Chapter 18 Enols and Enolates

18.1 The  -Carbon Atom and its Hydrogens

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

18.2  Halogenation of Aldehydes and Ketones

X 2 is Cl 2, Br 2, or I 2. Substitution is specific for replacement of  hydrogen. Catalyzed by acids. One of the products is an acid (HX); the reaction is autocatalytic. Not a free-radical reaction. General Reaction O R 2 CCR' H + X2X2X2X2O X + HXHXHXHX H+H+H+H+

Example H2OH2OH2OH2O (61-66%) Cl 2 +O HClOCl +

Example CHCl 3 (80%) Br 2 + HBr + H CHOBr CHO Notice that it is the proton on the  carbon that is replaced, not the one on the carbonyl carbon.

18.3 Mechanism of  Halogenation of Aldehydes and Ketones

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

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

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 Mechanism of  Halogenation Two stages:

Mechanism of  Halogenation RCH 2 CR' O RCH CR'OHslow X2X2X2X2 fast RCHCR'OX enol Enol is key intermediate

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 Mechanism of  Halogenation Two stages: examine second stage first

Reaction of enol with Br 2 BrBr R2CR2CR2CR2C CR' OH

Reaction of enol with Br 2 BrBr Br R2CR2CR2CR2C CR' OH + carbocation is stabilized by electron release from oxygen Br – R2CR2CR2CR2C CR' OH

Reaction of enol with Br 2 BrBr Br R2CR2CR2CR2C CR' OH + carbocation is stabilized by electron release from oxygen Br – R2CR2CR2CR2C CR' OH

Reaction of enol with Br 2 BrBr Br R2CR2CR2CR2C CR' OH + R2CR2CR2CR2C CR' OH Br R2CR2CR2CR2C CR' OH +

Loss of proton from oxygen completes the process Br – OCR' Br R2CR2CR2CR2C H Br HBr R2CR2CR2CR2C CR' O +

18.4 Enolization and Enol Content

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 Mechanism of  Halogenation Two stages: now examine first stage

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

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

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

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

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

O R2CR2CR2CR2C CR' H H + OHH

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

percent enol is usually very small keto form usually kJ/mol more stable than enol Enol Content R 2 CHCR' O R2CR2CR2CR2C CR'OHenolketo

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

18.5 Stabilized Enols

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

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

Enol form of 2,4-pentanedione

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