1 / 34

Chapter 18 Enols and Enolates

Chapter 18 Enols and Enolates. 18.1 The a -Carbon Atom and its Hydrogens. O. CH 3 CH 2 CH 2 CH. Terminology. The reference atom is the carbonyl carbon. Other carbons are designated a , b , g , etc. on the basis of their position with respect to the carbonyl carbon.

lis
Download Presentation

Chapter 18 Enols and Enolates

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 18Enols and Enolates

  2. 18.1The a-Carbon Atom and its Hydrogens

  3. O CH3CH2CH2CH Terminology • The reference atom is the carbonyl carbon. • Other carbons are designated a, b, g, 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. g b a

  4. 18.2a Halogenation ofAldehydes and Ketones

  5. O O H+ R2CCR' R2CCR' H X General Reaction + + X2 HX • X2 is Cl2, Br2, or I2. • Substitution is specific for replacement of a hydrogen. • Catalyzed by acids. One of the products is an acid (HX); the reaction is autocatalytic. • Nota free-radical reaction.

  6. O O Cl + HCl + Cl2 Example H2O (61-66%)

  7. O O CH CH H Br Example • Notice that it is the proton on the a carbon that is replaced, not the one on the carbonyl carbon. CHCl3 + HBr + Br2 (80%)

  8. 18.3 Mechanism of a Halogenation ofAldehydes and Ketones

  9. Mechanism of a Halogenation Experimental Facts • specific for replacement of H at the a carbon • equal rates for chlorination, bromination, and iodination • first order in ketone; zero order in halogen

  10. Mechanism of a Halogenation Experimental Facts • specific for replacement of H at the a carbon • equal rates for chlorination, bromination, and iodination • first order in ketone; zero order in halogen Interpretation no involvement of halogen until after therate-determining step

  11. Mechanism of a Halogenation Two stages: • first stage is conversion of aldehyde or ketone to the corresponding enol; is rate-determining • second stage is reaction of enol with halogen; is faster than the first stage

  12. OH O O RCH2CR' RCH CR' Mechanism of a Halogenation • Enol is key intermediate slow X2 RCHCR' fast X enol

  13. Mechanism of a Halogenation Two stages: • first stage is conversion of aldehyde or ketone to the corresponding enol; is rate-determining • second stage is reaction of enol with halogen; is faster than the first stage examine second stage first

  14. •• OH •• R2C CR' Reaction of enol with Br2 •• •• Br Br •• •• •• ••

  15. •• •• OH OH •• •• R2C R2C CR' CR' + Br •• •• •• – •• Br •• •• •• Reaction of enol with Br2 • carbocation is stabilized by electron release from oxygen •• •• Br Br •• •• •• ••

  16. •• •• OH OH •• •• R2C R2C CR' CR' + Br •• •• •• – •• Br •• •• •• Reaction of enol with Br2 • carbocation is stabilized by electron release from oxygen •• •• Br Br •• •• •• ••

  17. •• •• OH OH •• •• R2C R2C CR' CR' + Br •• •• •• •• + OH R2C CR' Br •• •• •• Reaction of enol with Br2 •• •• Br Br •• •• •• ••

  18. – •• •• Br Br H •• •• •• •• •• •• + H O O •• R2C R2C CR' CR' Br Br •• •• •• •• •• •• Loss of proton from oxygen completes the process ••

  19. 18.4 Enolization and Enol Content

  20. Mechanism of a Halogenation Two stages: • first stage is conversion of aldehyde or ketone to the corresponding enol; is rate-determining • second stage is reaction of enol with halogen; is faster than the first stage now examine first stage

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

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

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

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

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

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

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

  28. 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

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

  30. 18.5 Stabilized Enols

  31. 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

  32. O OH O O CH3C CH3CCH2CCH3 CHCCH3 1,3-Diketones(also called b-diketones) Example: 2,4-pentanedione • keto form is less stable than enol form (20%) (80%)

  33. Enol form of 2,4-pentanedione

  34. Enol form of 2,4-pentanedione intramolecular hydrogen bond 103 pm 166 pm H O O 133 pm 124 pm C C H3C C CH3 134 pm 141 pm H C=C and C=O are conjugated

More Related