Decoding the Question
Imagine you are standing in the chemistry lab, looking at a variety of cleansing agents. The question asks us to identify an anionic detergent from the given options. To solve this, we need to understand two key concepts: what makes a substance a detergent, and what makes it anionic.
Detergents are synthetic cleansing agents that have all the properties of soaps but do not actually contain any soap. They are incredibly useful because they can function even in hard water. Based on the charge of their active part, detergents are classified into three main categories: anionic, cationic, and non-ionic. An anionic detergent is one where the large part of the molecule involved in the cleansing action is negatively charged (an anion).
Analyzing the Options
Let's break down each option to see where it fits in our classification.
Option (a): Sodium lauryl sulphate
The chemical formula for sodium lauryl sulphate is CH3​(CH2​)10​CH2​OSO3−​Na+. When this molecule dissociates in water, it forms a large lauryl sulphate anion and a small sodium cation. Because the active cleansing part is the large anion, this is classified as an anionic detergent.
Option (b): Cetyltrimethyl ammonium bromide
This compound has the formula [CH3​(CH2​)15​N(CH3​)3​]+Br−. Here, the active part of the molecule is the large cetyltrimethyl ammonium cation. Therefore, it acts as a cationic detergent. These are often used in hair conditioners due to their germicidal properties.
Option (c): Glyceryl oleate
Glyceryl oleate is an ester formed from glycerol and oleic acid. It does not dissociate into ions in water. Since it lacks any ionic charge, it falls under the category of non-ionic detergents. These are commonly found in liquid dishwashing detergents.
Option (d): Sodium stearate
This is where many students make a silly mistake! The formula for sodium stearate is C17​H35​COO−Na+. While it does dissociate to form a large anion, it is actually the sodium salt of a long-chain fatty acid (stearic acid). This makes it an anionic soap, not a synthetic detergent. The question specifically asks for a detergent, so this option is incorrect.
The Final Verdict
After carefully analyzing all the options, it is clear that sodium lauryl sulphate is the only compound that fits the criteria of being both anionic and a synthetic detergent.
Therefore, the correct answer is Option (a).