Prerequisites
Before studying hydrogen bonding, review:
- Covalent Bonding — Polarity and electronegativity
- VSEPR Theory — Molecular shapes
- Dipole Moments — Understanding polar bonds
The Hook: The 104.5° That Gives You Life
A thought experiment:
Imagine winter comes, and lakes freeze from the bottom up instead of top down.
Result?
- All aquatic life dies (fish freeze solid) ✗
- Ocean floors become permanent ice ✗
- Earth’s climate becomes unstable ✗
- Life as we know it couldn’t exist ✗
What saves us? The fact that ice floats!
Why does ice float? Because solid water is less dense than liquid water — a property almost NO OTHER SUBSTANCE has! And this bizarre property exists because of one special interaction: Hydrogen Bonding.
In Interstellar (2014), they search for planets with liquid water. But it’s not just H₂O that matters — it’s the hydrogen bonding that makes water behave uniquely, creating conditions for life!
JEE Reality: 2-3 questions every year worth 8-12 marks. Hydrogen bonding explains boiling points, solubility, DNA structure, protein folding, and a dozen other “why” questions!
The Core Concept
What is Hydrogen Bonding?
Hydrogen bonding is a special type of dipole-dipole interaction between a hydrogen atom bonded to a highly electronegative atom (F, O, N) and a lone pair on another electronegative atom.
$$\boxed{X-H \cdots Y}$$Where:
- X = F, O, or N (highly electronegative)
- H = Hydrogen (becomes δ⁺)
- Y = F, O, or N (with lone pair, becomes δ⁻)
- ··· = Hydrogen bond (dashed line, weaker than covalent)
In simple terms: Imagine hydrogen as a “bridge” connecting two electronegative atoms. The hydrogen is so positive (because F, O, or N pull its electrons away) that it’s attracted to the lone pairs on another electronegative atom!
Why Only F, O, N?
Why can only F, O, N form hydrogen bonds?
Requirements:
High electronegativity (to make H very δ⁺)
- F: 4.0
- O: 3.5
- N: 3.0
- Compare: Cl: 3.0 (but too large!)
Small size (to approach closely)
- F, O, N are Period 2 → small atomic radii
- Cl, Br, I are too large → can’t get close enough
Lone pairs available (for H to attract to)
- F: 3 lone pairs
- O: 2 lone pairs
- N: 1 lone pair
Why not Cl? Although EN(Cl) = 3.0 ≈ EN(N), chlorine is too large. The H···Cl distance is too long for strong hydrogen bonding.
Memory trick: “Friends Often Need Help” → FON need H (hydrogen bonding!)
Interactive Demo: Visualize Hydrogen Bonding Networks
Explore hydrogen bonds in water, DNA base pairs, and protein structures.
Characteristics of Hydrogen Bonding
Strength
Hydrogen bond strength: 5-40 kJ/mol (typical: 10-40 kJ/mol)
Comparison:
- Covalent bond: 200-600 kJ/mol (10-50× stronger!)
- Hydrogen bond: 10-40 kJ/mol
- Van der Waals: 1-10 kJ/mol (weaker)
Key point: Hydrogen bonds are stronger than other intermolecular forces (van der Waals, London dispersion) but much weaker than covalent bonds.
Directionality
Hydrogen bonds are directional!
The strongest H-bond forms when X-H···Y is linear (180°).
Best: X—H · · · Y (180°, strongest)
Good: X—H ·· Y (150-180°, strong)
Weak: X—H ··Y (< 150°, weaker)
\_____/
Why? Maximum overlap of H δ⁺ with Y lone pair occurs at 180°.
Bond Length
Typical H-bond distances:
- O-H···O: 1.8-2.0 Å
- N-H···O: 1.9-2.1 Å
- F-H···F: 1.5-1.8 Å (shortest, strongest!)
Compare with covalent:
- O-H covalent: 0.96 Å
- N-H covalent: 1.01 Å
H-bonds are ~2× longer than covalent bonds → weaker!
