Q.Which of the following is used as a soap?

Biodegradability hinges on whether microorganisms can break down a molecule. Bacteria in soil and water possess enzymes that cleave long hydrocarbon chains step-by-step, typically starting from the terminal carbon and working inward through a process called β-oxidation. This works beautifully when the chain is linear—the enzyme can "walk" along it smoothly. But throw in extensive branching, and the enzyme's active site can no longer fit properly around the substrate. The branches create steric hindrance, blocking the enzyme's access.

Early synthetic detergents used alkylbenzene sulfonates (ABS) with highly branched alkyl side chains—often derived from propylene tetramers, which produced structures like this:

attached to a chain such as $\ce{-CH(CH3)-CH2-CH(CH3)-CH2-CH(CH3)-...}$

These branched chains accumulated in rivers and lakes, famously causing foam mountains in the 1960s because bacteria couldn't degrade them.

Why structure matters

1. Linear chains (found in soaps and modern biodegradable detergents like linear alkylbenzene sulfonates, LAS) present a smooth surface for enzymatic attack. Bacterial lipases and oxidases can sequentially remove two-carbon units.

2. Branched chains, especially those with quaternary carbons or methyl groups every few carbons, physically obstruct the enzyme. The branching points cannot be processed by the standard β-oxidation pathway, so the molecule persists. …

The question asks for the class of synthetic detergent used in toothpaste, not just a brand name. To answer this, we need to understand what detergents do in toothpaste and how they are classified.

Toothpaste needs a substance that lowers surface tension so that the paste can spread easily, create foam, and help dislodge debris from teeth. This is the job of a surfactant (surface-active agent). Synthetic detergents are a type of surfactant.

1. Classification of synthetic detergents: Synthetic detergents are broadly classified based on the charge on the hydrophilic (water-loving) part of the molecule:

   • Anionic detergents: The hydrophilic part carries a negative charge (e.g., sodium lauryl sulfate).
   • Cationic detergents: The hydrophilic part carries a positive charge (e.g., cetyltrimethylammonium bromide).
   • Non-ionic detergents: The hydrophilic part has no charge (e.g., polyoxyethylene alkyl ethers).

2. Why anionic detergents are used in toothpaste: The most common synthetic detergent in toothpaste is sodium lauryl sulfate (SLS). SLS is an anionic detergent because its lauryl sulfate ion ($\text{C}_{12}\text{H}_{25}\text{OSO}_3^-$) carries a negative charge. It is chosen because:

   • It is an excellent foaming agent, which gives the satisfying lather users expect.
   • It is effective at removing oily food residues and plaque from teeth.
   • It is relatively inexpensive and stable in the toothpaste formulation. …