CHAPTER 18 ENOLS AND ENOLATES SOLUTIONS TO TEXT PROBLEMS 18.1 (b) There are no a-hydrogen atoms in 2, 2-dimethylpropanal, because the a-carbon atom bears three meth CH. H 2. 2-Dimethylpropanal (c) All three protons of the methyl group, as well as the two benzylic protons, are a hydrogens Five a hydrogens (d) Cyclohexanone has four equivalent a hydrogens Cyclohexanone(the hydro 470 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
CHAPTER 18 ENOLS AND ENOLATES SOLUTIONS TO TEXT PROBLEMS 18.1 (b) There are no -hydrogen atoms in 2,2-dimethylpropanal, because the -carbon atom bears three methyl groups. (c) All three protons of the methyl group, as well as the two benzylic protons, are hydrogens. (d) Cyclohexanone has four equivalent hydrogens. O H H H H Cyclohexanone (the hydrogens indicated are the hydrogens) Benzyl methyl ketone O C6H5CH2CCH3 Five hydrogens H3C O H CH3 CH3 C C 2,2-Dimethylpropanal 470 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ENOLS AND ENOLATES 471 18.2 As shown in the general equation and the examples, halogen substitution is specific for the a-carbon tom. The ketone 2-butanone has two nonequivalent a carbons, and so substitution is possible at both positions. Both 1-chloro-2-butanone and 3-chloro-2-butanone are formed in the reaction CHa CCH,CH3 CICH, CCH,CH3 CH3 CCHCH 2-Butanone Chlorine 1-Chloro. 2-butanone 3-Chloro-2-butanone 18.3 The carbon-carbon double bond of the enol always involves the original carbonyl carbon and the a-carbon atom 2-Butanone can form two different enols, each of which yields a different a-chloro slow CH3 CCH,CH HC=CCHCH CICH,CCH, CH 2-Butanone 1-Buten-2-ol (enol) 1-Chloro-2-butanone CH CCH.CH CHC=CHCH CH CCHCH 2-Butanone 18.4 Chlorine attacks the carbon-carbon double bond of each enol H,=CCH, CH3 CICH,-CCHCH CI CH3C-CHCH3 CH2C—Cl cl-Cl 18.5 (b) Acetophenone can enolize only in the direction of the methyl group CCH H, Acetophenone Enol form of acetophenone (c) Enolization of 2-methylcyclohexanone can take place in two different directions H3 CH OH lethylcyclohex-l-enol 2-Methylcyclohexanone 6-Methylcyclohex-l-en (enol form Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
ENOLS AND ENOLATES 471 18.2 As shown in the general equation and the examples, halogen substitution is specific for the -carbon atom. The ketone 2-butanone has two nonequivalent carbons, and so substitution is possible at both positions. Both 1-chloro-2-butanone and 3-chloro-2-butanone are formed in the reaction. 18.3 The carbon–carbon double bond of the enol always involves the original carbonyl carbon and the -carbon atom. 2-Butanone can form two different enols, each of which yields a different -chloro ketone. 18.4 Chlorine attacks the carbon–carbon double bond of each enol. 18.5 (b) Acetophenone can enolize only in the direction of the methyl group. (c) Enolization of 2-methylcyclohexanone can take place in two different directions. CH3 OH 2-Methylcyclohex-1-enol (enol form) CH3 O 2-Methylcyclohexanone CH3 OH 6-Methylcyclohex-1-enol (enol form) Acetophenone CCH3 O Enol form of acetophenone C CH2 OH OH CH3C Cl CHCH3 Cl OH CH3C CHCH3 Cl Cl OH ClCH2 Cl CCH2CH3 OH H2C CCH2CH3 Cl Cl CH3CCH2CH3 O 2-Butanone CH3C CHCH3 OH 2-Buten-2-ol (enol) CH3CCHCH3 O Cl 3-Chloro-2-butanone slow Cl2 fast CH3CCH2CH3 O 2-Butanone H2C CCH2CH3 OH 1-Buten-2-ol (enol) ClCH2CCH2CH3 O 1-Chloro-2-butanone slow Cl2 fast H CH3CCHCH3 O Cl 3-Chloro-2-butanone ClCH2CCH2CH3 O 1-Chloro-2-butanone Cl2 Chlorine CH3CCH2CH3 O 2-Butanone Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
