Question

Which of the following is an aldehyde?

• A. CH₃–CO–H (acetaldehyde)
• B. CH₃–CH₂–CO–CH₃ (butan-2-one)
• C. CH₃–CH₂–CO–CH₂–CH₃ (pentan-3-one)
• D. CH₃–CO–CH₃ (acetone)

Hint:

An aldehyde has an H on the carbonyl carbon (–CHO); ketones have two carbons on it.

Answer 1

The Correct Option is A

Step 1: An aldehyde contains the carbonyl group with at least one hydrogen atom attached to the carbonyl carbon, i.e. the –CHO group. A ketone has the carbonyl carbon bonded to two carbon atoms (C–CO–C).
Step 2: Option (i) CH₃–CO–H is CH₃CHO (acetaldehyde). Its carbonyl carbon carries an H atom, so it is an aldehyde.
Step 3: Options (ii), (iii) and (iv) are butan-2-one, pentan-3-one and acetone respectively; in each the carbonyl carbon is flanked by two carbons, making them ketones, not aldehydes.
Step 4: Hence the aldehyde is option (i).