Question:
Why does an old sample of benzaldehyde turn blue litmus red?
Why does an old sample of benzaldehyde turn blue litmus red?
Show Hint
Aldehydes slowly react with atmospheric oxygen on standing, and benzaldehyde forms acidic benzoic acid this way.
Updated On: Jul 3, 2026
- Due to the conversion of benzaldehyde to phenol
- Due to the presence of acid impurity with benzaldehyde
- Due to aerial oxidation of benzaldehyde
- Benzaldehyde is acidic in nature
Show Solution
Verified By Collegedunia
The Correct Option is C
Solution and Explanation
Step 1: Benzaldehyde does not have a hydrogen directly bonded to its carbonyl oxygen, so pure benzaldehyde itself is essentially neutral and will not turn blue litmus red.
Step 2: Aldehydes are air-sensitive. On standing in contact with atmospheric oxygen over time, an aldehyde slowly undergoes autoxidation at the carbonyl carbon-hydrogen bond.
Step 3: For benzaldehyde this aerial oxidation converts some of the sample into benzoic acid: \[C_6H_5CHO + \tfrac{1}{2}O_2 \xrightarrow{\text{air}} C_6H_5COOH\]
Step 4: Benzoic acid is a genuine carboxylic acid, so once it accumulates in an old, air-exposed sample of benzaldehyde, it turns blue litmus red.
Step 5: The root cause is the slow reaction of benzaldehyde with atmospheric oxygen, aerial oxidation.
\[\boxed{\text{Aerial oxidation of benzaldehyde to benzoic acid}}\]Was this answer helpful?
0
0
Top CPET Chemistry Questions
- Which intermediate is formed during the reaction of acetamide with bromine in the presence of excess KOH solution?
- What is the major product of the reaction between benzene and iso-butyl bromide in the presence of AlCl3?
- Which product is obtained when 1-pentene reacts with HCl in the presence of benzoyl peroxide?
- What is the product of the reaction between n-butyl benzene and alcoholic KMnO4?
- Which alkene can be used for the synthesis of propanal exclusively using ozonolysis?
View More Questions
Top CPET Chemical Reactions of Aldehydes Ketones and Carboxylic Acids Questions
Top CPET Questions
- Which of the following is a key reason for choosing DRAM over SRAM for main memory?
- Which of the following components in an FPGA is responsible for implementing combinational and sequential logic?
- What is the main purpose of declaring a base class as virtual in multiple inheritance?
- What happens if a class contains at least one pure virtual function?
- What happens if a derived class defines a function with the same name as a base class function, but with a different parameter list?
View More Questions