1. 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

2. Mechanism of Enolization (In general) OCR' R2CR2CR2CR2C H OHH OHH

3. Mechanism of Enolization (In general) OCR' R2CR2CR2CR2C H R2CR2CR2CR2C CR' O H

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

5. Mechanism of Enolization (Acid-catalyzed) O R2CR2CR2CR2C CR' H O H H H +

6. Mechanism of Enolization (Acid-catalyzed) O R2CR2CR2CR2C CR' H H +

7. O R2CR2CR2CR2C CR' H H + OHH

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

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

11. 18.5 Stabilized Enols

12. 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. 1,3-Diketones (also called  -diketones) keto form is less stable than enol form CH 3 CCH 2 CCH 3 OO CH 3 C OH CHCCH 3 O(20%)(80%) Example: 2,4-pentanedione

14. Enol form of 2,4-pentanedione

15. 103 pm 133 pm 134 pm 141 pm 124 pm 166 pm H3CH3CH3CH3C C C C CH 3 H O O H C=C and C=O are conjugated intramolecular hydrogen bond