Q8(c)(ii). C₂H₂BrCl with J = 16 Hz; two dienones from UV data
(1) A compound with MF C₂H₂BrCl exhibits two doublets (J = 16 Hz) in its PMR spectrum. Suggest a suitable structure along with other possible structures. (2) How can the structures of A and B be decided based on their UV spectral data? [λmax = 296 nm (εmax = 10700) and λmax = 281 nm (εmax = 20800)] [structures of dienones A and B given in the paper]
Answer in brief
(1) (E)-1-bromo-2-chloroethene: J = 16 Hz is a trans vicinal coupling; (2) A absorbs at 296 nm (ε 10700), B at 281 nm (ε 20800).
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Coverscoupling constanttrans vicinal couplingWoodward–Fieser rules dienonesmolar absorptivityPMR spectroscopy
All parts of Question 8
- 8(a)(i) Carbonyl stretching frequency of two acetylcyclohexenes5 marks
- 8(a)(ii) Dehydration of 1,2-dimethylcyclohexanol; telling the alkenes apart by IR10 marks
- 8(b)(i) λmax of two polyenes by Woodward–Fieser rules10 marks
- 8(b)(ii) λmax of 1,3-butadiene and 1,3,5-hexatriene5 marks
- 8(c)(i) McLafferty rearrangement; fragmentation of butyl butyrate10 marks
- 8(c)(ii) C₂H₂BrCl with J = 16 Hz; two dienones from UV data10 marks