Why are the boiling points of aldehydes and ketones higher than hydrocarbons and ethers of comparable molecular mass?

The carbonyl group (>C=O) is strongly polar because oxygen is more electronegative than carbon. This creates permanent dipole–dipole attractions between neighbouring molecules — a form of weak molecular association that hydrocarbons (only weak van der Waals forces) and ethers (only weakly polar) lack. The extra energy needed to overcome this attraction raises the boiling point above that of a hydrocarbon or ether of similar molecular mass.

Why are the boiling points of aldehydes and ketones lower than alcohols of similar molecular mass?

Alcohols contain an O–H group and undergo extensive intermolecular hydrogen bonding, which is a stronger and more cohesive attraction than the dipole–dipole forces in carbonyl compounds. Aldehydes and ketones have no O–H hydrogen and therefore cannot hydrogen-bond among themselves. With only the weaker dipole–dipole association to overcome, they boil at lower temperatures than alcohols of comparable molecular mass.

Why are lower aldehydes and ketones soluble in water but higher members are not?

The carbonyl oxygen carries two lone pairs and a partial negative charge, so it accepts hydrogen bonds from water molecules. Lower members such as methanal, ethanal and propanone are miscible with water in all proportions for this reason. As the alkyl chain lengthens, the hydrophobic hydrocarbon portion grows and disrupts the water structure, so solubility falls rapidly with increasing molecular size.

How does the smell of aldehydes and ketones change with molecular size?

The lower aldehydes have sharp, pungent odours. As the size of the molecule increases, the odour becomes less pungent and more fragrant. Many naturally occurring higher aldehydes and ketones are pleasant enough to be used in blending perfumes and flavouring agents.

Is the carbonyl group polar, and what causes this polarity?

Yes. The carbon–oxygen double bond is polarised because oxygen is more electronegative than carbon, so the bonding electrons are pulled toward oxygen. The carbonyl carbon acquires a partial positive charge and the oxygen a partial negative charge. Carbonyl compounds therefore have substantial dipole moments and are more polar than ethers; this high polarity is described by resonance between a neutral and a dipolar structure.

How do the boiling points of n-butane, methoxyethane, propanal, acetone and propan-1-ol compare?

For these compounds of molecular mass 58–60, boiling point increases in the order n-butane < methoxyethane < acetone < propanal < propan-1-ol. n-Butane has only weak van der Waals forces; methoxyethane is only weakly polar; acetone and propanal are strongly polar carbonyls held by dipole–dipole forces; and propan-1-ol boils highest because of intermolecular hydrogen bonding through its O–H group.

Physical Properties of Aldehydes & Ketones — NEET Chemistry

The Polar Carbonyl: Root of All Trends

Aldehydes and ketones both carry the carbonyl group, $\ce{>C=O}$, in which the carbon is $sp^2$-hybridised and bonded to three atoms lying in one plane at angles of about $120^\circ$. The fourth valence electron of carbon occupies a $p$-orbital that overlaps sideways with a $p$-orbital of oxygen to form the $\pi$-bond, while oxygen retains two non-bonding lone pairs. This is the geometry NCERT fixes in §8.1.2 before any physical property is discussed.

Because oxygen is far more electronegative than carbon, the $\ce{C=O}$ bond is strongly polarised: the carbon acquires a partial positive charge ($\delta^+$) and the oxygen a partial negative charge ($\delta^-$). NCERT states plainly that carbonyl compounds have substantial dipole moments and are more polar than ethers, and explains this high polarity through resonance between a neutral structure and a dipolar one:

$\ce{R2C=O <-> R2C^{+}-O^{-}}$

Every physical property in this subtopic — the abnormally high boiling points relative to non-polar molecules, the inability to match alcohols, and the brisk water solubility of lower members — is downstream of this one fact. Hold the dipole picture in mind and the trends become deductions rather than things to memorise.

Figure 1 · Carbonyl polarity

The $\ce{C=O}$ dipole — bonding electrons drawn toward oxygen, leaving carbon electron-poor.