Types of Hydrogen Bonding
1. Intermolecular Hydrogen Bonding
Definition: Hydrogen bonding between different molecules of the same or different substances.
Effect: Molecules are held together → increases boiling point, viscosity, surface tension.
Example 1: Water (H₂O)
H H
\ /
O · · · O
/ \
H H
Each water molecule:
- Has 2 O-H bonds (can donate 2 H’s)
- Has 2 lone pairs on O (can accept 2 H-bonds)
- Can form up to 4 hydrogen bonds!
Result:
- High boiling point (100°C vs -60°C expected!)
- Ice structure: tetrahedral network, less dense than liquid
- Ice floats → life on Earth possible!
Example 2: Ammonia (NH₃)
H H H
\ | /
N· · ·N
/ | \
H H H
Each NH₃ molecule:
- Has 3 N-H bonds (can donate 3 H’s)
- Has 1 lone pair on N (can accept 1 H-bond)
Result:
- Boiling point: -33°C (higher than expected for such a small molecule)
- Good solvent for polar compounds
Example 3: Hydrogen Fluoride (HF)
H—F · · · H—F · · · H—F
HF forms strong, linear chains:
- Strongest H-bond (F most electronegative)
- Exists as (HF)ₙ polymers even in vapor phase
- Boiling point: 19.5°C (very high for diatomic!)
Example 4: Alcohols (R-OH)
R—O—H · · · O—R
| |
H H
Effect on boiling points:
| Compound | Formula | BP (°C) | H-bonding? |
|---|---|---|---|
| Ethane | C₂H₆ | -89 | No |
| Ethanol | C₂H₅OH | 78 | Yes! |
Ethanol vs ethane: Similar molecular weight, but ethanol has H-bonding → 167°C higher BP!
When comparing similar molecules:
With H-bonding > Without H-bonding
Example: Arrange by increasing BP: CH₄, CH₃OH, H₂O, NH₃
Analysis:
- CH₄: No H-bonding → Lowest BP
- NH₃: H-bonding (1 LP) → Medium
- CH₃OH: H-bonding (2 LP on O) → Higher
- H₂O: H-bonding (2 LP, small size) → Highest!
Answer: CH₄ < NH₃ < CH₃OH < H₂O
Quick rule: More H-bonds possible = Higher BP
2. Intramolecular Hydrogen Bonding
Definition: Hydrogen bonding within the same molecule (between different parts of one molecule).
Effect: Forms a “ring” or “loop” → decreases boiling point (molecules don’t stick together as much)!
Example 1: ortho-Nitrophenol
OH
/ \
/ \
| |
| NO₂
\ /··H (intramolecular H-bond)
\__/
Intramolecular H-bond:
- O-H hydrogen bonds with O of NO₂ group
- Forms a 6-membered ring
- Molecule is “self-satisfied” (doesn’t need other molecules)
Result:
- Lower boiling point (can’t form intermolecular H-bonds)
- Less soluble in water (no external H-bonding sites available)
- Steam volatile (evaporates easily)
Example 2: para-Nitrophenol
OH
/ \
/ \
| |
| |
\ /
\____/
|
NO₂
No intramolecular H-bond possible! (O-H and NO₂ too far apart)
Instead:
- Forms intermolecular H-bonds between molecules
- Molecules stick together strongly
Result:
- Higher boiling point than ortho-isomer!
- More soluble in water
- Not steam volatile
| Property | o-Nitrophenol | p-Nitrophenol | Reason |
|---|---|---|---|
| H-bonding type | Intramolecular | Intermolecular | Position of NO₂ |
| Boiling point | Lower | Higher | Inter > Intra |
| Solubility in water | Less | More | Inter can bond with H₂O |
| Steam volatility | Yes | No | Intra doesn’t hold molecules |
JEE Favorite Question: “Why is o-nitrophenol more volatile than p-nitrophenol?”
