Which of the following is incorrect about acetic acid? (A) It can be detected using a sodium bicarbonate solution. (B) It is a dibasic, aliphatic carboxylic acid. (C) It is less acidic than benzoic acid. (D) It reacts with ethanol, producing ethyl acetate and water. Or (E) it can be reduced, forming ethanol.
Acetic acid is the common name of ethanoic acid. The ending -oic acid indicates that this molecule is a carboxylic acid and contains a carboxyl group. The prefix eth- indicates that ethanoic acid contains two carbon atoms. The carboxyl group contains one carbon atom. This means that R must be a CH3 group. Now that we have determined the structure of acetic acid, we need to evaluate each of the statements given and identify which of the statements is incorrect.
Statement (A) states that acetic acid can be detected using a sodium bicarbonate solution. We know that acetic acid is a carboxylic acid. When carboxylic acids are combined with sodium bicarbonate, a salt, carbon dioxide gas, and water are produced. Visible bubbles of carbon dioxide gas or effervescence can be seen when performing this reaction. As such, addition of sodium bicarbonate to a solution is often used as a test to determine if the solution contains a carboxylic acid. So acetic acid can be detected using a sodium bicarbonate solution. Since this statement is true, it cannot be the correct answer to this question.
Now let’s take a look at statement (B). This statement has to do with the classification of acetic acid. We have already determined that acetic acid is a carboxylic acid. Carboxylic acids can be classified by their R group. If the R group consists of carbon atoms bonded in open chains, then the carboxylic acid is considered aliphatic. But if the R group contains at least one planar, cyclic structure of alternating single and double bonds, then the carboxylic acid is considered aromatic. Acetic acid does not contain a cyclic structure, so acetic acid is aliphatic.
As carboxylic acids are acids, they can also be classified by the number of acidic protons they can donate in an acid–base reaction. Carboxylic acids that are able to donate one proton are classified as monobasic. Carboxylic acids that are able to donate two protons are considered dibasic. And carboxylic acids that are able to donate two or more protons are considered polybasic. Acetic acid has one acidic proton, so it is classified as monobasic, not dibasic. This means that answer choice (B) is a false statement and is likely the answer to this question. But let’s take a look at answer choices (C), (D), and (E) just to be sure.
Answer choice (C) states that acetic acid is less acidic than benzoic acid. Benzoic acid is an aromatic carboxylic acid. Acetic acid and benzoic acid are both monobasic. Each species can donate one proton in an acid–base reaction to form a negatively charged carboxylate group. The negatively charged carboxylate group is particularly stable due to resonance effect. This means that the charge is delocalized over two oxygen atoms.
Since both benzoic acid and acetic acid are monobasic and the conjugate bases of each contain a carboxylate group, where the negative charge is delocalized over two oxygen atoms, we might expect both acids to have a similar acid strength. In fact, their pK a values are similar, but benzoic acid is slightly more acidic. This is because the adjacent unsaturated ring stabilizes the carboxylate ion more effectively than the adjacent alkyl group in acetate. As acetic acid is less acidic than benzoic acid, answer choice (C) cannot be the correct answer.
Answer choice (D) states that acetic acid reacts with ethanol. Ethanol is a two-carbon alcohol. Alcohols can react with carboxylic acids, like acetic acid, in the presence of an acid catalyst. This is an example of esterification, a type of chemical reaction where an ester is produced. Over the course of this reaction, the alkoxy group of the alcohol replaces the hydroxy group of the carboxylic acid. This produces an ester and water. The specific name of the ester is derived from the original alcohol and carboxylic acid. The ending of the alcohol name is changed to -yl, and the ending of the carboxylic acid name is changed to -ate. So the reaction between acetic acid and ethanol produces ethyl acetate and water. Therefore, statement (D) is true and cannot be the answer to this question.
Statement (E) says that acetic acid can be reduced. Carboxylic acids, like acetic acid, can be reduced using a variety of reducing agents. We can represent a generic reducing agent with a capital H in brackets. This reaction produces an aldehyde and water. But an aldehyde is rarely the final product of the reduction of a carboxylic acid. This is because aldehydes under standard conditions rapidly react with reducing agents to form a primary alcohol. We saw this two-carbon alcohol earlier in the video. This is the structure of ethanol. So acetic acid can be reduced to ethanol, meaning that answer choice (E) cannot be the correct answer to this question.
After reviewing all of the answer choices, we have determined that answer choice (B), it is a dibasic aliphatic carboxylic acid, is the statement about acetic acid that is incorrect.