Ch_322b_17.04 - Chapter 17.4 lecture note

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The Aldol Reaction: Addition of Enolate Anions to Aldehydes and Ketones Aldehydes and ketones with enolizable hydrogens ( ! -H) undergo a self-reaction to give a " -hydroxyaldehyde or " -hydroxyketone in the presence of base. This reaction is called the aldol condensation . Requirements for an aldol reaction: ! -H Dilute solution of base-usually HO - 2 CH 3 CH O = acetaldehyde 10% NaOH, H 2 O 5 o C CH 3 CHCH 2 CH O = OH 3-hydroxbutanal an "aldol" The aldol reaction does not occur with ArCHO HCHO (CH 3 ) 3 CCH O = ArCAr' O = ArCCR 3 O =
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A Mechanism for the Aldol Reaction In the dilute basic solution, an aldehyde or ketone with enolizable-H reacts to produce a low concentration of its enolate anion . The enolate anion is a good nucleophile and undergoes an addition reaction with the carbonyl compound. Note: Only a low conc. of the enolate anion is formed. RC CH 3 O = pKa ~ 20 + HO - RC CH 2 O - + H 2 O fast formation of the enolate anion pKa = 15.7 enolate anion RCCH 2 O = : - + RCCH 3 :O: = nucleophilic addition slow RCCH 2 CCH 3 O = : - R :O: H 2 O RCCH 2 CCH 3
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