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
ENOLS AND ENOLATES 475 This enolate adds to the carbonyl group of benzaldehyde to give the mixed aldol addition product, which then dehydrates under the reaction conditions oH O C6HS CH + CH, CC(CH3)3- CH_CHCH, CC(CH3) C6HsCHCH,CC(CH3)3 benzaldehyde Enolate of tert-butyl Product of mixed aldol addition C6HSCH=CHCC(CH3) 4, 4-Dimethyl-l-pl (product of mixed aldol condensation) (c) The enolate of cyclohexanone adds to benzaldehyde Dehydration of the mixed aldol addition product takes place under the reaction conditions to give the following mixed aldol condensa C6HSCH Cyclohexanone Benzaldehyde Benzylidenecyclohexanone 18.14 Mesityl oxide is an a B-unsaturated ketone. Traces of acids or bases can catalyze its isomerization so that some of the less stable B, y-unsaturated isomer is present H3C、 =CHECh= HCSC-CH,CCH, H3C H-C Mesityl oxide; 4-methyl- 4-Methyl-4-penten-2-one more stable) 18.15 The relationship between the molecular formula of acrolein(C3H4O)and the product(C3HSN3O) corresponds to the addition of HN3 to acrolein. Because propanal(CHCH, CH=o)does not react under these conditions, the carbon-carbon, not the carbon-oxygen, double bond of acrolein is the re- active site. Conjugate addition is the reaction that occurs H,C=CHCH N3,, CH Acrolein 3.Azidopropanal Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
This enolate adds to the carbonyl group of benzaldehyde to give the mixed aldol addition product, which then dehydrates under the reaction conditions. (c) The enolate of cyclohexanone adds to benzaldehyde. Dehydration of the mixed aldol addition product takes place under the reaction conditions to give the following mixed aldol condensation product. 18.14 Mesityl oxide is an ,-unsaturated ketone. Traces of acids or bases can catalyze its isomerization so that some of the less stable ,-unsaturated isomer is present. 18.15 The relationship between the molecular formula of acrolein (C3H4O) and the product (C3H5N3O) corresponds to the addition of HN3 to acrolein. Because propanal (CH3CH2CH?O) does not react under these conditions, the carbon-carbon, not the carbon-oxygen, double bond of acrolein is the reactive site. Conjugate addition is the reaction that occurs. N3CH2CH2CH O O H2C CHCH NaN3 acetic acid Acrolein 3-Azidopropanal O C CHCCH3 H3C H3C Mesityl oxide; 4-methyl- 3-penten-2-one (more stable) O C CH2CCH3 H3C H2C 4-Methyl-4-penten-2-one (less stable) HO O Cyclohexanone O C6H5CH Benzaldehyde H2O O CHC6H5 Benzylidenecyclohexanone O O C6H5CHCH2CC(CH3) 3 Enolate of tert-butyl methyl ketone CH2CC(CH3)3 O Benzaldehyde OH O C6H5CHCH2CC(CH3)3 Product of mixed aldol addition O C6H5CH H2O O C6H5CH CHCC(CH3)3 4,4-Dimethyl-1-phenyl-1-penten-3-one (product of mixed aldol condensation) H2O ENOLS AND ENOLATES 475 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
476 ENOLS AND ENOLATES 18.16 The enolate of dibenzyl ketone adds to methyl vinyl ketone in the conjugate addition step nOCH C6HS C6H5 H2C-CHCCH3CH, OH C&HSCH- CCHCH CHCHCCH Dibenzyl ketone Methyl vinyl ketone 1.3-Diphenyl-2, 6-heptanedione CBH CH,CCHCAH5 C6HSCH-CCHCH HC=CHCCH H,C--CHCCH3 The intramolecular aldol condensation that gives the observed product is HC、CH-C noCH CHCH CH,OH CHC&H HO CH,-CH, 1.3-Diphenyl-2, 6-heptanedione 18.17 A second solution to the synthesis of 4-methyl-2-octanone by conjugate addition of a lithium dialkylcuprate reagent to an a, B-unsaturated ketone is revealed by the disconnection shown CH, CH,CH,CH,CHCH, CCH3 CHa CH,CH, CH,CH=CHCCH3 H3 Disconnect this bond According to this disconnection, the methyl group is derived from lithium dimethylcuprate CH, CH, CH, CH, CH=CHCCH2 t LiCu(CHi) CH CH,CH,CH,CHCH, CCH 3-Octen-2-one Lithium 4.Methyl-2-octanone methy cuprate 18.18 (a) In addition to the double bond of the carbonyl group, there must be a double bond elsewher in the molecule in order to satisfy the molecular formula Cho (the problem states that the Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
