Chemistry - GOC Question with Solution | TestHub
Thyroxine, aklavinone, colchicine, callistephin and methoxatin are drawn below. How many aromatic rings are there in all five compounds together ? Count the benzene-type rings, the aromatic seven-membered ring of colchicine (do not count its C=O electrons), the pyrylium-type ring of callistephin, and the pyrrole and pyridine rings of methoxatin, but not the quinone-type and saturated rings.
(a) thyroxine 
(b) aklavinone 
(c) colchicine 
(d) callistephin 
(e) methoxatin 
Answer:
Solution:
(a) thyroxine : 2 benzene rings. (b) aklavinone : the two benzene rings (A and C). The quinone-type ring B (two C=O) is not aromatic and the ring with the ethyl and ester groups is saturated : 2. (c) colchicine : one benzene ring and one seven-membered ring with 6π : 2. (d) callistephin : the phenyl ring, the benzene ring and the pyrylium ring : 3. (e) methoxatin : the pyrrole ring (6π with the N lone pair) and the pyridine ring : 2. The quinone-type ring is not aromatic. Total = 2 + 2 + 2 + 3 + 2 = 11.
