Question Video: Predicting the Result of the Intramolecular Esterification of 5-Hydroxy Pentanoic Acid Chemistry

It is possible for a molecule containing an alcohol and a carboxylic acid group to react with itself to form cyclic esters known as lactones. Which structure would result from the reaction of 5-hydroxypentanoic acid with itself?

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Video Transcript

It is possible for a molecule containing an alcohol and a carboxylic acid group to react with itself to form cyclic esters known as lactones. Which structure would result from the reaction of 5-hydroxypentanoic acid with itself?

We have been given five possible solutions to this question labeled from (A) to (E). The question tells us that cyclic esters are formed. All options from (A) to (E) are cyclic because they contain a ring in their structure. But do they all contain the ester functional group? An ester has the following general displayed formula, where this R group, which represents the rest of the molecule, can be an alkyl group, an aryl group, or a hydrogen atom. And this R group can be an alkyl group or an aryl group.

Of the five possible answers, only two of them, (D) and (E), contain the ester functional group. The structures in (A), (B), and (C) contain the ether functional group. (A) also contains the carboxylic acid functional group, and the structures in (B) and (C) also contain the alcohol functional group. Therefore, structures (A), (B), and (C) cannot be the answer. Let’s remove these answer choices from the screen so that we have a bit more space.

Both (D) and (E) are cyclic esters. So, we need to figure out which one of these two structures would result from the reaction of 5-hydroxypentanoic acid with itself. The question tells us that to form these esters, the molecule must contain an alcohol and carboxylic acid functional group. Before we look at how to make cyclic esters, let’s look at how to make linear esters.

In this reaction, the carboxylic acid and alcohol functional groups would be on separate molecules. They react reversibly in the presence of a strong acid catalyst to produce an ester and the side product water. In this reaction, the hydroxy part of the carboxylic acid forms part of the water molecule, and the remaining part of the carboxylic acid forms part of the ester. The other part of the ester comes from the alkoxide part of the alcohol.

Now that we know how to form a linear ester, let’s look at how to form a cyclic ester. This is the structure of 5-hydroxypentanoic acid. Let’s start by identifying the key parts of the molecule. This is the hydroxy part that forms part of the water molecule. This is the other part of the carboxylic acid functional group and the start of the R group that will form part of the ester. And this is the alkoxide group, which forms the other part of the ester molecule. The part of the molecule that’s drawn in black is where the two R groups meet.

As shown in the formation of a linear ester, a bond will form between the oxygen atom in the alkoxide group of the alcohol and the carbonyl carbon in the carboxylic acid. In 5-hydroxypentanoic acid, these parts of the molecule are quite far away from each other. So, we should redraw the molecule to move these parts of the molecule closer together. To ensure that we don’t miss any parts of the molecule when it’s redrawn, let’s number the carbon atoms. There are five carbon atoms in this molecule. So when we redraw it, we need to make sure there are still five carbon atoms. Since we know that the end product is a cyclic ester, it makes sense for us to redraw the ester in a cyclical form.

When 5-hydroxypentanoic acid is reacted with itself, a bond forms between the oxygen atom in the alkoxide group of the alcohol and the carbonyl carbon in the carboxylic acid functional group. In this diagram, the carbon is labeled with the number five. These bonds break and the side product of water is formed. We now have the structure of the final product. And if we rotate that structure, we see that it matches the structure found in (D), where the ring is six-membered as it contains five carbon atoms and one oxygen atom, as opposed to the structure found in (E), which contains a five-membered ring as one of the carbon atom forms a side group.

Therefore, the answer to the question “which structure would result from the reaction of 5-hydroxypentanoic acid with itself?” is (D).

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