Problem 51 - 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

(d) I, II, III, IV

Step 1: Understanding Surfactants

Surfactants, or surface-active agents, are compounds that lower the surface tension between two substances, such as liquid and gas, liquid and liquid, or liquid and solid. They typically have a hydrophobic tail (often a long hydrocarbon chain) and a hydrophilic head (often charged or polar).

Step 2: Analyzing Compound I

The compound \( \mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{15}-\left(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3} \overline{\mathrm{Br}}\right) \) consists of a long hydrophobic hydrocarbon tail and a hydrophilic quaternary ammonium head with bromide ion as a counterion, making it a surfactant.

Step 3: Analyzing Compound II

The compound \( \mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2} \mathrm{NH}_{2} \) has a long hydrocarbon chain that is hydrophobic and an amine group \( \mathrm{NH}_{2} \) which can participate in hydrogen bonding, providing some hydrophilicity, making it a potential surfactant.

Step 4: Analyzing Compound III

The compound \( \mathrm{CH}_{3}-\left(\mathrm{CH}_{2}-\right)_{16}-\mathrm{CH}_{2} \mathrm{O} \mathrm{SO}_{3} \mathrm{Na} \) includes a long hydrophobic hydrocarbon tail and a hydrophilic sulfate group \( \mathrm{SO}_{3}^{-} \) with sodium as a counterion, making it a surfactant.

Step 5: Analyzing Compound IV

For the compound \( \mathrm{OHC}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}\left(\mathrm{COO}^{+} \mathrm{Na}\right) \), the long hydrocarbon tail is hydrophobic and the carboxylate group \( \mathrm{COO}^{-} \) with sodium as a counterion provides hydrophilicity, making it a surfactant.

Step 6: Conclusion: Selection of Correct Surfactants

All the compounds (I, II, III, and IV) have the structural characteristics of a surfactant with distinct hydrophobic and hydrophilic parts.

Key Concepts

Hydrophobic tail

In surfactants, the hydrophobic tail plays a crucial role. The term "hydrophobic" essentially means "water-fearing," and this part of the molecule is typically comprised of a long chain of hydrocarbons. These hydrocarbon chains are not attracted to water; instead, they prefer to associate with oils and other nonpolar substances. This property allows surfactants to mix and stabilize substances that would usually remain separate, such as oil and water. Surfactants achieve this by embedding their hydrophobic tails into oil droplets, while the other end of the surfactant remains in the water. This action is what enables surfactants to break up grease and form emulsions.

Hydrophilic head

The hydrophilic head of a surfactant is the component that is "water-loving." This head often contains polar or ionic groups that can form hydrogen bonds or ionic interactions with water molecules.
Here are some common characteristics:

• Polar or charged groups like -SO₃Na, -COO^-Na, or quaternary ammonium groups.
• These compositions grant the ability to interact favorably with water.
• Counterions such as Na⁺ or Br^- may also accompany the hydrophilic head to balance charge.

The interplay of the hydrophilic head and the hydrophobic tail allows surfactants to work effectively at interfaces between differing phases, thereby facilitating processes such as cleaning, emulsification, and dispersion.

Surface tension reduction

Lowering surface tension is a key function of surfactants. Surface tension, which results from the cohesive forces between liquid molecules, is the source of many fluids' resistance to external forces. Surfactants reduce this tension by positioning themselves at the interface, with hydrophobic tails in the air or oil and hydrophilic heads in water. This arrangement weakens the cohesive forces at the liquid's surface.

• This makes it easier for the liquid to spread or wet surfaces.
• It can help in spreading cleaners or creating bubbles and foams in soaps and detergents.
• This capacity to alter surface tension is essential in numerous applications, from cleaning agents to enhancing oil recovery.

This reduction has vast practical importance in both industrial and everyday contexts, making surfactants invaluable in multiple industries.

Chemical analysis

Understanding surfactants involves a good grasp of chemical analysis. This is the process of identifying the chemical components and functional groups in a compound to determine its properties and behavior. In the context of surfactants:

• Identifying hydrophobic and hydrophilic parts of a molecule aids in predicting its interfacial behavior.
• Techniques such as spectroscopy may be used to identify functional groups like -SO₃Na or quaternary ammonium.
• Analyzing the length and structure of hydrocarbon chains can provide insight into the overall efficiency and functionality of a surfactant.

These analyses help in designing surfactants that can efficiently lower surface tension and perform their intended function in a given application, such as in detergents or emulsifiers.

Organic chemistry

Organic chemistry is the backbone of understanding surfactants, which are organic compounds composed mainly of carbon and hydrogen atoms along with other elements. The field encompasses the study of the structure, composition, reactions, and properties of organic molecules:

• It provides insight into the behavior of the hydrocarbon chains and functional groups within surfactants.
• Understanding molecular interactions at the atomic level helps elucidate how surfactants arrange themselves at interfaces.
• Organic chemistry principles are used to synthesize new surfactants with improved properties or specific functions.

In sum, a strong foundation in organic chemistry enables chemists to innovate and create more effective surfactants that play crucial roles in industries like pharmaceuticals, cosmetics, and food production.