What are the essential conditions for hydrogen bond formation?

For a hydrogen bond to form, three key conditions must be met. First, there must be a hydrogen atom covalently bonded to a highly electronegative atom, typically Fluorine (F), Oxygen (O), or Nitrogen (N). This creates a highly polar bond, making the hydrogen partially positive. Second, there must be another highly electronegative atom (F, O, or N) with at least one lone pair of electrons, which acts as the hydrogen bond acceptor. This acceptor atom carries a partial negative charge. Finally, these two electronegative atoms and the hydrogen atom must be in close proximity, allowing for the electrostatic attraction to occur.

Why does water have an unusually high boiling point compared to H$_2$S?

Water (H$_2$O) has a significantly higher boiling point than hydrogen sulfide (H$_2$S), despite H$_2$S having a larger molecular mass. This anomaly is due to the extensive intermolecular hydrogen bonding present in water. Oxygen is much more electronegative than sulfur, leading to highly polar O-H bonds. These polar bonds allow water molecules to form strong hydrogen bonds with each other, creating a vast network that requires a substantial amount of energy to break during boiling. In contrast, sulfur is not electronegative enough to form hydrogen bonds, so H$_2$S molecules are held together only by weaker dipole-dipole interactions and London dispersion forces.

How does intramolecular hydrogen bonding affect a molecule's solubility in water?

Intramolecular hydrogen bonding generally decreases a molecule's solubility in water. When a molecule forms an internal hydrogen bond, the hydrogen donor and acceptor sites within that molecule are 'occupied' or 'satisfied'. This reduces their availability to form hydrogen bonds with water molecules. Since the ability to form hydrogen bonds with water is a major factor in aqueous solubility for many polar compounds, molecules exhibiting strong intramolecular hydrogen bonding tend to behave more like non-polar substances, thus showing reduced solubility in polar solvents like water.

Can a molecule exhibit both intermolecular and intramolecular hydrogen bonding?

Yes, it is possible for a molecule to exhibit both types of hydrogen bonding, though often one type dominates or is more significant under specific conditions. For example, a molecule with multiple hydroxyl groups, where some are in positions to form intramolecular hydrogen bonds (e.g., ortho-substituted) while others are not, might show both. However, the presence of strong intramolecular hydrogen bonding often reduces the extent of intermolecular hydrogen bonding by 'tying up' the potential bonding sites internally. The overall physical properties will reflect the net effect of both types of interactions.

Why are five- and six-membered rings preferred for intramolecular hydrogen bonding?

The formation of stable rings is crucial for intramolecular hydrogen bonding. Five- and six-membered rings are geometrically favorable because they minimize ring strain. In a five-membered ring, the atoms involved (H, the two electronegative atoms, and the two connecting atoms) can adopt a relatively strain-free conformation. Similarly, a six-membered ring allows for optimal bond angles and distances, leading to a stable chelate-like structure. Smaller rings (e.g., four-membered) would involve too much angle strain, while larger rings (e.g., seven-membered or more) would suffer from conformational flexibility and entropic disfavor, making the interaction less effective or stable.

Intermolecular and Intramolecular Hydrogen Bonding

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Hydrogen bonding is a special type of dipole-dipole interaction that occurs between a hydrogen atom covalently bonded to a highly electronegative atom (like nitrogen, oxygen, or fluorine) and another highly electronegative atom in the same or a different molecule. This interaction is stronger than typical dipole-dipole forces but weaker than covalent or ionic bonds. It plays a crucial role in dete…

Quick Summary

Hydrogen bonding is a special, strong type of dipole-dipole attraction involving a hydrogen atom covalently bonded to a highly electronegative atom (F, O, or N) and another electronegative atom. This creates a partially positive hydrogen and a partially negative electronegative atom, leading to an electrostatic attraction.

There are two main types: intermolecular and intramolecular. Intermolecular hydrogen bonding occurs *between* different molecules, leading to molecular association. This typically results in higher boiling points, melting points, viscosity, and increased solubility in polar solvents like water.

Examples include water, alcohols, and carboxylic acids. Intramolecular hydrogen bonding occurs *within* the same molecule, forming stable five- or six-membered rings. This internal bonding reduces the ability of molecules to interact with others, leading to lower boiling points, increased volatility, and decreased solubility in water.

Examples include o-nitrophenol and salicylaldehyde. The ability to distinguish between these types and predict their effects on physical properties is crucial for NEET.

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Key Concepts

Boiling Point Anomalies due to Intermolecular H-bonding

Intermolecular hydrogen bonding significantly increases the energy required to overcome attractive forces…

Volatility and Intramolecular H-bonding

Intramolecular hydrogen bonding reduces the ability of a molecule to form hydrogen bonds with *other*…

Solubility in Water and Hydrogen Bonding

The solubility of a substance in water is significantly influenced by its ability to form hydrogen bonds with…

Hydrogen Bond: — Electrostatic attraction between H (bonded to F, O, N) and another F, O, or N atom.

Donor: — H-X (X = F, O, N).

Acceptor: — Y (Y = F, O, N with lone pair).

Intermolecular H-bonding: — Between different molecules.

- $\uparrow$ Boiling Point, $\uparrow$ Melting Point, $\uparrow$ Viscosity, $\uparrow$ Water Solubility. - Examples: H $_2$ O, R-OH, R-COOH, NH $_3$ .

Intramolecular H-bonding: — Within the same molecule.

- Requires stable 5- or 6-membered ring. - $\downarrow$ Boiling Point, $\downarrow$ Melting Point, $\uparrow$ Volatility (steam volatile), $\downarrow$ Water Solubility. - Examples: o-Nitrophenol, Salicylaldehyde, o-Hydroxybenzoic acid.

Strength Order: — F-H

\cdots

F > O-H

\cdots

O > N-H

\cdots

To remember the effects of hydrogen bonding, think of 'I-I-I' for Intermolecular and 'I-A-D' for Intramolecular:

Intermolecular H-bonding: Increases BP, Increases Solubility, Increases Viscosity.

Intramolecular H-bonding: Always forms a ring, Decreases BP, Decreases Solubility, Decreases Intermolecular forces (making it more volatile).