Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following are surjactants? (I) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{15}-\left(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3} \overline{\mathrm{Br}}\right)\) (II) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2} \mathrm{NH}_{2}\) (III) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}-\right)_{16}-\mathrm{CH}_{2} \mathrm{O} \mathrm{SO}_{3} \mathrm{Na}\) (IV) \(\mathrm{OHC}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}\left(\mathrm{COO}^{-} \mathrm{Na}\right)\) (a) I, II, III (b) II, III, IV (c) I, III, IV (d) I, II, III, IV

Step-by-Step Solution

Answer

Compounds I, III, and IV are surfactants (option c).

Step 1: Understand Surfactant Structure

Surfactants generally consist of a hydrophobic tail and a hydrophilic head, which helps them reduce surface tension and mix water with oils. In this task, identify each compound as either possessing or lacking these properties.

Step 2: Analyze Compound I

Compound I, \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{15}-\left(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\overline{\mathrm{Br}}\right),\) has a long hydrophobic tail of 16 carbon atoms and a hydrophilic head (a quaternary ammonium group with a bromide ion as a counter ion). This compound acts effectively as a surfactant.

Step 3: Analyze Compound II

Compound II,\(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}\mathrm{NH}_{2},\) though possessing a long hydrocarbon chain, has an amino group at the end, which makes it more hydrophilic. However, the amino group is not as effective in surfactant action as other common hydrophilic heads, potentially limiting its efficacy as a surfactant unless it's a part of a better-designed molecule.

Step 4: Analyze Compound III

Compound III,\(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}-\right)_{16}-\mathrm{CH}_{2}\mathrm{O} \mathrm{SO}_{3} \mathrm{Na},\) has a hydrophobic tail along with a head containing the sulfate group attached via an ether linkage, which is typical in many common surfactants. This is a clear surfactant.

Step 5: Analyze Compound IV

Compound IV,\(\mathrm{OHC}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}\left(\mathrm{COO}^{-}\mathrm{Na}\right),\) contains a hydrophobic tail and a hydrophilic head (carboxylate anion with a sodium ion) capable of interacting with polar and nonpolar substances. It acts as a surfactant.

Step 6: Conclusion

Based on the analysis, compounds I, III, and IV demonstrate typical surfactant structures with effective hydrophobic and hydrophilic properties. Therefore, options II, III, and IV are actual surfactants considering the typical design.

Key Concepts

Understanding the Hydrophobic Tail

In the world of surfactants, the hydrophobic tail plays a crucial role in determining how effectively the surfactant can interact with oils and fats. Imagine this tail as a long chain made up of carbon and hydrogen atoms.
These long chains are not attracted to water, hence the term 'hydrophobic,' which literally means 'water fearing.' The primary task of the hydrophobic tail is to connect with oil molecules. By forming strong interactions with oils, these tails allow the surfactant to pull the oils into water. The longer the tail, the stronger the hydrophobic effect.
Examples of hydrophobic tails in the exercise include various hydrocarbon chains like \( \mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{15} \) found in several compounds assessed.

Exploring the Hydrophilic Head

The hydrophilic head of a surfactant is the component that loves water, i.e., 'water loving.' This is the part of the surfactant that interacts with water molecules, making oils and fats more soluble in water. In chemical terms, these heads often contain groups that can participate in hydrogen bonding or electrostatic interactions.Surfactants utilize these hydrophilic heads to attract water molecules, pulling them closer to the oil they are bonded to. This interaction is what allows surfactants to dissolve impurities in water, making them essential in cleaning agents.
In our exercise, examples of hydrophilic heads are the quaternary ammonium group \( \left(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\right)\) and the sulfate group \( \mathrm{O} \mathrm{SO}_{3} \mathrm{Na}\).

How Surfactants Reduce Surface Tension

Surface tension is a property of liquids that makes them behave like stretched elastic sheets. It arises because molecules in a liquid are more attracted to each other than they are to air molecules. Surfactants help reduce this surface tension by inserting themselves between water molecules at the surface, weakening their interactions. This makes water spread out more easily over surfaces and penetrate tiny spaces between particles of dirt and oil. By reducing surface tension, surfactants enhance the wetting ability of water and help in emulsifying oils, which significantly improves the cleaning process. When you think about the difference a drop of soap can make to a pool of water, it's all thanks to this property.

Understanding Chemical Structures of Surfactants

The ability of a compound to act as a surfactant depends largely on its chemical structure. For a molecule to be an effective surfactant, its structure must include both hydrophobic and hydrophilic components. In our exercise, the chemical structures presented have these elements in combinations that determine their potential to reduce surface tension.
Effective surfactants often have well-balanced structures that optimize their 'dual-nature'—one that allows them to dissolve in oil and water. This balance can be seen in common structural patterns involving quaternary ammonium, carboxylate, and sulfate groups. Understanding the chemical structures enables chemists to design surfactants that are targeted for specific applications, such as detergents, wetting agents, or dispersants.