Intramolecular versus Intermolecular Hydrogen Bond
Let's take a look at the melting points of some isomers of hydroxybenzoic acid.
We notice that the melting point is lowest (158.6oC) for 2-hydroxybenzoic acid, where the alcohol group is closest to the acid group.
Melting point is highest (214.5oC) for 4-hydroxybenzoic acid, where the alcohol group is farthest away from the acid group.
All isomers of hydroxybenzoic acid can form hydrogen bonds due to the presence of O-H bonds.
The position of the -OH group with respect to the -COOH group has an effect on the extensiveness of hydrogen bonds between molecules, and hence their melting points.
1. Intermolecular Hydrogen Bonds in 4-hydroxybenzoic acid
The -OH and -COOH groups are pointing opposite each other and are unable to interact with each other within the molecule.
Therefore both groups can be used for hydrogen bonds between molecules.
This results in more extensive hydrogen bonds and higher melting point for 4-hydroxybenzoic acid.
2. Intramolecular Hydrogen Bonds in 2-hydroxybenzoic acid
The -OH and -COOH groups are next to each other and can form hydrogen bond within the molecule.
Since hydrogen bonds are used up for intramolecular interaction, the intermolecular forces between 2-hydroxybenzoic acid molecules will be weaker instantaneous dipole-induced dipole (id-id) attractions, or dispersion forces or Van der Waals forces.
Hence the melting point of 2-hydroxybenzoic acid is lower.
In summary this is the comparison and reason why intramolecular hydrogen bond in 2-hydroxybenzoic acid lowers its melting point as compared to 4-hydroxybenzoic acid.
Topic: Intermolecular Forces, Physical Chemistry, A Level Chemistry, Singapore
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