Question

Propose a mechanism for formation of 2-carbethoxy-4-butanolactone and 4-butanolactone ( \(\gamma\)-butyrolactone) in the following sequence of reactions.

Short answer

Question: Describe the mechanism for the formation of 2-carbethoxy-4-butanolactone and then 4-butanolactone (γ-butyrolactone) from 4-hydroxy-4-pentenoic acid ethyl ester using an acylating agent. Answer: The mechanism involves an intramolecular nucleophilic substitution to form 2-carbethoxy-4-butanolactone, where the oxygen from the hydroxyl group attacks the carbonyl carbon of the ester linkage within the same molecule, forming a cyclic ester with a five-membered ring. This is followed by a decarboxylation reaction, which removes a carboxyl group to yield the final product, 4-butanolactone (γ-butyrolactone).

Step-by-step solution

Identify Starting Materials and Products

We are given the sequence of reactions that lead to the formation of 2-carbethoxy-4-butanolactone and then 4-butanolactone (γ-butyrolactone). The reactants in this sequence are 4-hydroxy-4-pentenoic acid ethyl ester and an acylating agent.

Formation of 2-carbethoxy-4-butanolactone

The first reaction in this sequence involves the intramolecular nucleophilic substitution reaction, where the oxygen from the hydroxyl group attacks the carbonyl carbon of the ester linkage within the same molecule. This process forms a cyclic ester with five-membered ring: 1. The oxygen atom from the hydroxyl group forms a bond with the carbonyl carbon of the ester linkage. 2. The carbonyl group's pi bond breaks, and the electrons move to the carbonyl oxygen, forming an intermediate with a negatively charged oxygen atom. 3. The negatively charged oxygen atom attacks the hydrogen of the hydroxyl group, forming 2-carbethoxy-4-butanolactone and releasing the extra water molecule: \(\underset{4-hydroxy-4-pentenoic\, acid\, ethyl\, ester}{\text{O}\ce{->C}\ce{-C}\ce{=}\ce{C}\ce{-C}\ce{-OH}} \underset{Acylating\, agent}{\text{--->[}} \underset{2-carbethoxy-4-butanolactone}{\text{O}\ce{->C}\ce{-C}\ce{=}\ce{C}\ce{-C}\ce{-O-}} + \ce{H2O}\)

Formation of γ-butyrolactone

To form the final product, 4-butanolactone (γ-butyrolactone), a decarboxylation reaction takes place, which involves the removal of a carboxyl group, -COOH: 1. The C-C bond between the carbon with the carbonyl oxygen and the adjacent carbon breaks. 2. The electrons from the broken bond form a pi bond between carbonyl carbon and its oxygen, releasing carbon dioxide (CO2) gas. 3. The remaining compound forms the product, 4-butanolactone or γ-butyrolactone: \(\underset{2-carbethoxy-4-butanolactone}{\text{O}\ce{->C}\ce{-C}\ce{=}\ce{C}\ce{-C}\ce{-O-}}\text{--->[}\underset{\gamma\, -\, butyrolactone}{\text{O}\ce{->C}\ce{-C}\ce{-C}\ce{-O-}} + \ce{CO2}\) Overall, the proposed mechanism for the given reaction sequence involves an intramolecular nucleophilic substitution to form 2-carbethoxy-4-butanolactone, followed by a decarboxylation reaction to yield the final product, 4-butanolactone (γ-butyrolactone).