Physical State and Smell

At room temperature, the very first member, methanal ($\ce{HCHO}$), is a gas. Ethanal ($\ce{CH3CHO}$) is a volatile liquid that boils just above room temperature. All other common aldehydes and ketones are liquids or, for the larger ones, solids. This progression — gas, then volatile liquid, then liquid, then solid — simply mirrors rising molecular mass and the strengthening grip of intermolecular forces.

Smell follows its own pattern. NCERT records that the lower aldehydes have sharp, pungent odours, but as molecular size increases the odour becomes less pungent and more fragrant. This is why several naturally occurring higher aldehydes and ketones — cinnamaldehyde, vanillin, and the like — are prized for blending perfumes and flavouring agents rather than avoided.

NEET Trap

"Aldehydes always smell unpleasant" — false

Only the lower aldehydes have sharp, irritating odours. The statement that odour becomes less pungent and more fragrant with increasing molecular size is the examinable line — many higher carbonyl compounds are pleasant enough for perfumery.

Pungent → fragrant as molecular size rises; lower members are the smelly ones.

Boiling Point: Higher Than Hydrocarbons & Ethers

The single most-tested fact in this subtopic: the boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers of comparable molecular mass. NCERT attributes this to the weak molecular association arising out of dipole–dipole interactions. Because each carbonyl molecule is a permanent dipole, the $\delta^+$ carbon end of one molecule is electrostatically attracted to the $\delta^-$ oxygen end of its neighbour.

A hydrocarbon of the same mass — say $n$-butane — is held together only by weak van der Waals (London dispersion) forces, the feeblest of the intermolecular attractions. An ether such as methoxyethane is only weakly polar and so musters only a modest dipole. The aldehyde or ketone, by contrast, must have its stronger dipole–dipole network broken before its molecules can escape into vapour, and that demands more thermal energy — a higher boiling point.

Figure 2 · Two kinds of association

Left: weak dipole–dipole alignment between carbonyl molecules. Right: the stronger O–H…O hydrogen bond of an alcohol — note the explicit bridging H.

This is exactly Statement I of NEET 2022 (Q.82), which the examiners marked correct: the higher boiling points are "because of weak molecular association in aldehydes and ketones due to dipole–dipole interactions."

Boiling Point: Lower Than Alcohols & Acids

The same molecules that out-boil hydrocarbons fall short of alcohols. NCERT is explicit: the boiling points of aldehydes and ketones are lower than those of alcohols of similar molecular masses due to the absence of intermolecular hydrogen bonding. An alcohol carries an $\ce{O-H}$ group, and the hydrogen of one molecule bonds to the electronegative oxygen of another, knitting the liquid into an extended hydrogen-bonded network that is markedly more cohesive than dipole–dipole alignment.

A carbonyl compound has no hydrogen attached to its oxygen, so although its oxygen can accept a hydrogen bond, it has no $\ce{O-H}$ hydrogen to donate. Carbonyls therefore cannot hydrogen-bond to one another. Their molecules part company at a lower temperature than alcohols, whose hydrogen-bond cage must be torn open first. This is Statement II of NEET 2022 (Q.82) — also marked correct.

The ceiling is set even higher by carboxylic acids. NCERT §8.8 notes that carboxylic acids are higher boiling than aldehydes, ketones and even alcohols of comparable molecular mass, because they associate through two hydrogen bonds at once, forming a dimer that survives even into the vapour phase. So the full hierarchy of association strength runs: van der Waals (hydrocarbon) < weak dipole (ether) < dipole–dipole (carbonyl) < single H-bond network (alcohol) < double H-bonded dimer (acid).

Connect the dots

The $\delta^+$ carbon that raises these boiling points is the same electrophilic centre attacked in nucleophilic addition reactions — physical and chemical behaviour share one cause.

Boiling-Point Trend With Molecular Mass

Within the aldehyde–ketone family itself, boiling point rises with molecular mass. A longer carbon chain means a larger, more polarisable molecule and so stronger van der Waals forces stacked on top of the constant dipole–dipole contribution. The dipole–dipole force per molecule does not grow much, but the dispersion contribution does, and that lifts the boiling point steadily up the homologous series.

