Why Do Aldehydes and Ketones Have Higher Boiling Points Than Hydrocarbons?
Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
Aldehydes and ketones have a polar C=O group. This lets their molecules attract each other with dipole-dipole forces, which are stronger than the weak forces between non-polar hydrocarbons. So they boil at a higher temperature. But they still boil lower than alcohols of the same mass, because they cannot form hydrogen bonds (they have no O-H). Memory hook: "Dipole beats nothing, but H-bond beats dipole" so the order is Hydrocarbon less than Aldehyde/Ketone less than Alcohol less than Acid.
For similar molecular mass, boiling point climbs as intermolecular forces get stronger: hydrocarbons (weak London forces) < aldehydes/ketones (polar C=O dipole-dipole) < alcohols (O-H hydrogen bonds) < carboxylic acids (strong H-bonded dimers).
Your doubts, answered
Why do aldehydes and ketones boil higher than alkanes of the same mass?
The C=O bond is polar: oxygen pulls the shared electrons, so oxygen is slightly negative and carbon is slightly positive. This makes each molecule a tiny dipole. The positive end of one molecule attracts the negative end of the next. These dipole-dipole forces are stronger than the very weak London (van der Waals) forces that hold non-polar alkane molecules together. More energy is needed to pull the molecules apart, so the boiling point is higher.
Why is the boiling point of an aldehyde or ketone LOWER than an alcohol of the same mass?
An alcohol has an O-H group. The H is bonded to a very electronegative O, so alcohol molecules link to each other by strong hydrogen bonds. Aldehydes and ketones have no O-H (their H is on carbon), so they CANNOT hydrogen-bond to each other. Hydrogen bonds are stronger than plain dipole-dipole forces, so alcohols hold together more tightly and boil higher. This is exactly what NEET 2022 tested.
So what is the full boiling point order for NEET?
For compounds of similar molecular mass: Hydrocarbon (alkane) < Ether ≈ weak < Aldehyde/Ketone < Alcohol < Carboxylic Acid. Hydrocarbons have only weak forces. Aldehydes/ketones add dipole-dipole. Alcohols add hydrogen bonding. Carboxylic acids form the strongest bonds of all, cyclic H-bonded dimers, so they boil the highest.
Are aldehydes and ketones soluble in water even though they can't H-bond with each other?
Yes, the lower members are quite soluble. An aldehyde or ketone cannot hydrogen-bond with another aldehyde/ketone, but the lone pairs on its oxygen CAN accept a hydrogen bond from a water molecule's O-H. So they mix well with water. As the carbon chain (hydrocarbon part) gets longer, solubility drops because the non-polar tail dominates.
Does branching change the boiling point of ketones and aldehydes?
Yes. For the same molecular formula, more branching gives a lower boiling point. Branched molecules are more compact and rounded, so they touch each other over a smaller surface area. Less surface contact means weaker total London forces, so the branched isomer boils lower than the straight-chain one.
⚠️ The NEET trap ✗ Aldehydes and ketones have higher boiling points than hydrocarbons because they form hydrogen bonds with each other. ✓ They boil higher than hydrocarbons because of dipole-dipole attraction from the polar C=O group, NOT hydrogen bonding. They cannot hydrogen-bond to each other because they have no O-H or N-H. That is exactly why they boil LOWER than alcohols. 🧠 No O-H = no self H-bond. Aldehydes/ketones only get dipole-dipole, so they sit between hydrocarbons and alcohols.
Real NEET questions
NEET 2022
Statement I: The boiling points of aldehydes and ketones are higher than hydrocarbons of comparable molecular masses because of weak molecular association due to dipole-dipole interactions. Statement II: The boiling points of aldehydes and ketones are lower than the alcohols of similar molecular masses due to the absence of H-bonding. In the light of the above statements, choose the most appropriate answer:
A · Both Statement I and Statement II are correct. ✓
B · Both Statement I and Statement II are incorrect.
C · Statement I is correct but Statement II is incorrect.
D · Statement I is incorrect but Statement II is correct.
Solution: Statement I is correct: the polar C=O group lets molecules attract through dipole-dipole forces, so they boil higher than non-polar hydrocarbons of similar mass. Statement II is also correct: aldehydes and ketones have no O-H, so they cannot form hydrogen bonds with each other, and they boil lower than alcohols of similar mass. Both statements are true, so the answer is (A).
NEET 2018
Carboxylic acids have higher boiling points than aldehydes, ketones and even alcohols of comparable molecular mass. It is due to their:
A · More extensive association of carboxylic acid via van der Waals force of attraction
B · Formation of carboxylate ion
C · Formation of intramolecular H-bonding
D · Formation of intermolecular H-bonding ✓
Solution: Carboxylic acids form strong intermolecular hydrogen bonds and exist largely as cyclic dimers (two molecules H-bonded together). This strong association needs a lot of energy to break, so acids boil higher than aldehydes, ketones and even alcohols of similar mass. This ranks acids at the TOP of the boiling point order. Answer (D).
ReNEET 2026
Arrange the following compounds in the increasing order of polarity: A. CH3CH2OCH2CH3, B. CH3CH2OH, C. CH3COCH3, D. CH3COOH.
A · A < B < C < D
B · C < A < D < B
C · C < A < B < D
D · A < C < B < D ✓
Solution: Diethyl ether (A) is least polar (weak dipole, no O-H). Acetone (C) has a polar carbonyl but no O-H. Ethanol (B) has an O-H that can hydrogen-bond, so it is more polar than acetone. Acetic acid (D) with -COOH is the most polar. Increasing order: A < C < B < D. This mirrors the boiling point trend: ether < ketone < alcohol < acid. Answer (D).
Solved Aldehydes, Ketones And Carboxylic Acid NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
They do not form hydrogen bonds with each other, because they have no O-H or N-H bond. But their oxygen lone pairs can accept a hydrogen bond from water, which is why the small ones dissolve in water.
Which boils higher, an aldehyde or a ketone of the same mass?
They are very close. Ketones often boil slightly higher than aldehydes of the same molecular mass because ketones usually have a slightly larger dipole moment. For NEET, focus on the big picture: both are between hydrocarbons and alcohols.
Why do boiling points rise as the aldehyde or ketone gets bigger?
A longer carbon chain has more electrons and more surface area, so the London (van der Waals) forces between molecules become stronger. Stronger total forces need more heat to break, so the boiling point rises with molecular mass.
Where do aldehydes and ketones sit in the full boiling point order?
Hydrocarbon < aldehyde/ketone < alcohol < carboxylic acid, for compounds of similar mass. Remember it as: weak forces, then dipole-dipole, then hydrogen bonds, then strongest dimer hydrogen bonds.
