The purine heterocyclic ring seen mainly in the structure of DNA. Identify number of ‘N’ atoms having localized lone pair of electron.

To answer the question based on the given purine ring structure:

Stepwise Solution

• The purine ring has four nitrogen atoms.
• Localized lone pair of electrons means a nitrogen whose lone pair is not involved in resonance (delocalization) with the aromatic π-system.
• Pyridine-like nitrogens (with lone pairs orthogonal to the ring π-system) have localized lone pairs. Pyrrole-like nitrogens have lone pairs delocalized.
• Analyzing the structure:
  • The nitrogen atoms at positions 1 and 7 (N-H above and N with two dots at the top right of the six-membered ring) have lone pairs not involved in the aromatic system; their lone pairs are localized.
  • The other two nitrogens (in the six-membered ring's left and bottom positions) have their lone pairs involved in the aromatic system (delocalized).

Final Answer

Number of N atoms having localized lone pair = 2.

These are the nitrogens at positions 1 and 7 in the purine ring.