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 Tests

The given options are chemical tests used to identify specific functional groups in molecules. The Iodoform test identifies methyl ketones or compounds convertible to methyl ketones. The Benedict's test detects reducing sugars. The Lucas test distinguishes alcohols based on reactivity. The Tollen's test identifies aldehydes and reducing sugars.

Step 2: Analyzing Acetaldehyde

Acetaldehyde (2H_4O ext{ or }CH_3CHO) is an aldehyde. It has a methyl group attached to the carbonyl carbon, allowing it to form iodoform in the Iodoform test. As an aldehyde, it can be oxidized, showing a positive reaction with Benedict's test. As an aldehyde, it can reduce Tollens' reagent, giving a positive test.

Step 3: Checking Lucas Test Applicability

The Lucas test is designed to differentiate primary, secondary, and tertiary alcohols based on solubility in the Lucas reagent, containing HCl ext{ and }ZnCl_2. Acetaldehyde isn't an alcohol, hence it cannot undergo the Lucas test effectively.

Key Concepts

Iodoform test

The Iodoform test is a specialized chemical test utilized to identify the presence of methyl ketones or compounds that can be converted into methyl ketones. During this test, iodine and a base, such as sodium hydroxide, are reacted with the compound in question. If a yellow precipitate, known as iodoform, is formed, it indicates a positive result for the presence of a methyl ketone. Acetaldehyde (CH₃CHO) can indeed exhibit a positive Iodoform test because it has a methyl group attached to its carbonyl carbon, which can be oxidized to form a methyl ketone. This structural feature allows acetaldehyde to undergo the reaction and produce iodoform, thus passing the test.

Tollen's test

Tollen's test is an essential test for identifying aldehydes. It utilizes Tollen's reagent, which is an ammoniacal silver nitrate solution. When exposed to an aldehyde, the aldehyde reduces the silver ion in the reagent to metallic silver. This results in a silver mirror forming on the inside of a test tube, indicating a positive result. For acetaldehyde, Tollen's test is significant because acetaldehyde possesses an aldehyde group. On reaction with Tollen's reagent, acetaldehyde reduces the silver ions, resulting in a visible silver mirror. This reaction confirms that acetaldehyde can indeed exhibit a positive result in Tollen's test.

Lucas test

The Lucas test is used primarily for identifying the type of alcohol present in a compound—either primary, secondary, or tertiary. It employs the Lucas reagent, which is a mixture of concentrated hydrochloric acid and zinc chloride. The test measures the reaction rate of alcohols with this reagent to form alkyl chlorides, which results in a two-phase system with a cloudy solution indicating a positive test. Acetaldehyde, however, is not an alcohol, but an aldehyde, making it inapplicable for the Lucas test. This is because the Lucas test is specifically designed for alcohols, and since acetaldehyde lacks the necessary functional group inherent in alcohols, it cannot react in this test.

Benedict's test

Benedict's test is a widely used method for detecting reducing sugars, as well as some aldehydes and alpha-hydroxy-ketones. The Benedict’s solution, typically a blue copper(II) sulfate solution, is reduced by reducing sugars and aldehydes to form a red or orange precipitate of copper(I) oxide, signaling a positive result. Acetaldehyde, featuring an aldehyde group, can indeed participate in Benedict's test, serving as a reducing agent to bring about this color change. The presence of a carbonyl group in acetaldehyde facilitates its ability to reduce the copper(II) ions to copper(I), yielding a positive Benedict's test, thus confirming that acetaldehyde behaves as a reducing agent in this context.