Problem 181 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Which of the following statements about benzyl chloride is incorrect? (a) It is less reactive than alkyl halides (b) It can be oxidised to benzaldehyde by boiling with copper nitrate solution (c) It is a lachrymatory liquid and answers Beilstein's test (d) It gives a white precipitate with alcoholie silver nitrate

Step-by-Step Solution

Answer

Statement (a) is incorrect.

Step 1: Understand the Question

The problem asks you to identify which statement about benzyl chloride is incorrect. We must evaluate the reactivity and chemical behavior of benzyl chloride to determine which statement is not true.

Step 2: Analyze Statement (a)

Statement (a) claims that benzyl chloride is less reactive than alkyl halides. Benzyl chloride, however, is more reactive due to the resonance stabilization of its intermediate carbocation. Hence, this statement is incorrect.

Step 3: Evaluate Statement (b)

Statement (b) asserts that benzyl chloride can be oxidized to benzaldehyde by boiling with copper nitrate solution. Benzyl chloride is typically not oxidized to benzaldehyde under these conditions, making this statement potentially incorrect or dubious depending on reaction specifics.

Step 4: Consider Statement (c)

Statement (c) provides two characteristics: benzyl chloride being a lachrymatory liquid and giving a positive Beilstein test (green flame test for halogen presence). Both of these properties are correct for benzyl chloride since it contains chlorine.

Step 5: Review Statement (d)

Statement (d) claims that benzyl chloride gives a white precipitate with alcoholic silver nitrate. This is true as benzyl chloride is a halide, which reacts with silver nitrate to form a precipitate of silver chloride.

Step 6: Determine the Answer

Based on our analysis, statement (a) is incorrect. Benzyl chloride is actually more reactive than alkyl halides, contrary to what the statement suggests.

Key Concepts

Reactivity of Benzyl Chloride

Benzyl chloride is known for its higher reactivity compared to other alkyl halides. This increased reactivity stems from its unique structural feature, which includes a benzene ring attached to the chloromethyl group. The benzene ring can stabilize the positive charge of the carbocation intermediate through resonance.
This stabilization allows benzyl chloride to undergo nucleophilic substitution reactions more readily than many other halides.
In essence, the presence of the benzene ring adjacent to the chloromethyl group modifies the electronic environment, adding to the reactivity of benzyl chloride.

• This makes benzyl chloride quite reactive, especially in substitution reactions.
• It is typically more reactive than simple alkyl halides like ethyl chloride.

Hence, the statement that benzyl chloride is less reactive than alkyl halides turns out to be incorrect.

Oxidation of Benzyl Chloride

Oxidation often involves converting an organic molecule to one with more oxygen atoms or fewer hydrogen atoms. Benzyl chloride can be transformed into benzaldehyde, but this transformation does not conventionally occur with copper nitrate solution.
Various chemicals can facilitate this type of oxidation, but copper nitrate is not commonly one of them.
The typical oxidation of benzyl chloride involves different oxidizing agents, such as peroxides, which help in achieving this transformation more effectively.

• In standard reactions, such conditions are avoided for converting benzyl chloride to benzaldehyde.
• The assertion that benzyl chloride can be oxidized to benzaldehyde by copper nitrate is misleading.

Oxidations usually require specific and appropriate conditions for the effective conversion of benzyl chloride.

Chemical Tests for Benzyl Chloride

Benzyl chloride is characterized using several chemical tests that confirm its composition and reactivity. One such test is the Beilstein test, where benzyl chloride produces a green flame when exposed to a copper wire flame — indicating the presence of a halogen, like chlorine.
Additionally, benzyl chloride reacts with alcoholic silver nitrate to form a white precipitate of silver chloride, confirming its status as a halogenated compound.
These reactions demonstrate the effectiveness of chemical tests in identifying specific structural aspects of benzyl chloride.

• The lachrymatory – or tear-inducing – nature of benzyl chloride is another signature trait confirming its identity.
• The white precipitate from the silver nitrate test is a conclusive indicator of chlorine presence.

Such chemical tests are crucial for the practical identification and confirmation of compounds like benzyl chloride in a lab setting.