Which class of organic compound must contain at least three carbon atoms? A - HSC - SSCE Chemistry - Question 10 - 2019 - Paper 1

To determine which class of organic compounds must contain at least three carbon atoms, we will evaluate each option:

1. Aldehydes: Aldehydes can have one or more carbon atoms, with the simplest being formaldehyde, which has only one carbon atom. Therefore, hydroxyl derivatives of aldehydes can contain fewer than three carbon atoms.

2. Alkenes: Similar to aldehydes, alkenes can have as few as two carbon atoms, with ethene being the simplest example. Thus, alkenes do not necessarily require three carbon atoms.

3. Carboxylic acids: The simplest carboxylic acid, formic acid, contains only one carbon atom. Therefore, carboxylic acids can also have fewer than three carbon atoms.

4. Ketones: Ketones need at least three carbon atoms because they are characterized by having a carbonyl group (C=O) between two carbon atoms. The simplest ketone, acetone, contains three carbon atoms.

In conclusion, the class of organic compound that must contain at least three carbon atoms is D. Ketones.