480 ENOLS AND ENOLATES (c) Dehydration of the aldol addition product occurs when the reaction is carried out at elevated C=0 2C6H3 CH2CH2CH ethanol 70C C6HSCHICHICH=CCH2C6H 3-Phenylpropanal Benzyl-S-phenyl-2-pentenal (d) Lithium aluminum hydride reduces the aldehyde function to the corresponding primary LAIH C6HS_ CH_ CH-CCH2C6H52.HO C6HSCHICHICH-CCH2C6H5 2-Benzyl-5-phenyl-2-penten-l-ol (e) Acharacteristic reaction of a B-unsaturated carbonyl compounds is their tendency to undergo conjugate addition on treatment with weakly basic nucleophiles. HC=O HC=O C6H CH,CH, CH=CCH C6H5 NaCN C6H_CH, CH,CHCHCH2C6H 2-Benzyl-5-phenyl-2-pentenal 2-Benzyl-3-cyano-5-phenylpentanal 18.23 (a) Ketones undergo a halogenation by way of their enol form HCH CH, C1, CCHCH a weakly basic nucleophile and adds to a, B-unsaturated ketones by conjugate addition. S (b) The combination of CHs CH SH and NaoH yields C6H_CH,S(as its sodium salt), which H-C F-C(CH) NOH.HO HC C(CH3)2 SCHCH c) Bromination occurs at the carbon atom that is a to the carbonyl group CH ChS Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(c) Dehydration of the aldol addition product occurs when the reaction is carried out at elevated temperature. (d) Lithium aluminum hydride reduces the aldehyde function to the corresponding primary alcohol. (e) A characteristic reaction of ,-unsaturated carbonyl compounds is their tendency to undergo conjugate addition on treatment with weakly basic nucleophiles. 18.23 (a) Ketones undergo halogenation by way of their enol form. (b) The combination of C6H5CH2SH and NaOH yields C6H5CH2S (as its sodium salt), which is a weakly basic nucleophile and adds to ,-unsaturated ketones by conjugate addition. (c) Bromination occurs at the carbon atom that is to the carbonyl group. Br2 diethyl ether C6H5 O C6H5 2,2-Diphenylcyclopentanone C6H5 O Br C6H5 2-Bromo-5,5- diphenylcyclopentanone (76%) H3C C(CH3)2 O 2-Isopropylidene-5- methylcyclohexanone C6H5CH2SH NaOH, H2O 2-(1-Benzylthio-1-methylethyl)-5- methylcyclohexanone (89–90%) H3C C(CH3)2 SCH2C6H5 H O CCH2CH3 Cl O 1-(o-Chlorophenyl)- 1-propanone CCHCH3 Cl Cl O 2-Chloro-1-(o-chlorophenyl)- 1-propanone Cl2 CH2Cl2 HC O C6H5CH2CH2CH CCH2C6H5 2-Benzyl-5-phenyl-2-pentenal HC CN O C6H5CH2CH2CHCHCH2C6H5 2-Benzyl-3-cyano-5-phenylpentanal NaCN H CH2OH C6H5CH2CH2CH CCH2C6H5 2-Benzyl-5-phenyl-2-penten-1-ol HC O C6H5CH2CH2CH CCH2C6H5 2-Benzyl-5-phenyl-2-pentenal 1. LiAlH4 2. H2O HC O C6H5CH2CH2CH CCH2C6H5 2-Benzyl-5-phenyl-2-pentenal O 2C6H5CH2CH2CH 3-Phenylpropanal NaOH ethanol, 70C 480 ENOLS AND ENOLATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ENOLS AND ENOLATES 481 (d) The reaction is a mixed aldol condensation. The enolate of 2, 2-diphenylcyclohexanone reacts ith p-chlorobenzaldehyde. Elimination of the aldol addition product occurs readily to yield the a B-unsaturated ketone as the isolated product CH OH Ch+ C6H5 ethanol O 人CH Hs 2-(p-Chlorobenzylidene (e) The aldehyde given as the starting material is called furfural and is based on a furan unit as an aromatic ring. Furfural cannot form an enolate. It reacts with the enolate of acetone in a manner much as benzaldehyde would CH CH NaOH O CHCH2CCH3 CHECHCCH Furfural Acetone 4-Fury Lithium dialkylcuprates transfer an alkyl group to the B-carbon atom of a B-unsaturated O CH CH t liCu(Ch,) 2.H2O H, C CH HC CH 2, 4, 4-Trimethy l-2 2, 3, 4, 4-Tetramethyl- A mixture of st obtained in 67% yield in this (g) Two nonequivalent a-carbon atoms occur in the starting ketone. Although enolate formation possible at either position, only reaction at the methylene carbon leads to an intermediate OH CHC Hs -HO CHCH Observed produc Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(d) The reaction is a mixed aldol condensation. The enolate of 2,2-diphenylcyclohexanone reacts with p-chlorobenzaldehyde. Elimination of the aldol addition product occurs readily to yield the ,-unsaturated ketone as the isolated product. (e) The aldehyde given as the starting material is called furfural and is based on a furan unit as an aromatic ring. Furfural cannot form an enolate. It reacts with the enolate of acetone in a manner much as benzaldehyde would. ( f) Lithium dialkylcuprates transfer an alkyl group to the -carbon atom of ,-unsaturated ketones. A mixture of stereoisomers was obtained in 67% yield in this reaction. (g) Two nonequivalent -carbon atoms occur in the starting ketone. Although enolate formation is possible at either position, only reaction at the methylene carbon leads to an intermediate that can undergo dehydration. O O CHC6H5 OH HO C6H5CHO H2O O CHC6H5 Observed product (75% yield) LiCu(CH3) 2 CH3 CH3 O H3C 1. diethyl ether 2. H2O 2,4,4-Trimethyl-2- cyclohexenone CH3 CH3 CH3 O H3C 2,3,4,4-Tetramethylcyclohexanone O NaOH water H2O CH O O Furfural CH3CCH3 Acetone CHCH2CCH3 OH O O (Not isolated) CH CHCCH3 O O 4-Furyl-3-buten-2-one (60–66%) Cl CH C6H5 C6H5 O KOH ethanol H2O CH O Cl p-Chlorobenzaldehyde C6H5 C6H5 O 2,2-Diphenylcyclohexanone CH OH Cl C6H5 C6H5 O (Not isolated) 2-(p-Chlorobenzylidene)-6,6- diphenylcyclohexanone (84%) ENOLS AND ENOLATES 481 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website