Crossed Aldol Condensation

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Crossed Aldol Condensation

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Crossed Aldol Condensation

  1. 1.  In the presence of a strong base, two different molecules of aldehyde or ketone combine to form a β-hydroxyaldehyde or a β-hydroxyketone.  This reaction is called as the aldol condensation.
  2. 2.  The enolate ion (carbanion) is the actual nucleophilic reagent.  The reaction breaks down to the following: The α-carbon of the donor attaches itself to the carbonyl carbon of the acceptor.
  3. 3. CH3 C H O CH3 C H O CH2 C H O CH2 C H O + _ _ .. _ acceptor donor
  4. 4.  The carbonyl group plays two roles in this reaction:  1. It provides a site for nucleophilic attack by the enolate ion.  2. It makes the a-hydrogens sufficiently acidic to enable the formation of reasonable concentrations of enolate ion.  NOTE: There must be a-hydrogens in order to undergo aldol condensation.  Without a-hydrogens, the only possible reaction is Cannizarro reaction
  5. 5.  If both carbonyl compounds have α-hydrogens, then the donor and acceptor could be controlled.  If one carbonyl compound that does not have α- hydrogens is chosen, it can only act as an acceptor.  Then, but adding the donor to a dilute solution of acceptor, we can control the reaction to give only one product.
  6. 6. C H O + CH3 C H O NaOH C CH2 OH H C H O - H2O (spontaneously) CH CH C H O CINNAMALDEHYDE
  7. 7.  Step 1: Deprotonation and formation of nucleophile. • Step 2: Attack by nucleophile: Formation of alkoxide ion.
  8. 8.  Step 3: Hydrolysis and formation of Aldol type product Step 4: Intramolecular dehydration
  9. 9.  The aldol condensation is very useful in the preparation of large molecules from simple starting materials.  In biosynthesis, where large molecules are products, condensation reactions provide the only reasonable route, since all of the starting materials are small molecules containing only two or three carbons.  In synthesis of insect repellants such as 2-Ethyl-3- hydroxy hexanal
  10. 10. CH2 CH C H O OHO P OH OO CH2 C CH2 O P OH OH O O O CH2 CH C CH C CH2 O P OH O O O OH OH HOHO P OH O O + Glyceraldehyde-3-phosphate Dihydroxyacetone phosphate Fructose-1,6-diphosphate a ALDOL CONDENSATION enzyme

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