Question

5) In the "reverse" of esterification, esters can be hydrolyzed with aqueous NaOH (as shown below). The factors that influence the rate of ester hydrolysis are similar to those that affect the rate of esterification described above in problem #4. The first step in this mechanism is nucleophilic attack of hydroxide on the carbonyl carbon for form a tetrahedral intermediate, followed by loss of the alcohol. OH/H20 OH/H, 0 0 0 ROH + R'OH - ROR too a. Which one of the following compounds is the correct structure for ethyl butanoate? London or for Answer: b. Based on their structures, which one of the following esters is expected to react the SLOWEST in the hydrolysis reaction with aqueous base? в

Answer 1

a) Option B) is ethyl butanoate.

Option A) is ethyl propanoate, C) propyl propanoate and D) is propyl ethanoate.

b) The first step in reaction mechanism is attack on the electrophilic carbon centre of C=O group by the nucleophile OH-. The reaction is nucleophilic addition reaction. The tetrahedral intermediate formed again collapses by re-forming C=O bond which results in the loss of the leaving group the alkoxide ion (-OR-). Carboxylic acid is formed and in the last step, which is irreversible, the alkoxide ion (a strong base) captures the acidic proton from the carboxylic acid, forming the carboxylate anion and Alcohol.

For step 1) Electron withdrawing group increases electrophilic character of this centre and hence reactivity in hydrolysis reaction.

For step 2) The loss of leaving group; It is same for all the esters given, that is ethoxide ion (-OEt-)

So, here the determining factor is electrophilicity of carbon.

For option B) The reaction would be fastest as, the tetrahedral intermediate formed would be additionally stabilized by conjugation with -Ph ring. For other options , Among the alkyl groups tertiary butyl (t-butyl) is most electron donating, hence would reduce the electrophilicity of carbon the most.

So D) is the correct option.

in minder Ernal Phenyl group isopropyl t-Butyl eletson donating power Electrophilicity fastest slowest of a tertiary butyl > isopropyl > ethyl of carbon n D is least a slowest reaction

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