18.16 The enolate of dibenzyl ketone adds to methyl vinyl ketone in the conjugate addition step. via The intramolecular aldol condensation that gives the observed product is 18.17 A second solution to the synthesis of 4-methyl-2-octanone by conjugate addition of a lithium dialkylcuprate reagent to an ,-unsaturated ketone is revealed by the disconnection shown: According to this disconnection, the methyl group is derived from lithium dimethylcuprate. 18.18 (a) In addition to the double bond of the carbonyl group, there must be a double bond elsewhere in the molecule in order to satisfy the molecular formula C4H6O (the problem states that the CH3CH2CH2CH2CH CHCCH3 O O LiCu(CH3 CH3CH2CH2CH2CHCH2CCH3 )2 3-Octen-2-one Lithium dimethylcuprate 4-Methyl-2-octanone CH3 CH3CH2CH2CH2CH CHCCH3 O O CH3CH2CH2CH2CHCH2CCH3 Disconnect this bond. CH3 CH3 1,3-Diphenyl-2,6-heptanedione C6H5CH2 C CHC6H5 O O CH2 CH2 CH3C H3C C6H5 C C CHC6H5 O HO CH2 CH2 CH 3-Methyl-2,6-diphenyl-2- cyclohexen-1-one NaOCH3 CH3OH H2O C6H5 C6H5 O H3C C6H5CH2CCHC6H5 H2C O O CHCCH3 C6H5CH2CCHC6H5 O H2C O CHCCH3 Dibenzyl ketone Methyl vinyl ketone 1,3-Diphenyl-2,6-heptanedione NaOCH3 CH3OH C6H5CH2CCH2C6H5 H2C CHCCH3 O O C6H5CH2CCHC6H5 CH2CH2CCH3 O O 476 ENOLS AND ENOLATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ENOLS AND ENOLATES 477 compounds are noncyclic). There are a total of five isomers: HC=CHCH.CH 3-Butenal (Er-2-Butenal (Z)-2-Butenal H, C=CCH H,C=CHCCH 2-Methy propenal 3-Buten-2-one b) The E and Z isomers of 2-butenal are stereoisomers. (c) None of the ChO aldehydes and ketones is chiral (d) The aB-unsaturated aldehydes are(E)-and(2)-CH3CH=CHCHO; and H,C=CCHO CH There is one a B-unsaturated ketone in the H,C=CHCCH (e) The E and z isomers of 2-butenal are formed by the aldol condensation of acetaldehyde 18.19 The main flavor component of the hazelnut has the structure shown. H C CHCH 18.20 The characteristic reaction of an alcohol on being heated with KHsO is acid-catalyzed dehydration Secondary alcohols dehydrate faster than primary alcohols, and so a reasonable first step is HOCHCHCHOH HOCHCHECHOH OH The product of this dehydration is an enol, which tautomerizes to an aldehyde. The aldehyde then undergoes dehydration to form acrolein HOCHCHECHOH HOCH. CHCH KHso→H2C= 3-Hydroxypropanal Acrolein Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
compounds are noncyclic). There are a total of five isomers: (b) The E and Z isomers of 2-butenal are stereoisomers. (c) None of the C4H6O aldehydes and ketones is chiral. (d) The ,-unsaturated aldehydes are (E)- and ; and . There is one ,-unsaturated ketone in the group: . (e) The E and Z isomers of 2-butenal are formed by the aldol condensation of acetaldehyde. 18.19 The main flavor component of the hazelnut has the structure shown. 18.20 The characteristic reaction of an alcohol on being heated with KHSO4 is acid-catalyzed dehydration. Secondary alcohols dehydrate faster than primary alcohols, and so a reasonable first step is The product of this dehydration is an enol, which tautomerizes to an aldehyde. The aldehyde then undergoes dehydration to form acrolein. KHSO4 heat (H2O) HOCH2CH CHOH Propene-1,3-diol 3-Hydroxypropanal HOCH2CH2CH O Acrolein H2C CHCH O KHSO4 heat HOCH2CHCH2OH OH 1,2,3-Propanetriol HOCH2CH CHOH Propene-1,3-diol C C H H3C H C C O H CH3 CH2CH3 (2E,5S)-5-Methyl-2-hepten-4-one H2C O CHCCH3 H2C CH3 (Z)-CH3CH CHCHO CCHO O H2C CH3 CCH 2-Methylpropenal O H2C CHCCH3 3-Buten-2-one (methyl vinyl ketone) O O H2C CHCH2CH 3-Butenal C C H3C H H CH (E)-2-Butenal C C H3C H H CH (Z)-2-Butenal O ENOLS AND ENOLATES 477 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
