Question

Which of the following sets of reagents convert toluene to benzaldehyde?

• A. \( B, C, D \)
• B. \( A, C \)
• C. \( A, D \)
• D. \( B, D \)

Hint:

To convert toluene to benzaldehyde, use selective oxidation methods such as the Etard reaction (CrO\(_2\)Cl\(_2\)) or controlled halogenation with Cl\(_2\)/hv, H\(_2\)O.

Answer 1

The Correct Option is C

Step 1: Identify the Reagents for Partial Oxidation
The conversion of toluene to benzaldehyde involves partial oxidation. The effective reagents for this reaction are:
- Chromyl chloride (\( CrO_2Cl_2 \)) in CS\(_2\), H\(_3O^+\) (Etard reaction)
- Chlorine (\( Cl_2 \)) in the presence of light and water Step 2: Explanation
- \( A: Cl_2/ hv, H_2O, \triangle \) → Effective (Partial oxidation of toluene to benzaldehyde)
- \( B: Cl_2/Fe; H_2O \) → Ineffective (Leads to benzoic acid)
- \( C: KMnO_4/ OH^-; H^+ \) → Ineffective (Complete oxidation to benzoic acid)
- \( D: CrO_2Cl_2/CS_2; H_3O^+ \) → Effective (Etard reaction forms benzaldehyde)
Thus, the correct answer is \( A, D \). \bigskip

Answer 2

To convert toluene to benzaldehyde, the methyl group must be selectively oxidized to an aldehyde group without further oxidation to the acid.

Option A: \( \text{Cl}_2 | h\nu; \; \text{H}_2\text{O}, \Delta \)
- Chlorination under light produces benzyl chloride.
- Hydrolysis and controlled oxidation convert benzyl chloride to benzaldehyde.
So, option A is correct.

Option B: \( \text{Cl}_2 | \text{Fe}; \; \text{H}_2\text{O} \)
- Chlorination with Fe catalyst mainly gives chlorobenzene via electrophilic substitution, not benzyl chloride.
So, option B is incorrect.

Option C: \( \text{KMnO}_4 | \text{OH}^-; \; \text{H}^+ \)
- Strong oxidation with KMnO4 converts methyl group fully to benzoic acid.
So, option C is incorrect.

Option D: \( \text{CrO}_2\text{Cl}_2 | \text{CS}_2; \; \text{H}_3\text{O}^+ \)
- This reagent is known for the Etard reaction, selectively oxidizing methyl to aldehyde.
So, option D is correct.

Therefore, the correct sets of reagents are:
\[ \boxed{A, D} \]