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Organic Chemistry Solutions Manual Purpose of the problem A more exacting exploration of the precise details of conjugation. Suggested solution Treating each molecule separately in the styles demanded by the question, the first compound is the guanidinium ion, a very stable cation because of conjugation. The curly arrows in the first set of diagrams show that each nitrogen has an equal amount of positive charge and this is reflected in the second diagram showing exactly one-third + on each N. The third diagram, of the lowest-energy MO, looks down on the molecule and shows the top lobe of the P orbital on each atom. NH₂ NH₂ NH₂ NH NH₂ NH₂ NH₂ NH₂ 1/3 (+) 1/3 (+) The second molecule is what we shall learn to call an enolate ion. The negative charge is delocalized throughout the conjugated system but is particularly on the two oxygen atoms and, to a lesser extent, on the central carbon atom. It is difficult to represent this accurately with partial charges but the charge on each oxygen atom is nearly a half. The lowest-energy MO has all the orbitals in phase. 0 0 0 0 (-) (-) The third compound is naphthalene. The structure drawn is the best; curly arrow diagrams do not do this molecule justice and other versions are less convincing. In particular, they do not show that the middle bond common to both rings is the shortest bond. There are no charges anywhere and all the P orbitals are conjugated. This is the basic ten-electron aromatic system. Problem 5 Which of these compounds are aromatic? Justify your answer with some electron counting. You are welcome to treat each ring separately or two or more rings together, whichever you prefer. 0 N N N OH N 0 0 MeO 0 H aklavinone: a tetracycline antibiotic NH NHAc MeO OMe Ho 0 N CO₂H 0 0 OMe OR methoxatin: coenzyme from colchicine: compound from Autumn bacteria living on methane crocus used to treat gout callistephin: natural red flower pigment