Video Transcript
Which of the following is incorrect about aliphatic carboxylic acids? (A) Aliphatic carboxylic acids are more acidic than alcohols that have the same number of carbon atoms. (B) Aliphatic carboxylic acids have higher boiling points than alcohols that have the same number of carbon atoms. (C) Aliphatic carboxylic acids are less acidic than hydrochloric acid. (D) Aliphatic carboxylic acids can be reduced, forming the corresponding alcohols. (E) Aliphatic carboxylic acids are generally more acidic than aromatic carboxylic acids.
Let’s begin by describing the general structure of a carboxylic acid molecule. Carboxylic acids are organic molecules that contain at least one carboxyl or COOH functional group. When the R group of the carboxylic acid is a hydrogen atom or an alkyl group, the carboxylic acid is called an aliphatic carboxylic acid. The alkyl groups may be linear, branched, or nonaromatic ring structures. Now that we know what an aliphatic carboxylic acid is, let’s compare it to an aromatic carboxylic acid.
You probably already guessed that the R group of an aromatic carboxylic acid is an aromatic ring, such as shown in the molecule below. Now, let’s discuss some of the properties of aliphatic carboxylic acids. Aliphatic carboxylic acids, such as ethanoic acid, can form dimers due to the presence of two hydrogen bonds between the carboxyl groups of two ethanoic acid molecules. Ethanol and alcohol with the same number of carbon atoms as ethanoic acid does exhibit hydrogen bonding between ethanol molecules. However, dimers do not form. Because the intermolecular attractions between the ethanoic acid molecules are stronger, ethanoic acid has a higher boiling point than ethanol.
This example illustrates that aliphatic carboxylic acids tend to have higher boiling points than alcohols with the same number of carbon atoms. Therefore, we can eliminate answer choice (B) because in this question, we are looking for an incorrect statement about aliphatic carboxylic acids. In general, carboxylic acids tend to be more acidic than alcohols with the same number of carbon atoms but less acidic than the mineral acids such as hydrochloric acid. For this reason, we can eliminate answer choices (A) and (C) because they are both correct statements.
But let’s get a little bit more specific. Aromatic carboxylic acids tend to be more acidic than aliphatic carboxylic acids. In water, an aromatic carboxylic acid such as the one shown donates a hydrogen ion from the carboxyl group to a water molecule forming a carboxylate anion and the positively charged hydronium ion.
Now, let’s have a look at a similar aliphatic carboxylic acid that has a six-membered nonaromatic ring. A similar reaction with water occurs. However, the anion produced is less stable than the one produced by the aromatic acid. The reason for this difference is that the negative charge in the anion can only be delocalized over the two oxygen atoms. In contrast, in the anion produced by the aromatic acid, delocalization of the negative charge can occur over both oxygen atoms and, to some degree, the benzene ring. Delocalization of the negative charge over a greater region of the molecule makes this anion more stable.
In summary, we’ve determined that aromatic carboxylic acids are more acidic than aliphatic carboxylic acids. Therefore, the statement that is incorrect about aliphatic carboxylic acids is answer choice (E).