478 ENOLS AND ENOLATES 18.21 (a) 2-Methylpropanal has the greater enol content CH,,CHCH (CH3)2C=CH 2-Methylpropanal Enol fo Although the enol content of 2-methylpropanal is quite small, the compound is nevertheless capable of enolization, whereas the other compound, 2, 2-dimethylpropanal, cannot enolize it has no a hydrogens CH H ( Enolization is impossible. (b) Benzophenone has no a hydrogens; it cannot form an enol. Enolization is impossible.) Dibenzyl ketone enolizes slightly to form a small amount of enol C6HSCH,CCH C6H5 C6H-CH=CCH, C6H5 conjugation of the double bond with the remaining carbonyl group and by intramolecular hy drogen bonding. C6HSCCH,CCHs C6HsC CC6H5 propanone Enol form (d) The enol content of cyclohexanone is quite small, whereas the enol form of 2, 4-cyclo- hexadienone is the aromatic compound phenol, and therefore enolization is essentially Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
18.21 (a) 2-Methylpropanal has the greater enol content. Although the enol content of 2-methylpropanal is quite small, the compound is nevertheless capable of enolization, whereas the other compound, 2,2-dimethylpropanal, cannot enolize— it has no hydrogens. (b) Benzophenone has no hydrogens; it cannot form an enol. Dibenzyl ketone enolizes slightly to form a small amount of enol. (c) Here we are comparing a simple ketone, dibenzyl ketone, with a -diketone. The -diketone enolizes to a much greater extent than the simple ketone because its enol form is stabilized by conjugation of the double bond with the remaining carbonyl group and by intramolecular hydrogen bonding. (d) The enol content of cyclohexanone is quite small, whereas the enol form of 2,4-cyclohexadienone is the aromatic compound phenol, and therefore enolization is essentially complete. O Keto form OH Enol form (aromatic; much more stable) 1,3-Diphenyl-1,3- propanedione C6H5CCH2CC6H5 O O Enol form O C6H5C CH CC6H5 O H Dibenzyl ketone C6H5CH2CCH2C6H5 O Enol form C6H5CH CCH2C6H5 OH C O (Enolization is impossible.) C H CH3C CH3 CH3 O (Enolization is impossible.) 2-Methylpropanal (CH3)2CHCH O Enol form (CH3)2C CH OH 478 ENOLS AND ENOLATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ENOLS AND ENOLATES 479 (e) A small amount of enol is in equilibrium with cyclopentanone Cyclopentanone Enol form Cyclopentadienone does not form a stable enol. Enolization would lead to a highly strained allene-type compound ot stabl (f The B-diketone is more extensively enolized. OH O 1. 3-Cyclohexanedione Enol form(double bond The double bond of the enol form of 1, 4-cyclohexanedione is not conjugated with the car bonyl group. Its enol content is expected to be similar to that of cyclohexanone 1. 4-Cyclohexanedione Enol form( double bond ar 18.22 (a) Chlorination of 3-phenylpropanal under conditions of acid catalysis occurs via the enol form and yields the a-chloro derivative acetic acid C6HSCHLCH,CH Cl C6H_CH, CHCH HCI 3-Phenylpropanal (b) Aldehydes undergo aldol addition on treatment with base 2C6H_ CH2CH2CH cthanol 10-c CgHSCH2CH2CHCHCH2C6H5 3-Phenylpropanal Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(e) A small amount of enol is in equilibrium with cyclopentanone. Cyclopentadienone does not form a stable enol. Enolization would lead to a highly strained allene-type compound. ( f) The -diketone is more extensively enolized. The double bond of the enol form of 1,4-cyclohexanedione is not conjugated with the carbonyl group. Its enol content is expected to be similar to that of cyclohexanone. 18.22 (a) Chlorination of 3-phenylpropanal under conditions of acid catalysis occurs via the enol form and yields the -chloro derivative. (b) Aldehydes undergo aldol addition on treatment with base. HC O C6H5CH2CH2CHCHCH2C6H5 OH 2-Benzyl-3-hydroxy-5-phenylpentanal O 2C6H5CH2CH2CH 3-Phenylpropanal NaOH ethanol, 10C Cl 2 3-Phenylpropanal C6H5CH2CH2CH O HCl 2-Chloro-3- phenylpropanal C6H5CH2CHCH O Cl acetic acid Enol form (not particularly stable; double bond and carbonyl group not conjugated) OH O 1,4-Cyclohexanedione O O O O 1,3-Cyclohexanedione OH O Enol form (double bond conjugated with carbonyl group) H O OH (Not stable; highly strained) O Cyclopentanone OH Enol form ENOLS AND ENOLATES 479 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
