chapter23 - Chapter 23 Carbonyl Condensation Reactions...

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Unformatted text preview: Chapter 23. Carbonyl Condensation Reactions Based on McMurry’s Organic Chemistry , 7 th edition 2 3 Housekeeping Items Housekeeping Items Exams Done – Returned today! Exams Done – Returned today! High 94 High 94 Low 24 Low 24 Average 61 Average 61 Exams Done – Returned today! Exams Done – Returned today! Scantron answers posted on Blackboard Scantron answers posted on Blackboard Exam Key posted on Blackboard Exam Key posted on Blackboard 4 Condensation Reactions Condensation Reactions Carbonyl compounds are both the electrophile and nucleophile in carbonyl condensation reactions 5 Why this Chapter? Carbonyl condensation reactions also occur often in metabolic pathways. Also one the general methods used to form C-C bonds. 6 23.1 Carbonyl Condensation: The Aldol 23.1 Carbonyl Condensation: The Aldol Reaction Reaction Acetaldehyde reacts in basic solution (NaOEt, NaOH) with another molecule of acetaldhyde The β-hydroxy aldehyde product is aldol ( ald ehyde + alcoh ol ) This is a general reaction of aldehydes and ketones Mechanism given on board in lecture. Mechanism given on board in lecture. 7 The Equilibrium of the Aldol The aldol reaction is reversible, favoring the condensation product only for aldehydes with no α substituent Steric factors are increased in the aldol product 8 Aldehydes and Ketones and the Aldehydes and Ketones and the Aldol Equilibrium Aldol Equilibrium Mechanism given on board in lecture. Mechanism given on board in lecture. 9 Mechanism of Aldol Reactions Mechanism of Aldol Reactions 10 23.2 Carbonyl Condensation versus Alpha- Substitution Carbonyl 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 11 Conditions for Condensations A 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 12 23.3 Dehydration of Aldol Products: Synthesis of Enones The β-hydroxy carbonyl products dehydrate to yield conjugated enones The term “condensation” refers to the net loss of water and combination of 2 molecules 13 Dehydration of b-Hydroxy Dehydration of b-Hydroxy Ketones and Aldehydes Ketones and Aldehydes The 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 14 Driving the Equilibrium Removal 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 15 Driving the Equilibrium Removal of water from the aldol reaction mixture can...
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