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Suggested solution Both compounds are quite polar and not very soluble in the usual organic NMR solvents. In addition, they have NH or OH protons that exchange and broaden the signals. If the exchange is made rapid by acid or base catalysis and the NHs and OHs exchanged for deuterium, all these problems disappear and clean sharp spectra result. The first compound is a guanidine and forms a cation in acid. The cation is symmetrical and a very simple spectrum results - just three types of carbon in the aromatic region and one very low field carbon (at large δ) for the carbon in the middle of the cation. D N D N DCI N D₂N D₂N N D₂O N N H D D The second compound loses a proton from the OH group in base to give a delocalized symmetrical anion. There will be five signals in the NMR spectrum: the two methyl groups are the same and the two CH₂ groups in the ring are equivalent. There is one unique carbon atom joined to the two methyl groups and another in the middle of the anion and, finally, the two carbonyl groups are equivalent. NaOD D₂O o Problem 9 The phenols shown here have approximate values of 4, 7, 9, 10, and 11. Suggest with explanations which pKₐ value belongs to which phenol. Me OH NO₂ Me Me Purpose of the problem A problem requiring detailed interpretation of electronic effects. Suggested solution Electron-withdrawing groups (Cl and even more so make phenols more acidic but electron- donating groups (Me) make them less acidic. The nitro group is very effective if it can stabilize the anion by conjugation. A pKₐ value of about 10 is normal for a phenol, so it is phenol itself that has that value. The compound less acidic than phenol 11) must be the trimethyl compound. One chlorine atom will make phenol slightly more acidic so 9 must be 3-chlorophenol and the compounds with one nitro group 7) or two (pKₐ 4) must be the very acidic phenols. Problem 10 Discuss the stabilization of the anions formed by the deprotonation of (a) and (b) and the cation formed by the protonation of (c). Consider delocalization in general and the possibility of aromaticity in particular. (a) (b) (c) N H N Ho N N N N H