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164 Organic Chemistry Solutions Manual Suggested solution Compound A is mostly enol because the enol is delocalized over five atoms. A minor reason for stability of this particular compound is that the enol has an internal hydrogen bond. The reason is conjugation. H. A 0 oH 0 0 0 OEt You may also have pointed out that there is an alternative and equally good enol that has the carbonyl group enolized. These two structures are tautomers of each other and of the keto-ester. oH 0 0 0 OH OEt OEt OEt That compound B is totally enolized shows that the conjugation is much more important the internal hydrogen bond, which is impossible with B. The two enols are the same and have conjugation from the lone pair electrons on the other oxygen atoms. © o OH OH The rest of the compounds have special problems with enolization. Compound C cannot enol between the two carbonyl groups at all as the alkene would be a bridgehead alkene unable get its P orbitals parallel (p. 484). It can, of course, form an ordinary enol on the other side ketone but this has no special stabilization. c Ho H Ho o 0 0 impossible structure Compounds D and E can form enols but the t-butyl group in D has to move into the plane the other atoms and there is a bad steric clash with the adjacent oxygen atoms. The t-butyl group out of the plane in the keto form. Compound E is strained but the enol is more strained as sp² atom has to fit into the four-membered ring. The dienol is worse - even more strained and aromatic (four electrons) to boot. D E o OH OH 0 Ho o Problem 10 Bromination of ketones can be carried out with molecular bromine in a carboxylic acid solution. Give a mechanism for the reaction. 0 0 Br₂ Br The rate of the reaction is not proportional to the concentration of bromine [Br₂]. Suggest an explanation. Why is the bromination of ketones carried out in acidic and not in basic solution?