Rank the Boiling Points
The three compounds in each problem differ in exactly one structural feature — chain length, branching, halogen size, O–H count, or the strongest force available — so a single trend decides the whole order. All boiling points are experimental values at atmospheric pressure.
Drag the structures into order, lowest to highest boiling point, then check. The explanation names the trend.
Common Questions
Why does boiling point increase along a homologous series?
Each additional CH2 unit adds electrons and surface area, and dispersion forces act through that surface, so every step up a homologous series raises the boiling point. Methanol, ethanol, and 1-propanol boil at 65, 78, and 97 °C — same functional group, one CH2 apart each time.
Why do alkyl iodides boil higher than alkyl chlorides?
A heavier halogen carries a larger, more polarizable electron cloud, so dispersion forces strengthen from Cl to Br to I: methyl chloride boils at −24 °C, methyl bromide at 4 °C, methyl iodide at 42 °C. Note that the dipole runs the other way — C–Cl is the most polar of the three bonds — so the trend is dispersion, not dipole strength.
Why do diols boil so much higher than alcohols?
Each O–H group is a hydrogen-bond donor, and every added donor ties the liquid together more tightly. On the same three-carbon skeleton, propane boils at −42 °C, 1-propanol at 97 °C, and propylene glycol at 188 °C — roughly a hundred degrees per hydroxyl.
What is the order of intermolecular force strength?
At similar molecular size, hydrogen bonding beats dipole–dipole attraction, which beats dispersion alone: propane boils at −42 °C, acetaldehyde at 20 °C, ethanol at 78 °C. The size condition matters — dispersion accumulates with surface area, so a large nonpolar molecule can out-boil a small polar one.