Question

Give reasons for the following : (a) Alkyl halides, though polar, yet their solubility in water is very low.

Hint:

Polarity alone does not guarantee water solubility. The ability to form hydrogen bonds with water is often more important.

Answer 1

Concept: The solubility of a substance in water depends not only on its polarity but also on its ability to form strong intermolecular interactions with water molecules. Water is a highly polar solvent and possesses extensive intermolecular hydrogen bonding. For a compound to dissolve readily in water, the new interactions formed between the solute and water molecules must compensate for the energy required to break the hydrogen bonds present among water molecules.

Step 1: Nature of alkyl halides. Alkyl halides contain a polar carbon-halogen bond: \[ R-X \] where \(X = F, Cl, Br, I\). Due to the difference in electronegativity between carbon and halogen, the bond possesses polarity. Thus alkyl halides are polar molecules.

Step 2: Interaction with water. Water molecules are strongly associated through hydrogen bonding. \[ H-O-H \cdots H-O-H \] To dissolve an alkyl halide, some of these hydrogen bonds must be broken. However, alkyl halides are unable to form hydrogen bonds with water molecules to any appreciable extent.

Step 3: Energy consideration. The energy required to break the strong hydrogen bonds between water molecules is not compensated by the weak interactions formed between alkyl halide molecules and water. Therefore, dissolution is not energetically favourable.

Step 4: Conclusion. Although alkyl halides are polar, they cannot form strong hydrogen bonds with water and hence possess very low solubility in water. \[ \boxed{\text{Alkyl halides are sparingly soluble in water because they cannot form strong hydrogen bonds with water molecules.}} \]