Answer in 1 line: o-Nitrophenol has intramolecular H-bonding (molecules don’t stick), while p-nitrophenol has intermolecular H-bonding (molecules stick together).
Example 3: Salicylic Acid
COOH
/ \
/ \
| |
| OH
\ /··H (intramolecular)
\__/
Intramolecular H-bond between -COOH and -OH groups.
Result: Less acidic than benzoic acid (H-bond stabilizes the molecule, reduces H⁺ donation)
Effects of Hydrogen Bonding
1. Anomalously High Boiling and Melting Points
Compounds with H-bonding have much higher BP than expected!
Group 16 Hydrides (O, S, Se, Te):
| Compound | Molecular Weight | BP (°C) | H-bonding? |
|---|---|---|---|
| H₂O | 18 | 100 | Yes! |
| H₂S | 34 | -60 | No |
| H₂Se | 81 | -41 | No |
| H₂Te | 130 | -2 | No |
Expected: H₂O should have lowest BP (smallest MW) Reality: H₂O has highest BP due to H-bonding!
Group 15 Hydrides (N, P, As, Sb):
| Compound | BP (°C) | H-bonding? |
|---|---|---|
| NH₃ | -33 | Yes! |
| PH₃ | -87 | No |
| AsH₃ | -62 | No |
Group 17 Hydrides (F, Cl, Br, I):
| Compound | BP (°C) | H-bonding? |
|---|---|---|
| HF | 20 | Yes! |
| HCl | -85 | No |
| HBr | -67 | No |
| HI | -35 | No |
For hydrides of F, O, N:
- Expected trend: BP increases with molecular weight down the group
- Reality: First member has HIGHEST BP!
Why? First members (HF, H₂O, NH₃) form H-bonds → anomalously high BP
Graphically:
BP ↑
| H₂O (100°C) ← Anomaly!
|
-60 | H₂S, H₂Se, H₂Te → Normal trend
|____________________________→ Group
JEE Trick: If first member has highest BP in a group → H-bonding!
2. Density of Ice < Density of Water
The famous anomaly!
Ice structure:
- Each H₂O forms 4 H-bonds (tetrahedral)
- Creates an open cage-like structure with lots of empty space
- Density ≈ 0.92 g/cm³
Liquid water:
- H-bonds constantly breaking and reforming
- More compact, molecules pack closer
- Density ≈ 1.00 g/cm³
Result: Ice floats!
Consequences:
- Lakes freeze from top (ice layer insulates water below)
- Aquatic life survives winters
- Climate stability (ice reflects sunlight)
Without H-bonding: Ice would sink → oceans freeze solid → no life!
3. High Heat of Vaporization
To boil water, you must break H-bonds!
Heat of vaporization (H₂O): 40.7 kJ/mol (very high!)
Compare:
- H₂S: 18.7 kJ/mol
- NH₃: 23.4 kJ/mol
- CH₄: 8.2 kJ/mol
Why high? Breaking H-bonds requires significant energy.
Practical consequence:
- Sweating cools you down (water evaporation absorbs heat)
- Water is excellent coolant (car radiators, power plants)
4. High Surface Tension and Viscosity
Water has high surface tension (72.8 mN/m at 20°C)
Why? Surface molecules form extra H-bonds with below → “skin” effect
Consequences:
- Water droplets are spherical (minimize surface area)
- Insects can walk on water
- Capillary action in plants
5. Solubility
“Like dissolves like” becomes “H-bonding dissolves H-bonding”
Soluble in water (H-bonding possible):
- Alcohols: R-OH (form H-bonds with water)
- Sugars: Multiple -OH groups (glucose very soluble!)
- Ammonia: NH₃ (H-bonds with water)
- HCl, HF: Strong H-bonding
Insoluble in water (no H-bonding):
- Hydrocarbons: C-H bonds (no polarity)
- CCl₄, benzene: Nonpolar
- Oils, fats: Long hydrocarbon chains
Question type: “Why is ethanol soluble in water but hexane is not?”
