Question

What happens when: p-fluorotoluene is treated with \( CrO_{3} \) in the presence of acetic anhydride followed by hydrolysis with aqueous acid?

Hint:

This is a variation of the Etard reaction. Using acetic anhydride is key to stopping the oxidation at the aldehyde stage.

Answer 1

Concept:

• Chromic anhydride (\( CrO_{3} \)) in acetic anhydride is used to partially oxidize the methyl group of an aromatic ring to an aldehyde.

• To prevent over-oxidation to a carboxylic acid, the intermediate is trapped as a gem-diacetate.

• Subsequent hydrolysis of the diacetate yields the aldehyde.
Step 1: Formation of the intermediate
p-fluorotoluene reacts with \( CrO_{3} \) and acetic anhydride to form p-fluorobenzylidene diacetate. \[ p\text{-F}-C_{6}H_{4}-CH_{3} \xrightarrow{CrO_{3}, (CH_{3}CO)_{2}O} p\text{-F}-C_{6}H_{4}-CH(OCOCH_{3})_{2} \]

Step 2: Hydrolysis
Acidic hydrolysis of the benzylidene diacetate replaces the two acetate groups with one oxygen atom, forming the aldehyde. \[ p\text{-F}-C_{6}H_{4}-CH(OCOCH_{3})_{2} \xrightarrow{H_{3}O^{+}} p\text{-F}-C_{6}H_{4}-CHO \]

Step 3: Identify the product
The resulting product is 4-fluorobenzaldehyde (or p-fluorobenzaldehyde). The final answer is 4-fluorobenzaldehyde.