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Enols

Enols. Keto-Enol Tautomerism?. Tautomerism v . - an interconversion between two structures that differ by the placement of an atom or group; in this case, an α hydrogen atom ( H ). The  -Carbon Atom and its Hydrogens. O. H. O. g. a. CH 3 CH 2 CH 2 CH. b. R. H. H. Terminology.

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Enols

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  1. Enols

  2. Keto-Enol Tautomerism? Tautomerism v. - an interconversion between two structures that differ by the placement of an atom or group; in this case, an α hydrogen atom (H).

  3. The -Carbon Atom and its Hydrogens

  4. O H O g a CH3CH2CH2CH b R H H Terminology Reference Atom (ipso) • The reference atom is the carbonyl carbon. • Other carbons are designated , , , etc. on the basis of their position with respect to the carbonyl carbon. • Hydrogens take the same Greek letter as the carbon to which they are attached.   

  5. Enolization (tautomerism)

  6. H •• O H O •• •• •• R2C CR' H O H •• •• H •• O H •• R2C CR' Mechanism of Enolization(In general)

  7. H + •• H O H O •• •• •• O H O H R2C •• CR' •• + H R2C CR' H H Mechanism of Enolization(Acid-catalyzed)

  8. •• H O •• R2C CR' H H + O O H •• •• •• H H Mechanism of Enolization(Acid-catalyzed) + •• H O R2C CR' H

  9. OH O R2CHCR' R2C CR' Enol Content • percent enol is usually very small • keto form usually 45-60 kJ/mol more stablethan enol keto enol

  10. OH O CH3CH H2C CH OH O Acetone CH3CCH3 H2C CCH3 K = 6 x 10-9 Enol Content Acetaldehyde K = 3 x 10-7

  11. Stabilized Enols

  12. O OH H H H H H H H H H H H 2,4-Cyclohexadienone • keto form is less stable than enol form • keto form is not aromatic • enol form is aromatic

  13. O H R ' R ' K e t o f o r m E n o l f o r m SUMMARY: Keto-Enol Tautomerism O R R H R R

  14. O O H R R R ' R ' H R R K e t o f o r m E n o l f o r m SUMMARY: Enol-Keto Tautomerism

  15. keto keto keto keto most stable most stable keto enol enol enol no α-H most stable

  16. enol enol enol enol keto keto keto keto

  17. keto keto keto keto keto enol enol enol

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