Hydrogen Bonding — Types, Conditions and Consequences
Hydrogen bonding is the one intermolecular force that changes an exam answer completely. Boiling points, solubility, the shape of DNA, why ice floats, why ortho-nitrophenol behaves differently from the para isomer — all of it turns on a force that is not a bond in the ordinary sense at all. This page explains exactly when a hydrogen bond forms, the two types you must be able to tell apart, and the consequences that examiners ask about again and again in Class 11, Class 12 and entrance papers.
What a hydrogen bond actually is
A hydrogen bond is an attraction between a hydrogen atom already bonded to a small, strongly electronegative atom and a lone pair on another such atom. It is much stronger than an ordinary dipole–dipole attraction, but far weaker than a covalent bond.
The dotted line is the hydrogen bond. The solid line is a normal covalent bond.
Three conditions must all be satisfied:
- The hydrogen must be covalently bonded to N, O or F. These three are small and highly electronegative, so they pull the shared pair strongly and leave the hydrogen nucleus unusually exposed.
- The acceptor atom Y must have a lone pair to offer.
- The acceptor must also be small. Chlorine has almost the same electronegativity as nitrogen, but its lone pair is spread over a much larger atom, so the attraction is weak. That is why HCl does not hydrogen bond appreciably while HF does.
Typical hydrogen bond energies lie roughly in the 10–40 kJ mol⁻¹ range, against about 350–450 kJ mol⁻¹ for a normal single covalent bond. So a hydrogen bond is worth around a tenth of a covalent bond — weak individually, decisive in bulk because there are so many of them.
The two types you must distinguish
| Intermolecular H-bond | Intramolecular H-bond | |
|---|---|---|
| Formed between | Two different molecules | Two groups inside the same molecule |
| Effect on boiling point | Raises it — molecules stick to each other | Lowers it — the molecule satisfies itself and ignores its neighbours |
| Effect on water solubility | Increases (can bond to water) | Decreases (the donor H is already used up) |
| Classic example | p-nitrophenol, water, ethanol, HF | o-nitrophenol, salicylaldehyde, 2-nitrophenol |
The ortho versus para nitrophenol comparison is the standard exam question. In o-nitrophenol the –OH and –NO₂ groups are next to each other, so the hydrogen bond closes a ring inside one molecule. In p-nitrophenol they are on opposite sides and can only reach a neighbouring molecule. The measured melting points show the consequence plainly: o-nitrophenol melts at about 45 °C, p-nitrophenol at about 114 °C. Same formula, same molar mass, roughly 70 degrees apart.
Consequence 1 — the boiling point anomalies
Down a group, boiling point normally rises with molar mass because London dispersion forces grow. In three groups the first member breaks the pattern badly, and hydrogen bonding is the only reason.
| Group 15 hydride | b.p. (°C) | Group 16 hydride | b.p. (°C) | Group 17 hydride | b.p. (°C) |
|---|---|---|---|---|---|
| NH₃ | −33 | H₂O | +100 | HF | +19.5 |
| PH₃ | −88 | H₂S | −60 | HCl | −85 |
| AsH₃ | −62 | H₂Se | −41 | HBr | −67 |
| SbH₃ | −18 | H₂Te | −2 | HI | −35 |
Read each column downwards from the second row and the trend is normal and rising. The first row sits far above where the trend predicts. Water is the extreme case because each molecule has two O–H bonds and two lone pairs, so on average it can form four hydrogen bonds. HF has only one hydrogen to donate; NH₃ has three hydrogens but only one lone pair, so the number of hydrogen bonds is limited by the lone pairs. That is exactly why water boils higher than HF even though fluorine is more electronegative than oxygen — a point that catches many students out.
Worked example 1 — ethanol against dimethyl ether
Both have the molecular formula C₂H₆O. Compute the molar mass to confirm they are truly identical in mass:
C: 2 × 12.011 = 24.022
H: 6 × 1.008 = 6.048
O: 1 × 15.999 = 15.999
M = 24.022 + 6.048 + 15.999 = 46.069 ≈ 46.07 g/mol for both.
Yet ethanol (CH₃CH₂–OH) boils at about 78 °C and dimethyl ether (CH₃–O–CH₃) at about −24 °C — a gap of roughly 102 degrees at the same molar mass.
