The hydrophilic end of a synthetic detergent is :

Question

Accepted Answer

The correct answer is Option 3: $SO\overline{_3} Na^+$.

1. **Explanation for Option 3 ($SO\overline{_3} Na^+$):**
   - Synthetic detergents are composed of molecules that have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts. The hydrophilic end is typically a polar or ionic group that can interact with water molecules.
   - The $SO\overline{_3} Na^+$ group is a sulfonate group, which is highly polar and ionic. This makes it very effective at interacting with water molecules, thus making it hydrophilic. The presence of the sodium ion ($Na^+$) further enhances its solubility in water, as it can dissociate and interact with water molecules.
   - Sulfonate groups are commonly used in synthetic detergents because they provide excellent solubility and cleaning properties, making them ideal for the hydrophilic end of detergent molecules.

2. **Explanation for why other options are incorrect:**

- **Option 1 ($CH_3-(CH_2)_{10}-CH_2-$):**
     - This is a hydrocarbon chain, which is non-polar and hydrophobic. Hydrocarbon chains do not interact well with water and are typically the hydrophobic part of a detergent molecule, not the hydrophilic part.

- **Option 2 ($CO\overline{_3} Na^+$):**
     - This represents a carbonate group. While carbonate ions can interact with water, they are not typically used as the hydrophilic end in synthetic detergents. The sulfonate group ($SO\overline{_3} Na^+$) is more effective and commonly used for this purpose due to its superior solubility and cleaning properties.

- **Option 4 ($COO^- Na^+$):**
     - This is a carboxylate group, which can be hydrophilic. However, in the context of synthetic detergents, sulfonate groups are preferred for their enhanced solubility and stability in various pH conditions. Carboxylate groups are more commonly found in soaps rather than synthetic detergents.

In conclusion, Option 3 ($SO\overline{_3} Na^+$) is the correct answer because it represents a sulfonate group, which is highly effective as the hydrophilic end of a synthetic detergent due to its ionic nature and excellent solubility in water.