THE CONCEPT
Three Forces, One Clear Hierarchy
Every physical property that depends on how strongly molecules attract each other — boiling point, melting point, water solubility, viscosity — traces back to which intermolecular forces (IMFs) a molecule can participate in. There are three IMFs relevant to organic molecules, and they form a clear, memorizable hierarchy from strongest to weakest.
Hydrogen bonding requires a hydrogen directly attached to a highly electronegative atom with a lone pair — specifically N-H, O-H, or F-H — interacting with a lone pair on a nearby electronegative atom. Dipole-dipole interactions occur between polar molecules that lack a hydrogen-bond donor (no N-H or O-H present), where the positive end of one molecule's permanent dipole attracts the negative end of a neighboring molecule's dipole. London dispersion forces (also called van der Waals forces) exist between literally every molecule, polar or nonpolar, arising from momentary, fluctuating electron distributions that induce a temporary dipole in a neighboring molecule.
💡 Memory Trick
The hub's trick states the ranking directly: H-bond > dipole-dipole > London dispersion, and ties it straight to the two physical properties it governs most: boiling point and solubility. Hold onto which functional groups belong to each category: alcohols, carboxylic acids, and amines (N-H) get hydrogen bonding; ketones, aldehydes, ethers, and esters (polar, but no H-bond donor) get dipole-dipole; and every molecule without exception gets at least London dispersion, which is why even nonpolar hydrocarbons still have a measurable boiling point that climbs with molecular size.
PREDICTING BOILING POINT FROM FIRST PRINCIPLES
Putting the Hierarchy to Work
Ranking two compounds' boiling points starts by identifying the strongest IMF each one can participate in. Between a carboxylic acid and a ketone of similar molecular weight, the acid wins decisively — its O-H hydrogen bonding (and carboxylic acids are even more strongly associated than simple alcohols, since they can form doubly hydrogen-bonded dimers) beats the ketone's dipole-dipole interaction every time. Between that same ketone and a similarly-sized nonpolar hydrocarbon, the ketone wins, since dipole-dipole interactions are stronger than dispersion forces alone.
Only once two molecules are tied on IMF category does molecular size become the deciding factor, since London dispersion forces (present in every molecule) scale with surface area — a larger molecule of the same functional-group class will generally have a somewhat higher boiling point than a smaller one, exactly as you saw with straight-chain alkanes in the Hydrocarbons unit. The same hierarchy governs water solubility directly: any functional group capable of hydrogen bonding with water dissolves best, dipole-dipole-only groups dissolve moderately (still able to interact with water's own polarity, just less strongly), and purely nonpolar hydrocarbons barely dissolve in water at all.
🧪 Lab Application
You're given three compounds of similar molecular weight — butanoic acid, butanal, and pentane — and need to rank their boiling points without a reference table.
1
Identify the strongest IMF available to each compound. Butanoic acid has an O-H group capable of hydrogen bonding. Butanal, an aldehyde, is polar (C=O) but has no N-H or O-H, so it's limited to dipole-dipole interactions. Pentane, a plain alkane, has neither — only London dispersion forces.
2
Rank by strongest available IMF. Hydrogen bonding beats dipole-dipole, which beats dispersion alone, so the predicted order is butanoic acid (highest boiling point) > butanal > pentane (lowest boiling point).
3
Note the added strength of carboxylic acid dimerization. Butanoic acid's boiling point is elevated even further than a simple alcohol of similar size would be, since two carboxylic acid molecules can hydrogen-bond to each other twice simultaneously, forming a stable dimer that takes extra energy to break apart during boiling.
4
State the final prediction. Butanoic acid boils highest, butanal boils at a clearly lower but still elevated temperature relative to a nonpolar hydrocarbon, and pentane boils lowest of the three.
📌 Exam Application
Boiling-point ranking questions are extremely common precisely because they test the full IMF hierarchy at once — always identify each compound's strongest available IMF category first before considering molecular size as a tiebreaker, since IMF category differences almost always outweigh modest size differences.
⚠️ Most Common Intermolecular Forces & Physical Properties Mistakes
The most common mistake is ranking compounds by molecular weight alone, ignoring functional group entirely — a smaller molecule capable of hydrogen bonding can easily out-boil a larger molecule limited to dispersion forces only. The other frequent trap is forgetting that a polar molecule without an N-H or O-H bond (a ketone, ester, or ether) is limited to dipole-dipole interactions, not hydrogen bonding, even though it looks 'polar' in a structure.
✓ Quick Self-Test
1) Rank the three intermolecular forces from strongest to weakest. 2) What structural feature is required for hydrogen bonding to occur? 3) Why do ketones and aldehydes only participate in dipole-dipole interactions rather than hydrogen bonding with themselves? 4) Why does every molecule, even a nonpolar one, still experience London dispersion forces? 5) Why does a carboxylic acid typically have an even higher boiling point than a simple alcohol of similar size?
Next Lesson
Carbon Chain Prefixes
→
← All Functional Groups Lessons