Arrange propan-1-ol, butanal and butane, all of comparable molar mass, in increasing order of boiling point, and explain the order in terms of intermolecular forces.

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Butane is non-polar and held together only by weak dispersion forces. Butanal is polar (the $\text{C=O}$ dipole gives dipole-dipole attraction between molecules) but has no $\text{O-H}$ of its own, so it cannot hydrogen-bond to a second molecule of itself. Propan-1-ol has an $\text{O-H}$ group and so hydrogen-bonds extensively between its own molecules, which is the strongest of the three intermolecular forces at comparable molar mass. Stronger intermolecular attraction requires more energy to overcome on boiling, …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set V11 mark
$Q. The lower aldehydes have sharp ________ odours.$
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The lower (smaller) aldehydes have sharp, unpleasant, pungent odours.

The lower members of the aldehyde series (e.g. formaldehyde, acetaldehyde) have sharp pungent odours. As the molecular size increases, the odour beco …
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$Q. True/False: Lower aldehydes are soluble in water.$
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True — lower aldehydes (e.g. formaldehyde, acetaldehyde) are water-soluble because they can hydrogen-bond with water.

Although aldehydes cannot hydrogen-bond with each other (no O-H or N-H), the carbonyl oxygen of an aldehyde can accept a hydrogen bond from a water molecule. For the first few members of the aldehyde series (up to about 4 carbons), this hydrogen bonding with water outweighs the hydrophobic effect of the small alkyl chain, so they are miscible with or highly soluble in water. Solubilit …

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CBSE 2025Set 56/5/11 markMCQ
$Q. Two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) as given below. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true. Assertion (A) : The solubility of aldehydes and ketones in water decreases with increase in size of the alkyl group. Reason (R) : Aldehydes and ketones have dipole-dipole interaction.$
›Reveal solutionSolution
The key idea is that solubility in water depends on hydrogen bonding between the polar carbonyl group and water. As the alkyl group grows larger, the nonpolar hydrocarbon part dominates, weakening this interaction — so Assertion is true. The Reason correctly identifies that aldehydes and ketones are polar (dipole-dipole), but that alone doesn't explain the trend with size; the real explanation involves the balance between polar and nonpolar parts. Hence both are true, but Reason is not the correct explanation.

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.

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.

Connecting the two statements

Assertion (A): True.

Reason (R): True (aldehydes and ketones do have dipole-dipole interactions). …
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Q.Arrange propan-1-ol, butanal and butane, all of comparable molar mass, in increasing order of boiling point, and explain the order in terms of intermolecular forces.

Concept understanding — Physical Properties of Aldehydes and Ketones

Q.Arrange propan-1-ol, butanal and butane, all of comparable molar mass, in increasing order of boiling point, and explain the order in terms of intermolecular forces.

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