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Why does ethanol boil far above an ether of similar mass, while both dissolve in water? Questions like this appear in JEE, NEET and board papers, and they reward a short chain of reasoning: which forces act between the molecules, and how strong are they? This post explains hydrogen bonding, works through NCERT boiling point data step by step and ends with a practice routine.
What does the syllabus ask for?
The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus list "hydrogen bonding and its applications" in chemical bonding. The JEE (Advanced) 2026 brochure goes further, with "intermolecular interactions: types, distance dependence, and their effect on properties" and "hydrogen bonding effects" in organic chemistry. Check the current documents for your exam.
What is a hydrogen bond?
The NCERT chapter Chemical Bonding and Molecular Structure defines it as the attractive force that binds a hydrogen atom of one molecule to an electronegative atom (F, O or N) of another. The hydrogen must itself be bonded to F, O or N, which pulls electron density away and leaves it partly positive. NCERT notes that a hydrogen bond is weaker than a covalent bond, and distinguishes two kinds:
Intermolecular: between different molecules, as in water or alcohols.
Intramolecular: within one molecule, as in o-nitrophenol, where the O–H hydrogen sits between two oxygens of the same molecule.
The weaker forces are dipole–dipole attractions between polar molecules and van der Waals (dispersion) forces, present between all molecules and larger for bigger, less branched ones.
What goes wrong in students' reasoning?
Schmidt, Kaufmann and Treagust (2009) gave written tests to German students aged 16 to 19 and found they had difficulty predicting relative boiling points. The most common alternative idea was that boiling breaks covalent bonds. Students also limited hydrogen bonds to molecules containing oxygen and hydrogen, or to any dipolar molecule. Henderleiter and colleagues (2001) interviewed students who had completed a year-long organic chemistry course and found something similar: many could define hydrogen bonding, but significant numbers could not apply it to boiling points or solubility.
So the first rule is simple. When a liquid boils, molecules move apart. Ethanol vapour is still made of ethanol molecules.
Worked example 1: five compounds of similar mass
The NCERT chapter on aldehydes, ketones and carboxylic acids ranks five compounds with molecular masses of 58 or 60:
n-butane: 273 K
methoxyethane: 281 K
propanal: 322 K
acetone: 329 K
propan-1-ol: 370 K
Work through it in order of the strongest force each can form. Butane has only dispersion forces. The ether is slightly polar. Propanal and acetone have a strongly polar C=O group, so dipole–dipole forces raise their boiling points. Only propan-1-ol has an O–H group, so only it forms hydrogen bonds between its own molecules, and it boils highest.
Worked example 2: isomeric amines
NCERT's amines chapter gives, for compounds of molar mass 72 to 74:
n-butylamine (primary): 350.8 K
diethylamine (secondary): 329.3 K
ethyldimethylamine (tertiary): 310.5 K
2-methylbutane: 300.8 K
butan-1-ol: 390.3 K
A primary amine has two N–H hydrogens to share, a secondary amine one and a tertiary amine none, so the boiling points fall in that order. Butan-1-ol is higher still because O is more electronegative than N, so its hydrogen bonds are stronger.
Worked example 3: same solubility, different boiling points
NCERT's alcohols, phenols and ethers chapter gives the boiling points of n-pentane (309.1 K), ethoxyethane (307.6 K) and butan-1-ol (390 K). Yet ethoxyethane and butan-1-ol dissolve to a similar extent in water, 7.5 g and 9 g per 100 mL, while pentane is essentially immiscible.
The explanation uses the callout above. Ether molecules have no O–H, so they cannot hydrogen bond to each other, and their boiling point is close to pentane's. But the ether oxygen can accept a hydrogen bond from water, so it dissolves about as well as the alcohol.
How does intramolecular bonding change properties?
The same NCERT chapter explains that o-nitrophenol and p-nitrophenol can be separated by steam distillation. The ortho isomer is steam volatile because its hydrogen bond is intramolecular. The para isomer forms intermolecular hydrogen bonds, which associate its molecules and make it less volatile. When a hydrogen bond stays inside one molecule, it is not holding molecules together.
Which mistakes cost marks?
Counting C–H hydrogens as hydrogen bond donors.
Comparing compounds of very different molar mass without saying so.
Forgetting branching: NCERT notes that boiling points of alcohols fall as branching increases, because surface area and van der Waals forces fall.
Writing that boiling breaks O–H bonds.
A practice routine
For any list of compounds, first write each one's strongest intermolecular force, then rank.
Redo the three examples above from memory, explaining each step aloud.
Add one solubility question a day: does it donate, accept, both or neither?
Review the alcohol, ether and amine chapters with these questions in mind. Our posts on alcohols, phenols and ethers and amines cover the reactions.
When the gap is in an earlier idea such as electronegativity or polarity, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can fill it before moving on.
Today, rank ethanol, methoxymethane and propane by boiling point, and write one reason for each step.
