Concept explainers
Suggest a reasonable explanation for each of the following observations:
The second-order rate constant
The second-order rate constant for saponification of ethyl
The second-order rate constant
The second-order rate constant
The second-order rate constant
The second-order rate constant
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
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
f) The rate constant for saponification of ethyl
Explanation of Solution
a) The saponification of esters involves cleavage of
In ethyl trifluoroacetate,
b) The saponification of esters involves cleavage of
In ethyl acetate,
c) The saponification of esters involves cleavage of
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
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
The anionic intermediate formed by the saponification of
f) The saponification of esters involves cleavage of
In ethyl
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