Question

Explain why o-hydroxybenzaldehyde is a liquid at room temperature while p-hydroxy benzaldehyde is a high melting solid.

Answer 1

Hint:There are two types of hydrogen bonding one is intramolecular and other is intermolecular. In o-hydroxy benzaldehyde intramolecular hydrogen bonding is observed and in p-hydroxy benzaldehyde intermolecular hydrogen bonding is observed.

Complete step by step answer:Hydrogen bond is defined as the electrostatic force of attraction taking place between the hydrogen atom and the most electronegative element present in the molecule.
Depending on the bonding type the hydrogen bond is divided into two forms as shown below.
Intermolecular hydrogen bonding: The intermolecular hydrogen bonding is described as the bonding taking place between the two or more than two different molecules.
Intramolecular hydrogen bonding: The intramolecular hydrogen bonding is described as the bonding taking place between the atoms of the same molecule.
The intramolecular hydrogen bonding is more stronger than the intermolecular hydrogen bonding as in intramolecular hydrogen bonding actual sharing of electrons takes place between the two groups, one group contains hydrogen atom bonded with the electronegative atom and other group consists of more electronegative atom bonded to atom with less electronegativity which results in a stronger bond formation.
In o-hydroxy benzaldehyde intramolecular hydrogen bonding is seen while in p-hydroxy benzaldehyde intermolecular hydrogen bonding is seen. As intramolecular hydrogen bonding is stronger than intermolecular hydrogen bonding p-hydroxy benzaldehyde has considerably high melting point as the bond cannot be broken easily and o-hydroxybenzaldehyde is liquid at room temperature.

Note:The intramolecular hydrogen bonding helps in the cyclization process of the molecule. The hydrogen bonds are weaker than the covalent bond. For forming a strong hydrogen bonding, the electronegative atom should be small in size.