Answer template:
Identify H-bonding capability
- Ethanol: -OH group → can H-bond with water ✓
- Hexane: Only C-H → no H-bonding ✗
Conclusion
- Ethanol dissolves (forms H-bonds with water)
- Hexane doesn’t dissolve (no favorable interactions)
Rule: Polar + H-bonding → Soluble in water
6. DNA Double Helix
Hydrogen bonds hold DNA strands together!
Base pairing:
- Adenine-Thymine: 2 H-bonds
- Guanine-Cytosine: 3 H-bonds
A = T (2 H-bonds)
G ≡ C (3 H-bonds, stronger!)
Importance:
- H-bonds strong enough to hold DNA together
- But weak enough to “unzip” for replication
- Temperature too high → H-bonds break → DNA denatures
Why not covalent bonds? Too strong! DNA couldn’t “open” for replication.
7. Protein Folding
Protein structure held by H-bonds!
Secondary structure:
- α-helix: H-bonds between C=O and N-H (4 residues apart)
- β-sheet: H-bonds between adjacent strands
Tertiary structure:
- H-bonds between different parts of protein chain
- Determines 3D shape (and function!)
Denaturation: Heat/acid breaks H-bonds → protein unfolds → loses function (e.g., cooking egg whites)
Comparison: Intermolecular vs Intramolecular
| Property | Intermolecular | Intramolecular |
|---|---|---|
| Location | Between molecules | Within one molecule |
| Effect on BP | Increases | Decreases |
| Effect on solubility | Increases (in water) | Decreases |
| Volatility | Decreases | Increases |
| Example | p-Nitrophenol, H₂O | o-Nitrophenol, Salicylic acid |
| Molecular association | High (molecules stick) | Low (self-satisfied) |
Memory Tricks & Patterns
The FON Rule
“FON are Fun for H-bonding!”
- Fluorine
- Oxygen
- Nitrogen
Only these three + H → hydrogen bonding
Boiling Point Trends
“H-bonds High, No H-bonds Low”
When comparing similar molecules:
- More H-bonds → Higher BP
- No H-bonds → Lower BP
Example ranking (increasing BP): Pentane < Diethyl ether < Butanol < Propanoic acid
Why?
- Pentane: No H-bonding (lowest)
- Ether: Polar but no H-bonding
- Butanol: 1 -OH (H-bonding)
- Propanoic acid: -COOH (2 H-bonding sites, highest!)
Ortho vs Para Isomers
“Ortho = Intra = Inside = Lower BP” “Para = Inter = Between = Higher BP”
Common Mistakes to Avoid
Mistake: “HCl has H-Cl bond, so it forms H-bonds like HF”
Wrong! Only F, O, N form H-bonds.
Why Cl doesn’t work:
- EN(Cl) = 3.0 (similar to N) ✓
- But Cl is too large (radius 99 pm vs F 64 pm) ✗
- H···Cl distance too long for strong attraction
JEE Trap: HCl is soluble in water (due to ionization H⁺ + Cl⁻), NOT due to H-bonding!
Mistake: “H-bonding always increases boiling point”
Correct:
- Intermolecular H-bonding: Increases BP (molecules stick)
- Intramolecular H-bonding: Decreases BP (molecules don’t stick)
Example:
- o-Nitrophenol (intramolecular): BP = 216°C
- p-Nitrophenol (intermolecular): BP = 279°C
JEE Trick: If H-bond forms a ring within molecule → Lowers BP!
Mistake: “H-bonds are as strong as covalent bonds”
Correct:
- Covalent bond: 200-600 kJ/mol
- Hydrogen bond: 10-40 kJ/mol
- Van der Waals: 1-10 kJ/mol
H-bonds are 10-50× weaker than covalent bonds!
But they’re stronger than other intermolecular forces → significant effects on properties.
