Question
Vinegar is dilute acetic acid (). Why does acetic acid have a higher boiling point () than ethanol (), even though they have similar molecular masses?
Solution — Step by Step
Acetic acid: , . Ethanol: , .
Acetic acid is heavier, but only by . That alone doesn’t explain a boiling point difference.
Both molecules have groups → hydrogen bonding. But acetic acid forms a special structure: a cyclic dimer held together by TWO hydrogen bonds simultaneously.
In the dimer, two acetic acid molecules link via two strong hydrogen bonds, effectively forming a six-membered ring.
The dimer’s two hydrogen bonds (each ) must both break before the molecules can vaporise. Total energy: in H-bonds alone.
Ethanol forms only a single H-bond per molecule (no carbonyl group to accept). Energy to vaporise: total enthalpy of vaporisation.
Acetic acid’s cyclic dimer structure (with two H-bonds) requires more energy to break than ethanol’s single H-bond network — hence its higher boiling point.
Why This Works
The cyclic dimer is unique to carboxylic acids. The combination of (H-bond acceptor) and (H-bond donor) on the same carbon allows two acid molecules to “lock” together face-to-face.
In gas phase, acetic acid persists as dimers up to high temperatures. This is why molecular mass measurements of carboxylic acids by vapor density often give double the expected value.
Memory hook: “Carboxylic acids party in pairs.” The dimer structure explains higher boiling points, lower volatility, and even the slightly anomalous ionization behaviour in concentrated solutions.
Alternative Method — Compare Functional Groups
Boiling point trend (similar molecular mass):
Carboxylic acid > Alcohol > Aldehyde/Ketone > Ether > Hydrocarbon
The order tracks the strength of intermolecular forces. Acids dimerize, alcohols H-bond, carbonyls have dipole-dipole, ethers have weak dipole, hydrocarbons have only London forces.
Common Mistake
Students often attribute acetic acid’s high BP solely to its molecular weight — ignoring the dimer effect. The dimer is the dominant factor; molecular weight is secondary.
Another classic: thinking aldehydes and ketones boil higher than alcohols of similar mass. Wrong — alcohols H-bond, aldehydes/ketones only have dipole-dipole forces. Acetone () boils at , lower than ethanol (, BP ).
NEET 2022 asked a similar boiling-point comparison among carboxylic acids, alcohols, and ethers. JEE Main 2023 included a dimer-related question on apparent molecular mass. Master the dimer concept and we cover both physical-property questions and “abnormal molecular mass” problems.
For Class 12 boards, “explain why carboxylic acids have higher BP than alcohols” is a guaranteed 2-mark question almost every year.