Question Video: Identifying the Structure of an Ester | Nagwa Question Video: Identifying the Structure of an Ester | Nagwa

Question Video: Identifying the Structure of an Ester Chemistry • Third Year of Secondary School

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Which of the following structures represents an ester? [A] (2) only [B] (1) only [C] (3) only [D] (4) only [E] (1) and (2)

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Video Transcript

Which of the following structures represents an ester? (1), (2), (3), or (4). Answer (A) (2) only, answer (B) (1) only, answer (C) (3) only, answer (D) (4) only, or answer (E) (1) and (2).

An ester is an organic compound with the general formula RCOOR prime. The structure of an ester includes a carbon with one double bond to oxygen and one single bond to oxygen. The ester carbon is also bonded to an R group. The capital letter R is a general representation that’s used by chemists. And in this case, it can mean a hydrogen, an alkyl functional group, or an aryl functional group. The R prime group, which is bonded to the oxygen of the ester, can be any alkyl or aryl functional group. You’ll notice that in the case of an ester, the R prime group cannot be a hydrogen.

Along with the R and R prime groups, there’s also an ester linkage. And this ester linkage is the functional group that makes this compound an ester. Esters are fragrant compounds that are found in many fresh foods, such as fruits and vegetables. For example, the compound ethyl pentanoate is a fragrant ester and is a key component to why apples smell and taste like apples. And you can also see that within ethyl pentanoate, we have an ester linkage; an R group, which in this case is an alkyl group bonded to the carbon of our ester linkage; and our R prime group, which in this case is an alkyl group bonded to the oxygen of the ester linkage.

And now with this information, we should be able to revisit the question. Which of the following structures represents an ester?

Structure one has a carbon with a double bond to one oxygen and a single bond to one oxygen. The R group bonded to the carbon of the ester linkage is composed of a two-carbon alkyl chain. But the R prime group bonded to the oxygen of the ester linkage is a hydrogen. And as we’ve already discussed, in an ester, the R prime group bonded to the oxygen of the ester linkage must be an alkyl group or an aryl group. But when the R prime group is a hydrogen, that makes this structure a carboxylic acid.

Structure number two has the same linkage with a carbon bonded to two oxygens, an R group that is an alkyl group, and the R prime group is also an alkyl group. And since we have an ester linkage with an alkyl group bonded on either side, we can identify structure number two as an ester.

Structure number three has a carbon with a double bond to one oxygen and a single bond to two different carbons. And the structure containing a carbon with a double bond to oxygen and carbons on either side is a ketone.

Structure number four has an oxygen atom with a single bond to two different carbons. But it does not contain a carbon with a double bond to oxygen required for an ester. And the functional group that is composed of an oxygen with a single bond to two different carbons and no C double bond O is an ether.

And so we can see that the only structure that contains an ester functional group is structure two. And that means the correct answer is answer choice (A) (2) only.

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