Melting & Boiling Points
The NCERT / JEE data for important organic compounds — every value in °C and K — with the intermolecular-force reasoning that turns numbers into marks.
The one idea behind every number · intermolecular forces
- H-bonding > dipole–dipole > London (van der Waals). Stronger force → more energy to separate → higher b.p.
- Same molar mass, b.p. order: acid > alcohol / phenol > amine > aldehyde ≈ ketone > ether ≈ alkane.
- ↑ Molar mass → ↑ surface area → stronger London forces → ↑ b.p. (down a homologous series).
- Branching ↓ b.p. (less contact) but a compact symmetrical shape ↑ m.p. (packs better).
- Carboxylic acids form cyclic dimers (two H-bonds) → behave like double the mass → top the b.p. list.
★ The comparison that wins marks · same mass (~72–74)
| Compound | Class | M | B.P. |
|---|
| Pentane | alkane | 72 | 36 °C · 309 K |
| Diethyl ether | ether | 74 | 35 °C · 308 K |
| Butanal | aldehyde | 72 | 76 °C · 349 K |
| Butanone | ketone | 72 | 80 °C · 353 K |
| Butan-1-ol | alcohol | 74 | 118 °C · 391 K |
| Propanoic acid | carboxylic acid | 74 | 141 °C · 414 K |
Same mass, b.p. climbs 35 → 141 °C purely as the force strengthens: London → dipole–dipole → H-bond → dimeric H-bond.
60-second recall before the exam
- Ladder (same mass): acid > alcohol ≈ phenol > amine > ketone ≈ aldehyde > ether ≈ alkane.
- Anchors: water 373 K (100 °C), benzene 353 K (80 °C), ethanol 351 K (78 °C).
- Branching lowers b.p., can raise m.p. (symmetry) — pentane vs neopentane.
- o-nitrophenol = intramolecular H-bond = volatile; para = intermolecular = high-melting. Steam-distil to separate.
- Acids dimerise → always out-boil the alcohol of equal mass.
- Diazonium salt: no b.p. to quote — keep below 278 K (5 °C) or it decomposes (N₂ off).