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Keto-enol Tautomerism The carbonyl group is electron withdrawing both by the -I and the -R effect. That makes the carbon more positive and withdraws electron density from the -C-H bond. The -hydrogens become more acidic. The acidity of hydrogens enables carbonyl groups (as in acetone) to undergo keto-enol tautomerism. Tautomers are rapidly interconverted constitutional isomers. They usually have the or -hydrogen atom on the O of the carbonyl group and the C=O double bond relocated between the carbonyl carbon and the -carbon. Tautomeric equilibria are catalyzed by traces of acids or bases that are generally present in most chemical samples. Mechanism for Enol Formation Acid conditions 1) Protonation of the Carbonyl 2) Removal of the -hydrogen Basic conditions 1) Removal of the -hydrogen 2) Protonation , -Unsaturated Carbonyl Compounds In , -unsaturated carbonyl compounds, the -R effect of the C=O group is also transmitted to the =carbon, as in pent-3-en-2_one, CH -CH=CH(C=O)CH . This makes the -carbon slightly more positive and the -H more acidic. Keto-enol tautomerism can occur by the same mechanisms as before. In 3-methylcyclopenten-2-one, there are three possible tautomeric forms, involving H atoms 1, 3, and 4, respectively. H atoms 2 and 5 cannot participate in keto-enol tautomerism. H-1 is an -hydrogen. H-3 and H-4 are -hydrogens. H-2 is a -hydrogen. It cannot participate because there is no way to delocalize its electrons to the O atom by resonance. Its electrons are localized in an sp orbital H-5 is an -hydrogen. Its electrons are localized in an sp orbital

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