17_ - P1: PCX/SRB JWDD052-17 P2: xxx JWDD052-Solomons-v3 17...

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P1: PCX/SRB P2: xxx Printer: Hamilton JWDD052-17 JWDD052-Solomons-v3 April 28, 2007 10:22 17 ALDEHYDES AND KETONES II. ENOLS AND ENOLATES SOLUTIONS TO PROBLEMS 17.1 Cyclohexa-2,4-dien-1-one (keto form) Phenol (enol form) O OH The enol form is aromatic, and it is therefore stabilized by the resonance energy of the benzene ring. 17.2 No. CC C 6 H 5 O C 3 H 7 C 2 H 5 CH 3 does not have a hydrogen attached to its α -carbon atom (which is a chirality center) and thus enol formation involving the chirality center is not possible. With C CH 2 CC 6 H 5 O C 2 H 5 CH 3 H the chirality center is a β carbon and thus enol formation does not affect it. 17.3 In OD /D 2 O: ( R ) or ( S ) D O C 6 H 5 + enantiomer - O C 6 H 5 D 2 O H O C 6 H 5 OD - + 337
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P1: PCX/SRB P2: xxx Printer: Hamilton JWDD052-17 JWDD052-Solomons-v3 April 28, 2007 10:22 338 ALDEHYDES AND KETONES II. ENOLS AND ENOLATES In D 3 O + /D 2 O: ( R ) or ( S ) H O C 6 H 5 H + C 6 H 5 D 3 O + D 3 O + + D 2 O C 6 H 5 D O D O D 3 O + D 2 O O + D D C 6 H 5 D O C 6 H 5 + enantiomer 17.4 H H H cis -Decalone H O H EtO O H H O H Protonation on the bottom face trans -Decalone + EtO EtO H O A large group is axial in cis -decalone. H O No bulky groups are axial in trans -decalone. 17.5 The reaction is said to be “base promoted” because base is consumed as the reaction takes place. A catalyst is, by deFnition, not consumed. 17.6 (a) The slow step in base-catalyzed racemization is the same as that in base-promoted halogenation— the formation of an enolate anion. (±ormation of an enolate anion from 2-methyl-1-phenylbutan-1-one leads to racemization because the enolate anion is achiral. When it accepts a proton, it yields a racemic form.) The slow step in acid-catalyzed racem- ization is also the same as that in acid-catalyzed halogenation— the formation of an enol. (The enol, like the enolate anion, is achiral and tautomerizes to yield a racemic form of the ketone.) (b) According to the mechanism given, the slow step for acid-catalyzed iodination (forma- tion of the enol) is the same as that for acid-catalyzed bromination. Thus, we would expect both reactions to occur at the same rate. (c) Again, the slow step for both reactions (formation of the enolate anion) is the same, and consequently, both reactions take place at the same rate.
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P1: PCX/SRB P2: xxx Printer: Hamilton JWDD052-17 JWDD052-Solomons-v3 April 28, 2007 10:22 ALDEHYDES AND KETONES II. ENOLS AND ENOLATES 339 17.7 (a) H OH O α β H O + H 2 O + H O + H O O O H HO H + O O H OH + OH O H (b) To form OH O H , a hydroxide ion would have to remove a β proton in the Frst step. This does not happen because the anion that would be produced, O - H , cannot be stabilized by resonance. (c) [both ( E ) and ( Z )] O H 17.8 H O H O [( E ) + ( Z )] OH (aldol) 10 % NaOH 5 ° C heat H Ni pressure H 2 OH O 17.9 (a) O H 2 O OH H OH - H 2 O
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P1: PCX/SRB P2: xxx Printer: Hamilton JWDD052-17 JWDD052-Solomons-v3 April 28, 2007 10:22 340 ALDEHYDES AND KETONES II. ENOLS AND ENOLATES OH (b) Product of (a) HA - H 2 O H (1) LiAlH 4 , Et 2 O (2) H 2 O [( E ) + ( Z )] O OH [( E ) + ( Z )] OH (c) Product of (b) H 2 Pt pressure (d) Product of (a) NaBH 4 OH 17.10 (shown in text) H Cl + CH 3 CH 3 CCH CCH 3 CH 3 Cl - OH + CH 3 CCH CCH 3 + OH CH 2
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17_ - P1: PCX/SRB JWDD052-17 P2: xxx JWDD052-Solomons-v3 17...

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