A-Level Amines, amides and amino acids: worked solution

Question

This question concerns heterocyclic compounds. The NAD+/NADH couple acts as a biological redox system. NAD+ can be drawn as a pyridinium ring (a positively charged nitrogen carrying an R group) that bears a carboxamide (-CONH2) group at position 3; NADH is the matching 1,4-dihydropyridine (reduced at ring position 4, so the nitrogen is neutral and one ring carbon has become an sp3 -CH2- centre). Decide whether each compound is aromatic or non-aromatic and give reasons for your decision.

NAD+ (pyridinium ring): aromatic. The ring is planar with every ring atom contributing a p orbital to a continuous cyclic overlap, giving a delocalised system of 6 π electrons (Hückel 4n+2, n=1). The positive charge on nitrogen does not disrupt this; the aromatic sextet is retained. NADH (1,4-dihydropyridine): non-aromatic. Ring position 4 has become an sp3 CH2 carbon, which has no p orbital in the ring plane, so the cyclic conjugation is broken and there is no continuous delocalised π ring, so it cannot be aromatic.