Practice Problems
Level 1: Foundation (NCERT Style)
Question: Explain why H₂O has a higher boiling point than H₂S despite having lower molecular weight.
Solution:
H₂O (MW = 18):
- O is highly electronegative (3.5)
- Forms H-bonds (O-H···O)
- Each molecule can form up to 4 H-bonds
- Molecules strongly associated
- BP = 100°C
H₂S (MW = 34):
- S has lower EN (2.5), larger size
- No H-bonding (S too large)
- Only weak van der Waals forces
- BP = -60°C
Conclusion: Despite lower MW, H₂O has much higher BP due to strong intermolecular H-bonding that must be overcome during boiling.
Answer: H₂O forms H-bonds (strong intermolecular forces), while H₂S only has weak van der Waals forces. Breaking H-bonds requires more energy → higher BP for H₂O.
Question: Which molecules can form hydrogen bonds with water? (A) CH₄ (B) NH₃ (C) HCl (D) CH₃OH
Solution:
(A) CH₄:
- No F, O, or N → Cannot H-bond ✗
(B) NH₃:
- Has N-H bonds + lone pair on N
- Can donate and accept H-bonds ✓
- NH₃ + H₂O → N-H···O-H and H-O···H-N
(C) HCl:
- Cl is not F, O, or N
- Cannot form H-bonds (only dipole-dipole) ✗
(D) CH₃OH:
- Has O-H bond + lone pairs on O
- Can donate and accept H-bonds ✓
- CH₃OH + H₂O → O-H···O-H interactions
Answer: (B) NH₃ and (D) CH₃OH can form H-bonds with water.
Level 2: JEE Main Type
Question: Arrange the following in order of increasing boiling point: CH₃CH₂CH₂CH₃ (butane), CH₃CH₂CH₂OH (1-propanol), CH₃CH₂OCH₂CH₃ (diethyl ether)
Solution:
Analysis:
Butane (C₄H₁₀):
- Only C-H bonds (nonpolar)
- No H-bonding, only London forces
- MW = 58
- BP ≈ 0°C (lowest)
Diethyl ether (C₄H₁₀O):
- Has O atom but no O-H bond
- Cannot donate H (no H attached to O)
- Can only accept H-bonds (has lone pairs on O)
- Polar, dipole-dipole + London forces
- MW = 74
- BP ≈ 35°C (medium)
1-Propanol (C₃H₈O):
- Has O-H bond
- Can donate and accept H-bonds
- Strong intermolecular H-bonding
- MW = 60
- BP ≈ 97°C (highest)
Order: Butane < Diethyl ether < 1-Propanol
Answer: CH₃CH₂CH₂CH₃ < CH₃CH₂OCH₂CH₃ < CH₃CH₂CH₂OH
Question: Why is o-nitrophenol steam volatile but p-nitrophenol is not?
Solution:
o-Nitrophenol (ortho):
- OH and NO₂ groups are adjacent
- Forms intramolecular H-bond (O-H···O=N)
- 6-membered ring structure
- Molecules are “self-satisfied” (no intermolecular H-bonding)
- Low intermolecular forces
- Steam volatile (evaporates easily)
p-Nitrophenol (para):
- OH and NO₂ groups are opposite (too far apart)
- Cannot form intramolecular H-bond
- Forms intermolecular H-bonds between molecules
- Molecules strongly associated
- High intermolecular forces
- Not steam volatile (requires higher temp to evaporate)
Answer: o-Nitrophenol has intramolecular H-bonding (weak intermolecular forces → steam volatile), while p-nitrophenol has intermolecular H-bonding (strong intermolecular forces → not steam volatile).
