Question
Distinguish between intermolecular and intramolecular hydrogen bonding. Give examples of each and explain how each type affects the physical properties of the compound.
Solution — Step by Step
Hydrogen bonding forms when a hydrogen atom covalently bonded to a highly electronegative atom (F, O, or N) is attracted to another electronegative atom in the same or a different molecule.
Requirements:
- Hydrogen must be bonded to F, O, or N (these are electronegative enough to make H partially positive, δ+)
- The hydrogen bond acceptor must be F, O, or N (must have lone pairs)
- The H…acceptor distance must be small enough (within van der Waals range)
The hydrogen bond is represented as X-H···Y, where X-H is the donor and Y is the acceptor.
Intermolecular hydrogen bonding forms between two different molecules (or between different parts of the same molecule that are not in the same chain segment).
Examples:
- Water (): each water molecule can form up to 4 H-bonds with neighboring molecules → extensive network → unusually high boiling point (100°C vs expected −80°C)
- Ethanol (): O-H of one molecule forms H-bond with O of another
- HF: strong zigzag chains of H-bonded molecules
- Carboxylic acids: form dimers through two O-H···O bonds between two molecules
Effect on properties: Intermolecular H-bonding requires more energy to overcome during melting/boiling → raises melting point, boiling point, viscosity, and surface tension. It also increases solubility in polar solvents like water.
Intramolecular hydrogen bonding forms within the same molecule, between an H-donor group and an H-acceptor group on different parts of the same molecule. This creates a ring-like structure (typically 5- or 6-membered ring).
Examples:
- o-nitrophenol: The O-H group forms a H-bond with the nearby N=O oxygen within the same molecule, forming a 6-membered ring
- o-hydroxybenzoic acid (salicylic acid): O-H of -OH forms H-bond with carbonyl O of -COOH group in the same molecule
- o-chlorophenol: weaker intramolecular H-bonding
- Proteins (α-helix): C=O group of one amino acid forms H-bond with N-H of another amino acid in the same polypeptide chain (4 residues away) — this stabilizes the secondary structure
Effect on properties: Intramolecular H-bonding “ties up” the donor and acceptor within the molecule. The molecule cannot form as many H-bonds with other molecules or with water → lowers melting point, boiling point compared to the isomer with intermolecular H-bonding; decreases solubility in water.
The classic exam comparison:
| Property | o-Nitrophenol | p-Nitrophenol |
|---|---|---|
| H-bonding type | Intramolecular | Intermolecular |
| Boiling point | Lower (~216°C) | Higher (~279°C) |
| Solubility in water | Lower | Higher |
| Volatility | More volatile (lower bp) | Less volatile |
Why: In o-nitrophenol, the -OH and -NO₂ groups are adjacent → form intramolecular H-bond → cannot H-bond with water effectively → less soluble, lower bp.
In p-nitrophenol, the groups are too far apart for intramolecular H-bonding → -OH freely H-bonds with water molecules → more soluble, higher bp.
Why This Works
The key insight is that hydrogen bonds are directional and form where donor and acceptor are geometrically close. Ortho-substituted compounds place two functional groups adjacent to each other on the benzene ring — if both are H-bond donors/acceptors, they naturally form intramolecular H-bonds.
The thermodynamic logic: a molecule that satisfies its H-bonding capacity internally doesn’t need to interact as strongly with other molecules. So it takes less energy to vaporize (lower bp) and it interacts less favorably with polar solvents like water (lower solubility).
This principle is exploited in drug design: drugs that form intramolecular H-bonds are more lipophilic (fat-soluble) and can cross cell membranes more easily than their isomers that H-bond with water.
Alternative Method
A quick way to predict which isomer has intramolecular vs intermolecular H-bonding: look at the molecular structure.
- Ortho (1,2) substitution: substituents are adjacent → intramolecular H-bonding possible if groups are complementary
- Meta (1,3) or Para (1,4) substitution: groups too far apart → only intermolecular H-bonding
Structural rule: intramolecular H-bonding is favored when it would form a 5- or 6-membered ring — these ring sizes minimize strain.
The o-nitrophenol vs p-nitrophenol comparison is asked in nearly every JEE Main and CBSE Class 12 exam cycle — at least as an MCQ. Memorize: ortho = intramolecular H-bond = lower bp, less water soluble. Para = intermolecular H-bond = higher bp, more water soluble. This same logic extends to o-hydroxybenzoic acid vs p-hydroxybenzoic acid, o-fluorophenol vs p-fluorophenol, etc.
Common Mistake
Students often confuse the effect on boiling point. When told “intramolecular H-bonding,” many incorrectly think it should increase the boiling point because H-bonds are “stronger.” The key is that intramolecular bonds are already satisfied internally — the molecule has fewer opportunities to form H-bonds with neighboring molecules. So it’s the intermolecular interactions that determine boiling point, not the presence of any H-bond per se. o-Nitrophenol has a lower boiling point than p-nitrophenol precisely because it forms intramolecular H-bonds (and thus fewer intermolecular ones).