1. Enols1 Enols

2. 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. Enols3 The  -Carbon Atom and its Hydrogens

4. Enols4  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. Terminology CH 3 CH 2 CH 2 CH O Reference Atom (ipso) O R H H H   

5. Enols5 Enolization (tautomerism)

6. Enols6 Mechanism of Enolization (In general) OCR' R2CR2CR2CR2C H OHH OHH R2CR2CR2CR2C CR' O H

7. Enols7 Mechanism of Enolization (Acid-catalyzed) OHH H + R2CR2CR2CR2C H OCR' O R2CR2CR2CR2C CR' H OHH H +

8. Enols8 Mechanism of Enolization (Acid-catalyzed) O R2CR2CR2CR2C CR' H H + OHH H O H H + O R2CR2CR2CR2C CR' H

9. Enols9  percent enol is usually very small  keto form usually 45-60 kJ/mol more stable than enol Enol Content R 2 CHCR' O R2CR2CR2CR2C CR'OHenolketo

10. Enols10 Enol Content CH 3 CH O H2CH2CH2CH2C CHOH K = 3 x 10 -7 Acetaldehyde CH 3 CCH 3 O H2CH2CH2CH2C CCH 3 OH K = 6 x 10 -9 Acetone

11. Enols11 Stabilized Enols Stabilized Enols

12. Enols12 2,4-CyclohexadienoneOH H H H H HOHH HH H H  keto form is less stable than enol form  keto form is not aromatic  enol form is aromatic

13. Enols13

14. Enols14 SUMMARY: Keto-Enol Tautomerism O R' R R H Keto form OH R' R R Enol form

15. Enols15 SUMMARY: Enol-Keto Tautomerism O R' R R H Keto form OH R' R R Enol form

16. Enols16 ketoketoketoketo ketoenolenolenol no α-Hmost stable most stable

17. Enols17 ketoketoketoketo enolenolenolenol

18. Enols18 ketoketoketoketo ketoenolenolenol