Chapter 17 Aldehydes and ketones I. Aldol reactions 17.1 The acidity of the a-hydrogens of carbonyl compounds: Enolate ions (烯醇负离子) Nucleophilic addition nu R a-hydrogen B-hydrogens are not acidic are unusually acidic pka=40-50 (pka=19-20
Chapter 17 Aldehydes and ketones II. Aldol reactions 17.1 The acidity of the α-hydrogens of carbonyl compounds: Enolate ions （烯醇负离子） O R H H - hydrogens are not acidic pka = 40 - 50 - Hydrogen are unusually acidic (pka = 19 - 20 Nucleophilic addition Nu: -
It can accept the proton to form the keto form, or it may accept the proton at oxygen to form an enol. B H Resonance-sta bilized anion HO Ht+ Enol fo (Unstable) Enolate .mml Keto form (Stable
It can accept the proton to form the keto form, or it may accept the proton at oxygen to form an enol. O H B: - O - -O H Resonance-stabilized anion O H Enolate HO O H H+ -H+ H + -H + Enol form Keto form (Unstable) (Stable)
17.2 Keto and enol tautomers (酮和烯醇互变异构体) Keto form Enol form Acetaldehyde CH3 H2c (-100%) ( almost 0%) OH Acetone CH H2c CH3 (99%) (0.00015% OH ycionexanone H (988%) (1.2%) OH O 2. 4-Pentanedione H3 C CH3 CCH, CCH3 CH3 C=CHCCH, The greater stability of the enol (76%) form
17.2 Keto and enol tautomers (酮和烯醇互变异构体） CH3 O H H2C OH H Keto form Enol form (- 100%) ( almost 0%) CH3 O CH3 H2C OH CH3 ( 99%) ( 0.00015%) O OH (98.8%) ( 1.2%) Acetaldehyde Acetone Cyclohexanone 2.4-Pentanedione CH3CCH2CCH3 O O CH3C=CHCCH3 OH O (24%) (76%) O O H H3C CH3 The greater stability of the enol form
17.3 Reactions via enols and enolate Ions 17.3 A Racemization(外消旋作用) Keto form ChG C2H OH CsH c CsH Or H3O k 5 H3C H HjCl H3C (R)-(+)-sec-Butyl phenyl ketone (+)-sec-Butyl phenyl ketone ( racemic form)(外消旋体) OH-1 Or H20 o→H ChG H3C a Acid or base can catalyze enolization Enol form
17.3 Reactions via enols and enolate ions 17.3A Racemization ( 外消旋作用) C O C6H5 Keto form C2H5 H3C H (R)-(+)-sec-Butyl phenyl ketone C O C6H5 C2H5 H3C H OH - Or H3O + + C O C6H5 C2H5 H H3C ( ± )-sec-Butyl phenyl ketone (racemic form)(外 消 旋 体 ） Enol form C O C6H5 C2H5 H3C H a b a b OH - Or H3O + Acid or base can catalyze enolization
17.3B Halogenation of ketones Ketones that have an a-hydrogen react readily with halogens by substitution. The rates of these halogenation reactions increase when acids or bases are added and substitution takes place almost exclusively at the a-carbon; X OH + x2 X Or H3O I
17.3B Halogenation of ketones Ketones that have an α-hydrogen react readily with halogens by substitution. The rates of these halogenation reactions increase when acids or bases are added and substitution takes place almost exclusively at the α-carbon; C O H OH - Or H3O + + X2 C O X + HX