472 ENOLS AND ENOLATES 18.6 (b) Eno sization of the central methylene group can involve either of the two carbonyl groups CH CCHECCH C6HSCCH, CCH C6HSC=CHCCH Enol form 1-Phenyl-1, 3-butanedione Enol form 18.7(b) Removal of a proton from 1-phenyl-1, 3-butanedione occurs on the methylene group between the carbonyls C6H CCH, CCH, HO- CH_CCHCCH3 H,O The three most stable resonance forms of this anion are 0: O CBH_ CCH=CCH3 +CH_ CCHCCH C6HSC=CHCCH (c) Deprotonation at C-2 of this B-dicarbonyl compound yields the carbanion shown The three most stable resonance forms of the anion are 18.8 Each of the five a hydrogens has been replaced by deuterium by base-catalyzed enolization. Only the OCH, hydrogens and the hydrogens on the aromatic ring are observed in the H NMr spectrum at 3.9 ppm and 8 6.7-6.9 ppm, respectively CHO H, O CHaCCH, 5D0 KCO, CH,O CD.CCD, SDO 18.9 ax-Chlorination of (R)-sec-butyl phenyl ketone in acetic acid proceeds via the enol. The enol is achi- ral and yields equal amounts of (R)-and(S)-2-chloro-2-methyl-I-phenyl-l-butanone. The product is chiral. It is formed as a racemic mixture, however, and this mixture is not optically active m CHCH CHCH o CH C6HSC-CCH,CH3 (R)-sec-Butyl phenyl ketone Enol phenyl-1-butanone Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
472 ENOLS AND ENOLATES 18.6 (b) Enolization of the central methylene group can involve either of the two carbonyl groups. 18.7 (b) Removal of a proton from 1-phenyl-1,3-butanedione occurs on the methylene group between the carbonyls. The three most stable resonance forms of this anion are (c) Deprotonation at C-2 of this -dicarbonyl compound yields the carbanion shown. The three most stable resonance forms of the anion are: 18.8 Each of the five hydrogens has been replaced by deuterium by base-catalyzed enolization. Only the OCH3 hydrogens and the hydrogens on the aromatic ring are observed in the 1 H NMR spectrum at 3.9 ppm and 6.7–6.9 ppm, respectively. 18.9 -Chlorination of (R)-sec-butyl phenyl ketone in acetic acid proceeds via the enol. The enol is achiral and yields equal amounts of (R)- and (S)-2-chloro-2-methyl-1-phenyl-1-butanone. The product is chiral. It is formed as a racemic mixture, however, and this mixture is not optically active. C C C6H5 HO CH3 CH2CH3 Enol C6H5C O CCH2CH3 CH3 Cl 2-Chloro-2-methyl-1- phenyl-1-butanone (50% R; 50% S) C O C6H5C H CH2CH3 CH3 (R)-sec-Butyl phenyl ketone acetic acid Cl2 K2CO3 CH2CCH3 5D2O CH3O CH3O O CD2CCD3 5DOH CH3O CH3O O O CH O O CH O O O CH H2O O H CH O O CH O HO O C6H5CCH O C6H5CCHCCH3 O O CCH3 C6H5C O O CHCCH3 O HO C6H5CCH2CCH3 C6H5CCHCCH3 H2O O O O O C6H5CCH CCH3 HO Enol form O O C6H5CCH2CCH3 1-Phenyl-1,3-butanedione OH O C6H5C CHCCH3 Enol form Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ENOLS AND ENOLATES 473 18.10 (b) Approaching this problem mechanistically in the same way as part(a), write the structure of the enolate ion from 2-methy lbutana CHaCH,CHCH HO- CHaCH, CCH →CH3CH2C=CH H3 2-Methylbutanal Enolate of 2-methylbutanal This enolate adds to the carbonyl group of the aldehyde CH CH3CH CHCH CCH,,CH3 CHCH2CHCH— CCH,CH CH 2-Methylbutanal Enolate of A proton transfer from solvent yields the product of aldol addition. 0 CH CH CHCHCH--CCH,CH HO CH-CH, CHCH--CCH,CH, +HO CH3 HC=O (c) The aldol addition product of 3-methylbutanal can be identified through the same mechanis (CH3)2 CHCH,CH HO (CH )2CHCHCH H,O 3-Methylbutanal Enolate of 3-methylbutanal H3) CH +CHCH(CH3)2(CH])2CHCH2 CH-CHCH(CH3) 3-Methylbutanal Enolate of 3-methylbutanal (CH), CHCH, CH--CHCH(CH)2+ OH 3-Hydroxy-2-isopropyl-5-methylhexanal Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
