羰基化合物缩合反应机理

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羰基化合物缩合 反应机理Based on McMurry’s Organic Chemistry, 6th edition

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Condensation ReactionsCarbonyl compounds are both the electrophile and nucleophile in carbonyl condensation reactions

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23.1 Mechanism of Carbonyl Condensation ReactionsCarbonyl condensation reactions utilize α-substitution steps An enolate ion adds as a nucleophile to the electrophilic acceptor

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23.2 Condensations of Aldehydes and Ketones: The Aldol ReactionAcetaldehyde reacts in basic solution (NaOEt, NaOH) with another molecule of acetaldhyde The β-hydroxy aldehyde product is aldol (aldehyde + alcohol) This is a general reaction of aldehydes and ketones

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The Equilibrium of the AldolThe aldol reaction is reversible, favoring the condensation product only for aldehydes with no α substituent Steric factors are increased in the aldol product

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Aldehydes and the Aldol Equilibrium

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Ketones and the Aldol Equilibrium

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Mechanism of Aldol ReactionsAldol reactions, like all carbonyl condensations, occur by nucleophilic addition of the enolate ion of the donor molecule to the carbonyl group of the acceptor molecule The addition intermediate is protonated to give an alcohol product

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23.3 Carbonyl Condensation Reactions versus Alpha-Substitution ReactionsCarbonyl condensations and α substitutions both involve formation of enolate ion intermediates Alpha-substitution reactions are accomplished by converting all of the carbonyl compound to enolate form so it is not an electrophile Immediate addition of an alkyl halide to completes the alkylation reaction

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Conditions for CondensationsA small amount of base is used to generate a small amount of enolate in the presence of unreacted carbonyl compound After the condensation, the basic catalyst is regenerated

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23.4 Dehydration of Aldol Products: Synthesis of EnonesThe β-hydroxy carbonyl products dehydrate to yield conjugated enones The term “condensation” refers to the net loss of water and combination of 2 molecules

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Dehydration of β-Hydoxy Ketones and AldehydesThe α hydrogen is removed by a base, yielding an enolate ion that expels the OH leaving group Under acidic conditions the OH group is protonated and water is expelled

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Driving the EquilbriumRemoval of water from the aldol reaction mixture can be used to drive the reaction toward products Even if the initial aldol favors reactants, the subsequent dehydration step pushes the reaction to completion

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23.6 Mixed Aldol ReactionsA mixed aldol reaction between two similar aldehyde or ketone partners leads to a mixture of four possible products This is not useful

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Practical Mixed AldolsIf one of the carbonyl partners contains no α hydrogens and the carbonyl is unhindered (such as benzaldehyde and formaldehyde) it is a good electrophile and can react with enolates hen a mixed aldol reaction is likely to be successful 2-methylcyclohexanone gives the mixed aldol product on reaction with benzaldehyde

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Mixed Aldols With Acidic Carbonyl Compou

ndsEthyl acetoacetate is completely converted into its enolate ion under less basic conditions than monocarbonyl partners Aldol condensations with ethyl acetoacetate occur preferentially to give the mixed product