Level 3: JEE Advanced Type
Question: Compare the following properties of o-hydroxybenzoic acid (salicylic acid) and p-hydroxybenzoic acid: (a) Boiling point (b) Solubility in water (c) Acidity
Solution:
Structure:
- o-Hydroxybenzoic acid: -COOH and -OH are adjacent (ortho)
- p-Hydroxybenzoic acid: -COOH and -OH are opposite (para)
(a) Boiling Point:
o-Hydroxybenzoic acid:
- Forms intramolecular H-bond (-COOH···OH)
- Molecules don’t associate strongly
- Lower BP
p-Hydroxybenzoic acid:
- Forms intermolecular H-bonds
- Molecules associate strongly
- Higher BP
Conclusion: p-isomer > o-isomer
(b) Solubility in Water:
o-Hydroxybenzoic acid:
- Intramolecular H-bond “uses up” H-bonding sites
- Less available for H-bonding with water
- Lower solubility
p-Hydroxybenzoic acid:
- No intramolecular H-bond
- Both -OH and -COOH available for H-bonding with water
- Higher solubility
Conclusion: p-isomer > o-isomer
(c) Acidity:
o-Hydroxybenzoic acid:
- Intramolecular H-bond between -COOH and -OH
- Stabilizes the molecule
- Makes it harder to lose H⁺ from COOH
- Lower acidity (pKa ≈ 3.0)
Wait, actually opposite!
Correction: o-isomer has intramolecular H-bond in anion (after losing H⁺):
- Anion (COO⁻···HO-) is stabilized by H-bond
- More stable anion → easier to lose H⁺
- Higher acidity (pKa ≈ 2.98)
p-Hydroxybenzoic acid:
- No intramolecular stabilization of anion
- Lower acidity (pKa ≈ 4.58)
Conclusion: o-isomer > p-isomer (more acidic!)
Answer: (a) BP: p > o (b) Solubility: p > o (c) Acidity: o > p (intramolecular H-bond stabilizes conjugate base!)
Question: (a) Explain why ice floats on water (b) Predict what would happen if ice were denser than water (c) At what temperature is water densest?
Solution:
(a) Why ice floats:
Ice structure:
- Each H₂O molecule forms 4 H-bonds (tetrahedral)
- Creates open cage-like structure (hexagonal crystals)
- Lots of empty space trapped inside
- Density ≈ 0.92 g/cm³
Liquid water:
- H-bonds constantly breaking and reforming
- Molecules pack more closely (less organized)
- Density ≈ 1.00 g/cm³
Result: ρ(ice) < ρ(water) → ice floats!
(b) If ice were denser:
Consequences:
- Ice would sink to the bottom
- Lakes/oceans would freeze from bottom up
- Ice layer would keep growing upward
- All aquatic life would die (frozen solid)
- Earth’s albedo (reflectivity) would decrease → climate disaster
- Life on Earth likely impossible
(c) Maximum density temperature:
Water is densest at 4°C (277 K)
Why?
- At 0°C: Ice structure (low density)
- 0-4°C: Ice structure breaks → density increases
- 4°C: Maximum density (optimal packing)
- Above 4°C: Thermal expansion → density decreases
Graph:
Density ↑
1.000 | /—\ ← Max at 4°C
| / \
0.917 |____/ \____
0°C 4°C 100°C
Biological importance: Deep water stays at 4°C (densest layer sinks) → aquatic life survives in deep water even when surface freezes!
Answer: (a) Ice has open H-bonded structure (less dense) → floats (b) If ice sank → oceans freeze solid → no aquatic life → likely no life on Earth (c) Water is densest at 4°C (not 0°C!)
