Question:

The boiling point of para nitrophenol is greater than ortho nitrophenol, because:

KEAM

Updated On: Jun 7, 2024

there is intermolecular hydrogen bonding in para nitrophenol and intramolecular hydrogen bonding in ortho nitrophenol
there is intramolecular hydrogen bonding in para nitrophenol and intermolecular hydrogen bonding in ortho nitrophenol
both have the same kind of hydrogen bonding
para nitrophenol is polar, while ortho nitrophenol is non-polar

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The Correct Option is A

Solution and Explanation

Phenols containing group like $ N{{O}_{2}} $ in the ortho position to the $ -OH $ group can form intramolecular hydrogen bonds (e. g., o-nitrophenol) which is responsible for their lower boiling points and less solubility in water than the corresponding meta or para isomer. Due to possibility of intramolecular H-bonding (also known as chelation) in the ortho isomer, intermolecular H-bonding is not possible and hence, the ortho isomer can neither associate nor can form H-bonding with $ {{H}_{2}}O $ with the result it has a low boiling point and less solubility in $ {{H}_{2}}O $ than the meta and para isomers which can associate (union of 2 or more molecules of the same species) as well as can form H-bonding with $ {{H}_{2}}O $ .

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Concepts Used:

Hydrogen Bonding

Hydrogen bonding implies the formation of hydrogen bonds which are an attractive intermolecular force. An example of hydrogen bonding is the bond between the H atom and the O atom in water.

A special type of intermolecular attractive force arises only in the compounds having Hydrogen atoms bonded to an electronegative atom. This force is known as the Hydrogen bond. For instance, in water molecules, the hydrogen atom is bonded to a highly electronegative Oxygen.

The conditions for hydrogen bonding are:

The molecule must contain a strongly electronegative atom that is bound to the hydrogen atom. The higher the electronegativity, the more polarized is the molecule.
The electronegative atom must be small. The smaller the size, the greater the electrostatic magnetism.

Effects of Hydrogen Bonding on Elements:

Association: The molecules of carboxylic acids exist as dimer because of the hydrogen bonding. The molecular masses of such compounds are found to be double than those calculated from their simple formula.

Dissociation: In aqueous solution, HF dissociates and gives the difluoride ion instead of fluoride ion. This is due to hydrogen bonding in HF. The molecules of HCl, HBr, HI do not form a hydrogen bond. This explains the non-existence of compounds like KHCl2, KHBr2, KHI2.

Types of Hydrogen bonding

Intramolecular Hydrogen bonding: When hydrogen bonding takes place between different molecules of the same or different compounds, it is called intermolecular hydrogen bonding.
Intermolecular hydrogen bonding: The hydrogen bonding which takes place within a molecule itself is called intramolecular hydrogen bonding.
Symmetrical Hydrogen bonding: The symmetric hydrogen bond is a type of a three-centre four-electron bond.