CHE202 - Chapter 22 Lecture Notes

CHE202 - Chapter 22 Lecture Notes - Organic Chemistry, 6th...

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Chapter 22 Alpha Substitution and Condensations of Enols and Enolate Ions Lecture Date April 14 and 16, 2009 Review Chapter 18 Organic Chemistry , 6 th Edition L. G. Wade, Jr. McMurry’s Organic Chemistry , 6 th edition
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2 Nucleophilic Addition to the Carbonyl Groups Addition of a nucleophile to a carbonyl carbon occurs because of the δ + charge at the carbon Nucleophilic Addition-Elimination at the Acyl Carbon The carbonyl group of carboxylic acids and their derivatives undergo nucleophilic addition-elimination Dr. K Ahsan SP2009
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The α Position The carbon next to the carbonyl group is designated as being in the α position A carbonyl compound with hydrogen atoms on its α carbon rapidly equilibrates with its corresponding enol Compounds that differ only by the position of a moveable proton are called tautomers Note : Tautomers Are Not Resonance Forms Tautomers are structural isomers Resonance forms are representations of contributors to a single structure Tautomers interconvert rapidly while ordinary isomers do not Resonance forms Tautomers Dr. K Ahsan SP2009 3
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22-1 Keto and Enol Tautomers Enol-keto tautomers are constitutional isomers that are easily interconverted by a trace of acid or base For aldehydes and ketones, the keto form is greatly favored at equilibrium. An enantiomer with an enolizable hydrogen can form a racemic mixture. β -Dicarbonyl compounds exist primarily in the enol form The enol is more stable because it has a conjugated p system and because of stabilization of the enol through hydrogen bonding Dr. K Ahsan SP2009 4
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Keto-Enol Tautomers Basic Conditions Acid Conditions Dr. K Ahsan SP2009 5
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Acidity of α -Hydrogens Hydrogens on carbons α to carbonyls are unusually acidic The resulting enolate anion is stabilized by resonance to the carbonyl p K a for α -H of aldehyde or ketone ~20. alkane or alkene (p K a > 40) or alkyne (p K a = 25). Less acidic than water (p K a = 15.7) or alcohol (p K a = 16-19). Dr. K Ahsan SP2009 6
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Enolate Reaction In the presence of hydroxide or alkoxide ions , only a small amount of enolate ion is present at equilibrium. As enolate ion reacts with the electrophile, the equilibrium shifts to produce more. Dr. K Ahsan SP2009 7
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Reagents for Enolate Formation : Acid-Base Reaction Ketones are weaker acids than the OH of alcohols so a more powerful base than an alkoxide is needed to form the enolate Sodium hydride (NaH) or lithium diisopropylamide [LiN( i -C 3 H 7 ) 2 ] – ”LDA- (p K a 40)” are strong enough to form the enolate Soluble in organic solvents and effective at low temperature with many compounds Not nucleophilic Dr. K Ahsan SP2009 8
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Reactions via Enols and Enolate Anions Racemization An optically active aldehyde or ketone with a stereocenter at the α -carbon can racemize in the presence of catalytic acid or base The intermediate enol or enolate has no stereocenter at the α position Dr. K Ahsan SP2009 9
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Racemization : Mechanisms • Base catalysed • Acid catalysed Dr. K Ahsan SP2009 10
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CHE202 - Chapter 22 Lecture Notes - Organic Chemistry, 6th...

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