18.10 (b) Approaching this problem mechanistically in the same way as part (a), write the structure of the enolate ion from 2-methylbutanal. This enolate adds to the carbonyl group of the aldehyde. A proton transfer from solvent yields the product of aldol addition. (c) The aldol addition product of 3-methylbutanal can be identified through the same mechanistic approach. O (CH3)2CHCH2CH CHCH(CH3)2 HC O 3-Hydroxy-2-isopropyl-5-methylhexanal OH (CH3)2CHCH2CH CHCH(CH3)2 OH HC O 3-Methylbutanal Enolate of 3-methylbutanal O (CH3)2CHCH2CH CHCH(CH3)2 HC O H2O HO H2O O (CH3)2CHCH2CH 3-Methylbutanal O (CH3)2CHCHCH Enolate of 3-methylbutanal HO HO CH3CH2CHCH CH3 CCH2CH3 CH3 HC O H2O O CH3CH2CHCH CH3 CCH2CH3 CH3 HC O 2-Ethyl-3-hydroxy-2,4- dimethylhexanal O CH3CH2CHCH CH3 2-Methylbutanal Enolate of 2-methylbutanal CCH2CH3 CH3 HC O O CH3CH2CHCH CH3 CCH2CH3 CH3 HC O HO O CH3CH2CHCH CH3 2-Methylbutanal Enolate of 2-methylbutanal O CH3CH2CCH CH3 O CH3CH2C CH CH3 ENOLS AND ENOLATES 473 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
474 ENOLS AND ENOLATES 18.11 Dehydration of the aldol addition product involves loss of a proton from the a-carbon atom and hydroxide from the B-carbon atom oH O R.C-CHCH R, C=CHCH H,O +HO H H (b) The product of aldol addition of 2-methylbutanal has no a hydrogens. It cannot dehydrate to an aldol condensation product. 2CH3 CH,CHCH CH3CH2CHCH—CCH2CH CH CH HC=O 2-Methylbutanal (c) Aldol condensation is possible with 3-methylbutanal. 2(CH3)2CHCH,CH (CH3)2CHCH, CHCHCH(CH3)h2 (CH3) CHCH,CH=CCH(CH3)2 HC=O 3-Methy butanal Aldol addition product 2-Isopropyl-5-methyl-2-hexenal 18.12 The carbon skeleton of 2-ethyl-1-hexanol is the same as that of the aldol condensation product derived from butanal. Hydrogenation of this compound under conditions in which both the -carbon double bond and the carbonyl group are reduced gives 2-ethyl-l-hexanol CH..CH CHaCH,CH,CH=CCH CHaCH,CH,CH,CHCH,OH CH,CH CHCH 2-Ethyl-2-hexenal 2-Ethyl-1-hexanol 18.13 (b) The only enolate that can be formed from tert-butyl methyl ketone arises by proton abstrac tion from the methyl group. (CH,2 CCCH2 + HO tert-Butyl methyl ate of tert-butyl methyl ketone Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
18.11 Dehydration of the aldol addition product involves loss of a proton from the -carbon atom and hydroxide from the -carbon atom. (b) The product of aldol addition of 2-methylbutanal has no hydrogens. It cannot dehydrate to an aldol condensation product. (c) Aldol condensation is possible with 3-methylbutanal. 18.12 The carbon skeleton of 2-ethyl-1-hexanol is the same as that of the aldol condensation product derived from butanal. Hydrogenation of this compound under conditions in which both the carbon–carbon double bond and the carbonyl group are reduced gives 2-ethyl-1-hexanol. 18.13 (b) The only enolate that can be formed from tert-butyl methyl ketone arises by proton abstraction from the methyl group. HO O (CH3)3CCCH3 tert-Butyl methyl ketone Enolate of tert-butyl methyl ketone O (CH3)3CCCH2 CH3CH2CH2CH O Butanal 2-Ethyl-1-hexanol CH3CH2CH2CH2CHCH2OH CH2CH3 CH3CH2CH2CH O CCH CH2CH3 2-Ethyl-2-hexenal H2 NaOH, H , Ni 2O heat HO 2(CH3)2CHCH2CH O 3-Methylbutanal (CH3)2CHCH2CHCHCH(CH3)2 HO HC O Aldol addition product (CH3)2CHCH2CH CCH(CH3)2 HC O 2-Isopropyl-5-methyl-2-hexenal H2O 2CH3CH2CHCH CH3 O HO 2-Methylbutanal CH3CH2CHCH CH3 HO CCH2CH3 HC O CH3 (No protons on -carbon atom) H2O HO R2C CHCH O CHCH H O OH R2C OH heat 474 ENOLS AND ENOLATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website