Question

Acetaldehyde and Propyne can be distinguish by: (i) Tollen's reagent (ii) \(\mathrm{I}_{2} / \mathrm{NaOH}\) (iii) Lucas reagent (iv) neutral \(\mathrm{FeCl}_{3}\) (1) (i), (ii) \& (iii) (2) (ii) \& (iii) (3) (i) \& (ii) (4) (iii) \& (iv)

Short answer

The correct answer is (3) (i) & (ii).

Step-by-step solution

- Understanding Tollen's Reagent

Tollen's reagent (Ammoniacal Silver Nitrate) is specific for aldehydes. Acetaldehyde, being an aldehyde, will reduce Tollen's reagent to form a silver mirror, while Propyne will not react.

- Reaction with \mathrm{I}_{2} / \mathrm{NaOH}

The \mathrm{I}_{2} / \mathrm{NaOH} (Iodoform test) distinguishes methyl ketones and secondary alcohols having the methyl group at the alpha position, which forms a yellow precipitate of Iodoform (CH\(_3\)I). Acetaldehyde gives a positive iodoform test due to the presence of a \mathrm{C=O}-CH\(_3\) group, whereas Propyne does not.

- Understanding Lucas Reagent

Lucas reagent (ZnCl\(_2\)/HCl) is used to distinguish between primary, secondary, and tertiary alcohols based on their reactivity and formation of alkyl chlorides. Neither Acetaldehyde (an aldehyde) nor Propyne (an alkyne) react with Lucas reagent.

- Reaction with neutral FeCl\(_3\)

Neutral FeCl\(_3\) test is used to detect phenols, which form a colored complex. Neither Acetaldehyde nor Propyne give a positive test with neutral FeCl\(_3\).

- Concluding the Results

From the information gathered, Tollen's reagent and \mathbf{I}_{2} / \mathbf{NaOH} can distinguish Acetaldehyde from Propyne. Lucas reagent and neutral FeCl\(_3\) are not useful in this context.

Key concepts

Tollen's Reagent Test

The Tollen's reagent test is a classical test used in organic chemistry to distinguish aldehydes from other carbonyl compounds. Tollen's reagent, consisting of ammoniacal silver nitrate, reacts specifically with aldehydes. When an aldehyde, like acetaldehyde, is present, it reduces the Tollen's reagent to metallic silver. This reaction forms a characteristic silver mirror on the container's inner surface. This visual cue helps confirm the presence of an aldehyde. In contrast, propyne, an alkyne, does not react with Tollen's reagent. Therefore, the Tollen's reagent test is highly effective for differentiating acetaldehyde from propyne.

Iodoform Test

The iodoform test uses iodine and sodium hydroxide to identify methyl ketones and specific secondary alcohols. If a compound has a methyl group directly bonded to a carbonyl group (C=O-CH3), it will yield yellow iodoform (CH3I) precipitate upon reaction. Acetaldehyde, being a molecule with C=O-CH3, gives a positive iodoform test. The reaction creates a noticeable yellow precipitate. However, propyne, an alkyne without the required structure, does not produce iodoform and fails this test. This makes the iodoform test a handy method for distinguishing acetaldehyde from propyne.

Lucas Reagent Test

The Lucas reagent test is designed to differentiate alcohols based on their reaction with Lucas reagent, a mixture of zinc chloride (ZnCl2) and hydrochloric acid (HCl). This test categorizes primary, secondary, and tertiary alcohols based on how quickly they form an alkyl chloride precipitate. Tertiary alcohols react fastest, followed by secondary, then primary. However, aldehydes like acetaldehyde and alkynes like propyne do not react with Lucas reagent. Thus, this test is ineffective in distinguishing acetaldehyde from propyne.

Neutral FeCl3 Test

The neutral ferric chloride (FeCl3) test is commonly employed to detect phenols. When phenols react with neutral FeCl3, they form a colored complex ranging from purple to green to red. Neither acetaldehyde nor propyne qualifies as a phenol, so neither reacts with neutral FeCl3. This reaction's absence makes this test unhelpful in distinguishing between acetaldehyde and propyne. For identifying phenols, it remains a robust and reliable test, but not suitable for our given compounds.