Quick Revision Box
H-Bonding Requirements
| Requirement | Details |
|---|---|
| Atoms | Only F, O, N (+ H) |
| Reason | High EN + Small size + Lone pairs |
| Strength | 10-40 kJ/mol (weaker than covalent) |
| Geometry | Best when linear (180°) |
Effects Summary
| Effect | Explanation |
|---|---|
| High BP | Must break H-bonds to boil |
| High viscosity | H-bonds resist flow |
| High surface tension | Surface molecules H-bond downward |
| Solubility | H-bonding compounds dissolve in water |
| Ice floats | Open H-bonded structure (less dense) |
| DNA stability | A-T, G-C base pairing via H-bonds |
Intermolecular vs Intramolecular
| Intermolecular | Intramolecular | |
|---|---|---|
| Between/Within | Between molecules | Within one molecule |
| BP | ↑ Increases | ↓ Decreases |
| Solubility | ↑ Increases | ↓ Decreases |
| Example | H₂O, p-nitrophenol | o-nitrophenol |
Real-World Applications
1. Biology:
- DNA: H-bonds hold double helix (A-T, G-C pairing)
- Proteins: H-bonds create α-helix, β-sheet structures
- Enzyme-substrate: H-bonds help specific binding
2. Climate:
- Water cycle: High heat of vaporization moderates temperature
- Ice caps: Ice floats → reflects sunlight → cools Earth
- Ocean currents: Density differences drive circulation
3. Industry:
- Pharmaceuticals: Drug design considers H-bonding for solubility
- Polymers: Nylon, Kevlar use H-bonds for strength
- Chromatography: Separation based on H-bonding differences
4. Daily Life:
- Cooking: Boiling water (break H-bonds)
- Sweating: Evaporative cooling (break H-bonds = absorbs heat)
- Alcohol effects: Ethanol H-bonds with brain proteins → impairment
- Hand sanitizer: Alcohols denature proteins via H-bond disruption
5. Materials:
- Kevlar: Strong H-bonding between polymer chains → bullet-proof
- Paper: Cellulose fibers H-bond → strength when wet
- Adhesives: H-bonds to surfaces (glue stickiness)
Teacher’s Summary
1. H-bonding = special dipole-dipole interaction
- Requires: X-H···Y where X, Y = F, O, or N
- Only FON work (high EN, small size, lone pairs)
- Strength: 10-40 kJ/mol (weaker than covalent, stronger than van der Waals)
2. Two types with OPPOSITE effects:
- Intermolecular: Between molecules → Increases BP, viscosity, solubility
- Intramolecular: Within molecule → Decreases BP (molecules self-satisfied)
3. H-bonding explains water’s unique properties:
- High BP (100°C vs -60°C expected)
- Ice floats (open structure, 4 H-bonds per molecule)
- High heat of vaporization (cooling via sweat)
- Maximum density at 4°C (not 0°C!)
4. Biological importance:
- DNA: A-T (2 H-bonds), G-C (3 H-bonds) hold strands
- Proteins: α-helix, β-sheet from H-bonds
- Enzyme specificity: H-bonding in active sites
5. JEE Patterns:
- Boiling point: H-bonding > No H-bonding (always!)
- Ortho vs Para: Intramolecular (ortho) → lower BP than intermolecular (para)
- Anomalies: First member of group (HF, H₂O, NH₃) has highest BP
- Solubility: “H-bonds with water” = soluble
6. Quick predictions:
- Molecule has -OH, -NH, -FH → Likely H-bonding
- o-isomer with nearby -OH, =O → Intramolecular H-bond
- More H-bonding sites → Higher BP
“Hydrogen bonding: The weak force that makes life possible!”
Related Topics
Within Chemical Bonding
- Covalent Bonding — Polarity and dipole moments
- VSEPR Theory — Shapes that enable H-bonding (bent H₂O)
- Ionic Bonding — Contrast: electron transfer vs H-bonding
Cross-Chapter Links
- Solutions — Solubility depends on H-bonding
- Liquid State — Viscosity, surface tension from H-bonds
- Biomolecules — DNA, proteins, carbohydrates
- Alcohols, Phenols — Classic H-bonding examples
- Amines — NH₂ group H-bonding
Cross-Subject: Biology
- DNA structure — Double helix held by H-bonds
- Protein folding — Secondary/tertiary structure
- Cell membranes — Phospholipids and H-bonding
Cross-Subject: Physics
- Intermolecular forces — H-bonding as electrostatic interaction
- Thermal properties — Heat capacity, phase changes