Video Transcript
Oil of wintergreen is an ester that can be prepared from salicylic acid. The structures of both molecules are shown. How might oil of wintergreen be prepared directly from salicylic acid? (A) By the reaction of salicylic acid with methane. (B) By the reaction of salicylic acid with methanol. (C) By the reaction of salicylic acid with ethane. (D) By the reaction of salicylic acid with ethanol. Or (E) by the reaction of salicylic acid with propanol.
An ester is an organic compound with a general formula of RCOOR prime. The general structure of an ester, as shown, includes a carbon with one double bond to oxygen and one single bond to oxygen. The ester carbon is also connected to an R group. This R group is a general representation that can either be a hydrogen, an alkyl group, or an aryl group. And the R prime group bonded to the oxygen can be either an alkyl or an aryl group, but in this case it cannot be a hydrogen.
The identifying functionality however is the ester linkage. You can see the same type of ester linkage in the oil of wintergreen shown above. In this example, the R group is the aryl group shown here, and the R prime group is the CH3 attached to the oxygen. When making esters in a chemistry lab, we often will react a carboxylic acid, such as our starting material salicylic acid, with an alcohol. In this reaction, a new bond is formed between the oxygen of the alcohol and the carbon of the carboxylic acid.
This new bond creates the ester linkage that connects the R and R prime groups. During this reaction, there is also a loss of water or H2O. And this type of reaction is called a condensation or dehydration reaction, which is a reaction that joins molecules in a chemical bond and results in the formation of a water molecule.
At this point, let’s revisit the question. How might oil of wintergreen be prepared directly from salicylic acid? We know that the correct answer must include a reaction between a carboxylic acid and an alcohol. We can see that answer choices (A) and (C) react salicylic acid with methane and ethane, respectively. And methane and ethane are both alkanes not alcohols. So, we can eliminate both answer choices (A) and (C). Answer choice (B) states that we can make oil of wintergreen by the reaction of salicylic acid with methanol. If we break down the compound methanol, we can see that it starts with the prefix meth-, which means that it has a one-carbon chain, and the suffix -anol, which means an alkane with an alcohol.
Answer choice (D) states that you’ll react salicylic acid with ethanol. The prefix eth- in this case meaning CH2CH3. And answer choice (E) states that you can make oil of wintergreen by the reaction of salicylic acid with propanol. In this case, the prefix prop- means a three-carbon chain or CH2CH2CH3. Circled in orange is the R prime group of the ester. This group was a part of the alcohol that reacted with salicylic acid to form the ester linkage. And because the R prime group is from methanol, we can see that it’s a CH3 group. And so, we can be sure that answer choices (D) and (E) are incorrect. And so, we can choose answer choice (B) as the correct answer, the reaction of salicylic acid with methanol.
