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CHMB42H3 (16)
Chapter 18

CHMB42 Chapter 18.doc

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Wanda/ Lana

Carbonyl Compounds IIIThe Acidity of an aHydrogen3A hydrogen bonded to an sp carbon adjacent to a carbonyl carbon is much 3more acidic than hydrogens bonded to other sp carbonsCarbon Acid a compound that contains a relatively acidic hydrogen bonded to 3an sp carbonThe ahydrogen is more acidic because the base formed when a proton is removed from anacarbon is more stable than a base formed when proton is 3removed from other sp carbonsoThe electrons left behind when the proton is removed are delocalized and electron delocalization increases stabilityoThe electrons are delocalized onto the oxygen an atom that is better able to accommodate them because it is more electronegative than carbonAldehydes and ketones are more acidic than esters because the oxygen of the OR group of the ester also has a lone pair that can be delocalized onto the carbonyl oxygen competesIf the acarbon is between two carbonyl groups the acidity of an ahydrogen is even greater because the electrons left behind when the proton is removed can be delocalized onto two oxygen atomsKetoEnol TautomersA ketone exists in equilibrium with its enol tautomerFor most ketones the enol tautomer OH and CC is much less stable than the keto tautomer COThe fraction of the enol tautomer in an aqueous solution is considerably greater for a bdiketone acid anhydride because the enol tautomer is stabilized by intramolecular hydrogen bonding and by conjugation of the carboncarbon double bond with the second carbonyl groupPhenol is an enol tautomer and is more stable than its keto tautomer because it is aromatic and the keto tautomer is notEnolizationKetoEnol Tautomerization or Enolization interconversion of the keto and enol tautomersMechanism for basecatalyzed ketoenol interconversionoHydroxide ion removes a proton from the acarbon of the keto tautomer forming an anion called an enolate ion The enolate has two resonance contributorsoProtonation on oxygen forms the enol tautomer whereas protonation on the acarbon reforms the keto tautomerMechanism for acidcatalyzed ketoenol interconversionoThe carbonyl oxygen of the keto tautomer is protonatedoWater removes a proton from the acarbon forming the enolHow Enols and Enolate Ions ReactCarbonyl compounds with an ahydrogen can undergo substitution reaction at the acarbonMechanism for basecatalyzed asubstitutionoA base removes a proton from the acarbon forming an enolate ion
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