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