-nitrobenzoate. Therefore, the second order rate constant for saponification of methyl m-nitrobenzoate is more than methyl benzoate. The structure of methyl m-nitrobenzoate and methyl benzoate is shown below.

e) The saponification of esters involves cleavage of $-\text{C}\left(\text{O}\right)-\text{O}-$ bond. The products obtained from the reaction are carboxylic acid and alcohol. It can take place under acidic and basic condition. In the first step of saponification, the incoming nucleophile attack on the carbonyl carbon and generate anionic intermediate. The stability of anionic intermediate is determined by the alkyl group attached to carbonyl carbon.

The anionic intermediate formed by the saponification of $5-\text{pentanolide}$ is stabilized more than the intermediate formed by the saponification of $4-\text{butanolide}$. This is because the angle strain in $5-\text{pentanolide}$ is less. Therefore, the rate constant for saponification of $5-\text{pentanolide}$ is very high than $4-\text{butanolide}$.

f) The saponification of esters involves cleavage of $-\text{C}\left(\text{O}\right)-\text{O}-$ bond. The products obtained from the reaction are carboxylic acid and alcohol. It can take place under acidic and basic condition. The mechanism followed by the saponification reaction is ${\text{S}}_{\text{N}}\text{2}$.

In ethyl $\text{trans}-4-\text{tert}-\text{butylcyclohexane}-\text{carboxylate}$, the bulkier groups are present opposite to each other. The attack of nucleophile becomes easier in trans diastereomer. However, in ethyl $\text{cis}-4-\text{tert}-\text{butylcyclohexane}-\text{carboxylate}$, bulkier groups are present on the axial position. The incoming nucleophile is sterically hindered in cis diastereomer. Therefore, the rate constant for saponification of ethyl $\text{trans}-4-\text{tert}-\text{butylcyclohexane}-\text{carboxylate}$ is more than ethyl $\text{cis}-4-\text{tert}-\text{butylcyclohexane}-\text{carboxylate}$.