Q.Explain why the lower aldehydes and ketones (up to about four carbons) are miscible with water, while their solubility falls away sharply as the carbon chain lengthens.

The lower members of the aldehyde series (e.g. formaldehyde, acetaldehyde) have sharp pungent odours. As the molecular size increases, the odour beco …

1. Understanding the Assertion: solubility trend with alkyl size

   Aldehydes and ketones contain a carbonyl group ($\ce{C=O}$) which is polar — the oxygen is partially negative and the carbon partially positive. This polarity allows the carbonyl oxygen to form hydrogen bonds with water molecules. For small aldehydes (like formaldehyde, $\ce{HCHO}$) and small ketones (like acetone, $\ce{CH3COCH3}$), the polar carbonyl group dominates, so they mix freely with water.

   As the alkyl chain (the R groups attached to the carbonyl) gets longer, the molecule becomes more hydrophobic — the nonpolar hydrocarbon part grows, and it becomes harder for water molecules to surround and solvate the entire molecule. The hydrogen bonding with water is still there, but the large nonpolar region disrupts water's structure, making dissolution less favourable. So solubility decreases as the alkyl group size increases. This is a well-known trend: formaldehyde is infinitely soluble, acetone is miscible, but hexanal is only slightly soluble.

2. Examining the Reason: dipole-dipole interaction

   The Reason states that aldehydes and ketones have dipole-dipole interaction. This is true — the $\ce{C=O}$ bond is polar, so there is a permanent dipole moment. In the pure liquid, molecules interact via dipole-dipole forces. However, this fact alone does not explain why solubility in water changes with alkyl size. The solubility trend is governed by the competition between hydrogen bonding with water (which favours solubility) and the hydrophobic effect of the alkyl chain (which opposes it). The Reason is a true statement, but it is not the correct explanation for the Assertion.

3. Connecting the two statements

   • Assertion (A): True.
   • Reason (R): True (aldehydes and ketones do have dipole-dipole interactions). …