Question

Which of the following reagents is used to identify acetic acid? (a) Tollen's reagent (b) Neutral \(\mathrm{FeCl}_{3}\) solution (c) Ceric ammonium nitrate (d) \(\mathrm{NaOH}\) and \(\mathrm{I}_{2}\)

Short answer

The reagent used to identify acetic acid is NaOH and I2 (option d).

Step-by-step solution

Understand the Analytical Techniques

To identify acetic acid, we need to choose a reagent that reacts specifically with it. Acetic acid is a carboxylic acid, and certain reagents can be used to detect it. Here, we will evaluate each option for its applicability to acetic acid.

Analyze Tollen's Reagent

Tollen's reagent is used to test for aldehydes, giving a silver mirror with them. Acetic acid does not react with Tollen's reagent because it is not an aldehyde. Hence, option (a) is incorrect.

Analyze Neutral FeCl3 Solution

Neutral \(\mathrm{FeCl}_{3}\\) gives a color change (usually violet) with phenolic compounds but not with acetic acid. Thus, option (b) does not identify acetic acid.

Analyze Ceric Ammonium Nitrate

Ceric ammonium nitrate is used to identify alcohols and not typically used for carboxylic acids. Acetic acid will not give a characteristic reaction with this reagent, making option (c) incorrect.

Analyze Reaction with NaOH and I2

Combining \(\mathrm{NaOH}\) and \(\mathrm{I}_{2}\) forms a test known as the Iodoform Test. Acetic acid reacts through an intermediate to produce yellow precipitate of iodoform (CHI3), indicating a positive test. This confirms option (d) as correct.

Key concepts

Iodoform Test

The Iodoform Test is a classic chemical reaction used to identify certain methyl ketones and alcohols containing methyl groups. When acetic acid, which has the structure CH₃COOH, is subjected to the Iodoform Test, it reacts through an intermediate step to produce a yellow precipitate of iodoform (CHI₃). This yellow precipitate is a positive indication of the presence of acetic acid.
The reaction involves the halogenation of methyl groups adjacent to the carbonyl group when the compound is treated with iodine ( I₂ ) and a base, usually sodium hydroxide ( NaOH ). The methyl groups are converted into iodoform, which has a distinct yellow color and odor. This property makes the Iodoform Test a reliable method to identify compounds like acetic acid, specifically among other carboxylic acids that do not yield iodoform under similar conditions.

Tollen's Reagent

Tollen's reagent is widely known for its use in distinguishing aldehydes from ketones. It contains a complex of silver ammonium ion, Ag(NH₃)₂⁺, which readily oxidizes aldehydes to carboxylic acids, forming a characteristic silver mirror coating inside the test tube.
However, Tollen's reagent is not suitable for identifying acetic acid, a carboxylic acid, because acetic acid does not possess an aldehyde group needed to reduce the silver ions to elemental silver. Therefore, acetic acid will not lead to any visible change when treated with Tollen's reagent, unlike compounds that contain an aldehyde group.
This distinction makes Tollen's reagent ineffective for acetic acid identification, as it remains non-reactive in the presence of carboxylic acids.

Neutral FeCl3 Solution

Neutral ferric chloride ( FeCl₃ ) is primarily used in the detection of phenolic compounds due to its ability to form colored complexes with them. When phenols are mixed with FeCl₃ , a color change, such as a violet or green hue, typically occurs, indicating a positive test.
Acetic acid, however, is not a phenol. Therefore, neutral FeCl₃ does not produce any significant color change when reacted with acetic acid. Instead, it remains unchanged, making it an ineffective reagent for detecting acetic acid presence in a sample.
This characteristic renders neutral ferric chloride unsuitable for specifically identifying acetic acid within a mixture or on its own.

Ceric Ammonium Nitrate

Ceric Ammonium Nitrate (CAN) is commonly utilized to detect the presence of alcohols. When mixed with alcohols, this reagent forms a red complex, which serves as an indication of a positive test. CAN's primary application revolves around testing for hydroxyl groups rather than carboxyl groups.
Since acetic acid is a carboxylic acid and lacks the alcohol functional group, it will not react or form the red complex when treated with Ceric Ammonium Nitrate. Therefore, it is not suitable for identifying acetic acid.
In organic chemistry, distinguishing between different types of functional groups is crucial, and the specificity of CAN for alcohols underscores its inapplicability for acetic acid testing.