2,4-pentanedioneSection 19.2 Keto–Enol Tautomerism 791If the is betweentwo carbonyl groups, the acidity of an is
even greater (Table 19.1). For example, an of ethyl 3-oxobutyrate, a com-
pound with an between a ketone carbonyl group and an ester carbonyl group,
has a of 10.7. An of 2,4-pentanedione, a compound with an
between two ketone carbonyl groups, has a of 8.9. Ethyl 3-oxobutyrate is classi-
fied as a because the ester has a carbonyl group at the
2,4-Pentanedione is a
The acidity of bonded to carbons flanked by two carbonyl groups in-
creases because the electrons left behind when the proton is removed can be delocal-
ized onto twooxygen atoms. have lower values than esters
because electrons are more readily delocalized onto ketone carbonyl groups than they
are onto ester carbonyl groups.
PROBLEM 2Give an example of
a. a nitrile b. aPROBLEM 3List the compounds in each of the following groups in order of decreasing acidity:19.2 Keto–Enol Tautomerism
A ketone exists in equilibrium with its enol tautomer. You were introduced to tau-
tomers in Section 6.6. Recall that tautomersare isomers that are in rapid equilibrium.
Keto–enol tautomers differ in the location of a double bond and a hydrogen.
RCH 2 CROHR
keto tautomer enol tautomerORCH CONCH 3OOOc.CH 3 OCCH 2 COCH 3OOb. CH 3 CCH 2 CCH 3OOCH 3 CCH 2 COCH 3OOCH 3 CCH 3OCH 3 CH 3 CH 3 CH HCOa. CH 2 CH 2 CHb-keto b-diesterb-Diketones pKa b-ketoa-hydrogensCH 3 CCH 2O2,4-pentanedione
acetylacetone
a -diketoneOCH 3 CCH 2 CCH 3Oethyl 3-oxobutyrate
ethyl acetoacetate
a -keto esterOC OCH 2 CH 3pKa = 10.7 pKa^ = 8.9B-diketone.B-keto ester b-position.pKapKa a-hydrogen a-carbona-carbona-hydrogena-carbon a-hydrogenCH 3 CCHOCCH 3O− −CH 3 CCHOCCH 3O−CH 3 CCHOCCH 3Oresonance contributors for the 2,4-pentanedione anion