However the diphenol has a very simple nmr spectrum

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Unformatted text preview: eason—conjugation of C=C and C=O— and a minor reason – intramolecular hydrogen bonding. OH O O O CO2Et H O O OEt H O OEt 3 Solutions for Chapter 20 – Formation and Reactions of Enols and Enolates PROBLEM 3 The NMR spectrum of this dimethyl ether is complicated: the two MeO groups are different as are all the hydrogen atoms on the rings. However the diphenol has a very simple NMR spectrum – there are only two types of proton on the rings marked ‘a’ and ‘b’ on the diagram. Explain. Ha O Hb O Hb OH Ha OMe Ha dimethyl ether diphenol OMe OH Hb Purpose of the problem Exploring the way that tautomerism leads to equivalence. Suggested solution The protons in the ether are obviously all different as it has no symmetry. Tautomerization interconverts carbonyl groups and enols, and can make either of the enols in the diphenol into a carbonyl group and can make the carbonyl group into an enol, so all structures are equivalent. If this proton transfer (note that it is not a delocalization) is fast on the NMR time scale, all the Has will appear in one signal and all the Hbs will appear in another signal. Hb O Hb Ha Hb Ha Hb HO O Ha interconversions take place too fast to be detected by NMR (they are fast on the NMR timescale) is covered in the blue boxes on p. 363 and p. 374 of the textbook. Ha Ha OH ■ The idea that some H Hb O H O Hb Hb Ha Ha Hb HO Ha OH Ha O Hb 4 Solutions Manual to accompany Organic Chemistry 2e PROBLEM 4 Suggest mechanisms for these...
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