11. Carbonyl alpha Substitution reactions and condensations
11. Carbonyl AlphaSubstitution Reactions and Condensations H O E + O E
The a position H E 0 OH H tOh Ht O E
The a Position H O OH . . E + OH E －H + O E + －H + H O O - E + O E O -
11.1 Keto-Enol p.345 Tautomerism OH H or oh H OH R R" R R R H2 R
11.1 Keto–Enol Tautomerism p.345 R R' OH R R R' O R H1 H2 R R' R OH H + or OHH O OH
Tautomers are not Resonance forms H H 且CC3≠ H C CH C C C HHH HHH 1-Butene 2-Butene a2004 Thomson/Brooks C
Tautomers Are Not Resonance Forms
Enols The enol tautomer is usually present to a very small extent and cannot be isolated However since it is formed rapidly, it can serve as a reaction intermediate
Enols: • The enol tautomer is usually present to a very small extent and cannot be isolated • However, since it is formed rapidly, it can serve as a reaction intermediate
.346 Contents of enols at equilibrium (%h,COCH, CH,(CO, C,Hh NCCH,CO, CH 0.00015 0.0077 0.25 CH3COCH, CO2C2H5 CH2COCHCHsCOCH CH3COCH2COC6H5 76.4 89.2
Contents of enols at equilibrium （%） p.346 CH3COCH3 CH2 (CO2C2H5 )2 NCCH2CO2C2H5 CH3COCH2CO2C2H5 0.00015 0.0077 0.25 8 CH3COCHC6H5CO2C2H O O CH3COCH2COC6H5 1.2 30 76.4 89.2 O
Acid-catalyzed ketone-enol tautomerism →+OH OH H H u H Base-catalyzed ketone-enol tautomerism 0 BH 一BH OH B H H
B H O O - O - －BH H + OH BH －B Acid-catalyzed ketone-enol tautomerism H O H + H OH H OH + + －H + OH H + －H + Base-catalyzed ketone-enol tautomerism
C=0 C-0 CC0H→CC=0H hs attached to heteroatoms. as in the enol form are more acidic than c-h's as in the ketone form
* H’s attached to heteroatoms , as in the enol form, are more acidic than C-H’s, as in the ketone form * C O C + O - C C O H C - C O + H
11.2 Reactivity of Enols: The Mechanism of Alpha-Substitution Reactions oH +Oh +OH H E E E H OH E OH E
11.2 Reactivity of Enols: The Mechanism of Alpha-Substitution Reactions . .OH OH - + H O OH . . E + OH E －H + O E + OH OH + E . . E + －H +
11.3 Alpha Halogenation of Aldehydes and Ketones p.348 Aldehydes and ketones can be halogenated at their -positions by reaction with cl. Br, or h in acidic solution H H+x x +HX
11.3 Alpha Halogenation of Aldehydes and Ketones • Aldehydes and ketones can be halogenated at their a -positions by reaction with Cl2 , Br2 , or I2 in acidic solution p.348 H O + X2 X HX O + H +