ALDOL condensation A property of carobnylcompounds having α-hydrogen
ALDOL condensation• Aldol means aldehyde and alcohol groups on the same molecule• May occur between two aldehydes (aldols) or ketones (ketols) in the presence of a catalytic base• Reaction is only possible between two components having α-hydrogen• Such reactions are reversible• Mechanism: involves enolate formation and nucleophilic attack at a carbonyl carbon• Aldols and ketols loss water in the presence of an acid forming α-ß-unsaturated aldehydes or ketones and ß- hydroxy aldehydes or ketones
Aldol condensation• Aldehydes containing α-hydrogen undergo self addition in the presence of a base to form aldols• For example2 molecules of acetaldehyde condense to form aldol haviving name 3 hydroxy butanal
MechanismStep-1 One carbonyl partner with an alpha hydrogen atom isconverted by base into its enolate ion. O H C OH R C
O Nucleophilic OH2 Donor C : O: R C:Step 2 This enolate ion C Electrophilicacts as a nucleophilic Acceptordonor and adds to theelectrophilic carbonylgroup of the acceptorpartner O .. C O : C .. R C
OH2 O C C C OH -OH R + New C-C BondStep 3 addition of water resulting a neutralcondensation product.
Mechanism of the Aldol Reaction O HO:Base removes an H Cacidic alpha hydrogen Cfrom one aldehyde Hmolecule, yieldinga resonance- H Hstabilized eno-late ion.
O : O: .. C + H2O H C C H The enolate ion attacksCH3 H H a second aldehyde molecule in a nucleophilic addition reaction to give a tetrahedral alkoxide ion intermediate.
H 2OProtonation of the alkoxide ion intermediate yieldsneutral aldol product and regenerates the basecatalyst.
Aldols loss water in the presence of an acid forming α-ß-unsaturated aldehydes Heat/acid
Ketol condensation• Like aldehydes, ketones having alpha hydrogen condense to form ketolsFor example 2 molecules of acetone condense to produce 4-hydroxy, 4-methyl, 2-pentanone
4-hydroxy, 4-methyl, 2-pentanone 4 methyl-3 pentene-2 one
Mixed aldol condensation• If two different aldehydes condense, the process is called mixed aldol condensation.• For example• Benzaldehyde and acetaldehyde condense to form cinnamaldehyde