NCERT illustrates the cross-family ordering with five compounds of molecular mass 58–60 — $n$-butane, methoxyethane, propanal, acetone (propanone) and propan-1-ol. Ranked by increasing boiling point they line up as:

$\ce{CH3CH2CH2CH3} < \ce{C2H5OC2H5}\text{-type ether} < \text{acetone} < \text{propanal} < \ce{CH3CH2CH2OH}$

The hydrocarbon sits lowest (van der Waals only), the ether next (weakly polar), the two carbonyls in the middle (dipole–dipole), and the alcohol on top (hydrogen bonding). The bar chart below renders this same ladder so the gaps between association types are visible at a glance.

Figure 3 · Boiling-point ladder (mass ≈ 58–60)

Schematic boiling-point ranking for comparable-mass molecules; heights show relative order, not exact values.

NEET Trap

Propanal vs acetone — which boils higher?

In NCERT's worked example, the aldehyde is placed above the ketone of the same mass because butanal is more polar than the ether and aldehydes carry a slightly larger dipole than their isomeric ketones. Do not assume the ketone always wins — check polarity, and never rank either above the alcohol.

Order for C₃H₆O isomers: ether < ketone ≤ aldehyde < alcohol (by molecular mass class).

Solubility in Water

Lower aldehydes and ketones dissolve in water with surprising ease. Methanal, ethanal and propanone are miscible with water in all proportions. The reason, again, is the carbonyl oxygen: with its lone pairs and partial negative charge it readily accepts a hydrogen bond from a water molecule's $\ce{O-H}$. NIOS §27.1.3 draws this explicitly — the aldehyde or ketone oxygen bridges to the hydrogen of water.

But solubility falls away sharply as the alkyl chain lengthens. Each added $\ce{-CH2-}$ unit enlarges the hydrophobic hydrocarbon tail, which cannot hydrogen-bond and instead disrupts the surrounding water lattice. Beyond the small members the hydrophobic part dominates and the compound becomes only sparingly soluble; benzaldehyde, for instance, is poorly soluble. All aldehydes and ketones, however, remain freely soluble in organic solvents such as benzene, ether, methanol and chloroform.

Worked Example

Arrange in increasing order of boiling point: $\ce{CH3CHO}$, $\ce{CH3CH2OH}$, $\ce{CH3OCH3}$, $\ce{CH3CH2CH3}$.

Reasoning. Propane has only van der Waals forces, so it is lowest. Dimethyl ether is weakly polar, ranking next. Ethanal is a strongly polar carbonyl held by dipole–dipole forces, placing it above the ether. Ethanol hydrogen-bonds through its $\ce{O-H}$, so it boils highest.

Answer (NCERT Intext 8.3). $\ce{CH3CH2CH3} < \ce{CH3OCH3} < \ce{CH3CHO} < \ce{CH3CH2OH}$.

Side-by-Side Comparison Table

The whole subtopic compresses into one comparison across four classes at roughly the same molecular mass (around 58–62). Read the dominant intermolecular force column and the boiling-point and solubility entries fall out of it directly.

Class (example, M ≈ 58–62)	Dominant intermolecular force	Relative boiling point	Water solubility (lower member)
Alkane — `n-butane`	van der Waals only	Lowest	Essentially insoluble
Ether — `methoxyethane`	Weak dipole + van der Waals	Low	Slightly soluble
Ketone — `acetone`	Dipole–dipole (no self H-bond)	Moderate	Miscible (accepts H-bond from water)
Aldehyde — `propanal`	Dipole–dipole (no self H-bond)	Moderate, slightly > ketone	Miscible (accepts H-bond from water)
Alcohol — `propan-1-ol`	Intermolecular H-bonding (O–H)	High	Miscible (donates & accepts H-bond)
Carboxylic acid — `acetic acid`	Double H-bonded dimer	Highest	Miscible (lower members)

Notice the symmetry between the two halves of the story. The carbonyl oxygen can accept hydrogen bonds — that is why lower members dissolve in water. But the molecule has no $\ce{O-H}$ hydrogen to donate — that is why it cannot match an alcohol or acid on boiling point. One structural absence explains a presence (solubility) and an absence (the H-bonding boiling-point ceiling) at the same time.

Quick Recap