Introduction:
Purine and pyrimidine are two types of nitrogen-containing bases that are essential components of nucleotides, the building blocks of nucleic acids such as DNA and RNA. Both purine and pyrimidine are hydrophobic in nature, meaning they repel water. This hydrophobicity is mainly due to the presence of aromatic rings in their chemical structures.

Structure of purine and pyrimidine:
1. Purine: Purine is a heterocyclic compound consisting of a pyrimidine ring fused with an imidazole ring. It has a double-ring structure, with a six-membered ring fused to a five-membered ring.
2. Pyrimidine: Pyrimidine is a heterocyclic compound consisting of a six-membered ring containing four carbon atoms and two nitrogen atoms.

Hydrophobicity of purine and pyrimidine:
1. Aromatic rings: Both purine and pyrimidine contain aromatic rings in their structures. Aromatic rings are flat and planar structures with a delocalized electron system. These delocalized electrons create a stable electron cloud that contributes to the hydrophobicity of these molecules.
2. Nonpolar nature: Aromatic rings are composed of carbon and hydrogen atoms, which are both nonpolar elements. Nonpolar molecules do not have a charge separation and do not readily interact with polar molecules like water.
3. Hydrogen bonding: Purine and pyrimidine can form hydrogen bonds with other molecules or functional groups, but these hydrogen bonds are not as energetically favorable as the interactions between water molecules, which form extensive hydrogen bonding networks.
4. Repulsion of water: Due to their hydrophobic nature, purine and pyrimidine molecules tend to cluster together and exclude water molecules. This clustering helps to minimize the unfavorable interactions between the hydrophobic bases and the polar water molecules.

Conclusion:
In summary, purine and pyrimidine are hydrophobic due to the presence of aromatic rings in their structures, which are nonpolar and repel water. Their hydrophobic nature allows them to form stable interactions with other hydrophobic molecules, contributing to their role as the building blocks of nucleic acids.