Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

Acetaldehyde cannot exhibit (a) Iodoform test (b) Benedict's test (c) Lucas test (d) Tollen's test

Step-by-Step Solution

Answer

Acetaldehyde cannot exhibit the Lucas test.

Step 1: Understanding the Iodoform Test

The iodoform test is used to identify the presence of a methyl ketone group or ethanol in a compound. Compounds containing a CH3-C=O group adjacent to a carbon atom will give a positive iodoform test. Acetaldehyde (CH3CHO) has this functional group, hence it can exhibit the iodoform test.

Step 2: Analyzing the Benedict's Test

The Benedict's test is a test for reducing sugars and aldehydes with an oxidizable group, specifically an aldehyde group. Acetaldehyde contains an aldehyde group, allowing it to give a positive Benedict's test.

Step 3: Evaluating the Lucas Test

The Lucas test is used to distinguish between primary, secondary, and tertiary alcohols based on their reactivity with Lucas reagent. Since acetaldehyde is an aldehyde and not an alcohol, it cannot exhibit a positive Lucas test.

Step 4: Considering the Tollen's Test

The Tollen's test involves the reaction of a compound with Tollen's reagent, which oxidizes aldehydes to carboxylic acids, resulting in a silver mirror formation. Since acetaldehyde is an aldehyde, it will give a positive Tollen's test.

Key Concepts

Iodoform Test

The Iodoform Test is a chemical reaction used to identify the presence of a specific group in hydrocarbons: the methyl ketone group or ethanol. This test relies on the structure having a CH3-C=O group adjacent to a carbon atom. When a compound like acetaldehyde (CH3CHO) has this group, it reacts with iodine in the presence of a base, usually sodium hydroxide, resulting in the formation of iodoform (CHI3). This is a yellow crystalline solid, which indicates a positive iodoform reaction.
This test is quite useful in organic chemistry for determining the presence of secondary alcohols or methyl ketones. But remember, the test is not applicable to compounds without this specific group, like non-methyl ketones or primary alcohols.

Benedict's Test

Benedict's Test is a simple and rapid test used primarily to detect reducing sugars and aldehydes. Reducing sugars like glucose, fructose, and aldehydes like acetaldehyde readily undergo oxidation and provide this test its utility. The presence of the aldehyde group in acetaldehyde means it can react with Benedict's reagent — a solution containing cupric ions (Cu²⁺).
When acetaldehyde is heated with Benedict's solution, the aldehyde group is oxidized, while the cupric ions are reduced to red cuprous oxide (Cu₂O). This result is visually identified by a color change from blue to red, green, or yellow, depending on the concentration of the aldehyde present. However, it's important to note that not all compounds will yield a positive result; especially non-reducing sugars and compounds lacking an oxidizable group.

Lucas Test

The Lucas Test is a crucial laboratory technique to distinguish between primary, secondary, and tertiary alcohols based on their reactivity with the Lucas reagent, which is a mixture of concentrated hydrochloric acid and zinc chloride (ZnCl₂). Tertiary alcohols react immediately to form a cloudiness or emulsion, secondary alcohols take several minutes, whereas primary alcohols react very slowly or not at all.
Since acetaldehyde is an aldehyde and not an alcohol, it doesn't react with the Lucas reagent. This absence of reactivity is due to the structural difference; the Lucas test requires the presence of an alcohol group (\[\text{R-OH}\]). Therefore, acetaldehyde does not give a positive Lucas Test, demonstrating its utility in differentiating aldehydes from alcohols.

Tollen's Test

Tollen's Test is a classic laboratory test used to identify the presence of the aldehyde group by leveraging its ability to be oxidized. The reagent involved is Tollen's reagent, a mildly basic solution of silver nitrate (AgNO₃) and ammonia (NH₃). When an aldehyde like acetaldehyde is introduced, it gets oxidized to its corresponding carboxylic acid, facilitating the reduction of silver ions to metallic silver.
This reaction results in the formation of a silver mirror on the surface of the test container, a distinct indicator of a positive Tollen's Test. Acetaldehyde, having an aldehyde group, exhibits this spectacular silver mirror effect. Such reactions are commonly used to differentiate aldehydes from ketones, as ketones generally do not react. However, caution should be exercised as some keto-enols may also give false positives.