Reason: ethanol has an O–H bond, so it can donate a hydrogen bond. Dimethyl ether has oxygen lone pairs and can only accept one; with no O–H, ether molecules cannot hydrogen bond to each other at all. Molar mass is identical, so mass cannot be the explanation — the hydrogen bond is.
Consequence 2 — why ice floats, with the arithmetic
In liquid water the hydrogen bonds are constantly breaking and reforming, so molecules pack fairly closely. On freezing, each molecule locks into four hydrogen bonds arranged tetrahedrally, and that open cage structure contains empty space. The solid is therefore less dense than the liquid — almost unique among substances.
Density of ice at 0 °C ≈ 0.917 g cm⁻³; density of liquid water near 4 °C ≈ 1.000 g cm⁻³.
Volume of 1 g of water = 1 ÷ 1.000 = 1.000 cm³
Volume of 1 g of ice = 1 ÷ 0.917 = 1.0905 cm³
Expansion = 1.0905 − 1.000 = 0.0905 cm³
Percentage expansion on freezing = (0.0905 ÷ 1.000) × 100 = 9.05 ≈ 9%.
That 9% is why a water pipe or a sealed glass bottle bursts in a freezer, and why lakes freeze from the top down, leaving fish alive underneath.
Consequence 3 — solubility, viscosity and biology
- Solubility. Ethanol, glucose, ammonia, urea and acetic acid dissolve readily in water because they can hydrogen bond to it. Hydrocarbons cannot, so they do not. "Like dissolves like" is, for these cases, simply a statement about hydrogen bonding.
- Alcohol boiling points rise then behave normally. As the carbon chain grows, the –OH becomes a smaller fraction of the molecule and dispersion forces take over.
- Viscosity and surface tension. Glycerol has three –OH groups per molecule and is noticeably viscous; water's high surface tension lets a needle float and insects walk on a pond.
- Biology. The two strands of DNA are held together by hydrogen bonds between base pairs — two between adenine and thymine, three between guanine and cytosine. Protein α-helices and β-sheets are held in shape the same way. These bonds are weak enough to be opened by an enzyme and strong enough to hold the structure at body temperature, which is precisely why life uses them rather than covalent bonds.
Mistakes that cost marks
- Calling it a bond between molecules of any polar compound. A hydrogen bond needs H attached to N, O or F. CHCl₃ and acetone are polar and attract one another, but that is dipole–dipole attraction, not hydrogen bonding.
- Forgetting the acceptor lone pair. Both a donor (X–H) and an acceptor (a lone pair on N, O or F) are required. Writing "ethers cannot hydrogen bond" is wrong — they cannot hydrogen bond to each other, but they accept hydrogen bonds from water, which is why small ethers dissolve.
- Saying HF boils higher than H₂O because F is most electronegative. Electronegativity sets the strength of one bond; the number of bonds per molecule sets the bulk property. Water forms about twice as many, so water wins.
- Mixing up the two types. Intramolecular hydrogen bonding lowers boiling point and water solubility. Students routinely write the opposite because they remember "hydrogen bonding raises boiling point" without the qualifier.
- Drawing the hydrogen bond as a solid line. In a diagram it must be a dotted or dashed line — a solid line means a covalent bond and loses the mark.
Where this is asked in exams
| Exam | Typical question |
|---|---|
| CBSE / ICSE Class 11 | Explain the anomalous boiling point of H₂O, NH₃ and HF; classify given molecules as H-bonding or not |
| CBSE / ICSE Class 12 | o- vs p-nitrophenol volatility and solubility; alcohol vs ether boiling points |
| JEE / NEET | Order the given set by boiling point; identify which compound shows intramolecular H-bonding |
| IIT-JAM / CUET-PG | Hydrogen bonding in solvent effects, association and deviations from Raoult's law |
| GATE / CSIR-NET | Hydrogen bonding in spectroscopy (O–H stretch broadening), crystal packing and biomolecular structure |
Check where the H-bonding elements sit. Hydrogen bonding is decided by which atom the hydrogen is attached to, so the fastest way to settle a doubtful case is to look up N, O and F and compare them with the larger atoms below them. The Interactive Periodic Table in the suite covers all 118 elements.
Open the Interactive Periodic Table →Struggling to hold bonding, structure and organic reactions together for boards? ABC Chemistry runs Class 11–12 chemistry coaching at the Gurugram centre and online classes across India — details at abcchemistry.in. For students in Delhi, Noida or Gurgaon who want one-to-one teaching at home, delhihometutor.com arranges home tuition in the NCR.