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Unformatted text preview: The self-aldol reaction of aldehydes an eton with enolizable-H also occurs under acid-catalyzed conditions. The nudeophile in the addition reaction is the.!:!!.2L(not the enolate anion), and the rate of the addition step is increased by acid catalysis, as shown below. Overall Reaction + + H-B While only a (ow concentration of enol is present under equilibrium conditions, it is a reactive nudeophile. ~~~2:~~' aldol slow ~C1J step rust ~ l1ucleopllilic a,lt/ilion :O-H (9-H RC=CH 2 + RtCH, ~ ~/"f The E'nol is i l Protonation of the nudeophile hy .va}' carbonyl fum"lion ofthe ~carbon. promote!' nucleophilk a Hack. Q W 2RtCH, ~ The Acid-Catalyzed Aldol Condensation A Mechanism acitl-catnlyzell enol fonnatiOJ~ :0 : fast :< i/-H R~CH, + H.B' ~ RCCH, + :C j?-H RCCH, + B: sluw :O-H RC=CH 2 enol lowconc. + B: + H-B' While only a low concenltation of enol is present under equilibrium conditions, it is a reactive nudeophile. H' catalyzes enol formation. The enoi is the nucleophiie in the addition step. The protonated carbonyl is more reactive than the unprotonated function. /,~rd" In general the dehydration product is tormed in the acid catalyzed mechanism-;==-- A Directed ~molecuIar Aldol Reaction Four Intramolecular Aldol Pathways 9 (I\~ STra(nCt! fJlcyc!fc .- ~H ring system o; 2.CH:g~H3 r-r-?I2 high!li strailleli / ... __.__ ~ cycfolmtllne ,'i)- . H O 'c = 0 ~~ ~ The acid-catalyzed intramolecular aldol reaction of the diketone below yields a single product even though four different reaction pathways are possible. a H2CH2~CH3 ~ O~ . 'b V b To understand the selectivity of this reaction, the four possible reaction pathways need to be evaluated. Also, favorable inltamolecular reactions proceed much faster than comparable intermolecular reactions....
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- Spring '09