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.
