Which of the following types of molecules can react together to form an ester? (A) Alcohol and alcohol, (B) alkane and alkane, (C) carboxylic acid and alkene, (D) carboxylic acid and alcohol, or (E) alcohol and halogenoalkane.
An ester is an organic compound with the general formula of RCOOR prime. The structure of an ester consists of a carbon with one double bond to oxygen and one single bond to oxygen. This ester carbon is also connected to an R group, which is representative of either a hydrogen, an alkyl group, or an aryl group. The R prime group, which is bonded to the oxygen of the ester, can either be an alkyl group or an aryl group. You’ll also notice that the R prime group cannot be a hydrogen. Well, R and R prime can be a variety of different types of groups. The consistent functionality of any ester group is the ester linkage.
Ester compounds are commonly found in fruits and vegetables, and they tend to be very fragrant. And so they tend to be a large part of what you smell and therefore taste in fresh produce. In a chemistry lab, we often try to recreate esters due to their smell, taste, and other desirable properties. And to make esters in lab, we will often take carboxylic acids, such as acetic acid, and react it with an alcohol, like methanol. And the reaction of these two types of compounds creates an ester linkage, just like the one shown in the general example above.
In this example, the R group connected to the ester linkage comes from the CH3 group of acetic acid. And the R prime group comes from the CH3 from methanol. In this reaction, the OH from the carboxylic acid and the hydrogen from the alcohol are also lost as water. This is known as a condensation reaction, which is a reaction that joins molecules in a chemical bond — in this case, we’re talking about an esterification reaction — and results in the formation of a water molecule.
And with that, we can revisit the question. Which of the following types of molecules can react together to form an ester? And as we see exemplified in our condensation reaction of acetic acid with methanol, we can correctly choose answer choice (D) carboxylic acid and alcohol.