Micro-Scale Cannizzaro Reaction

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Micro-scale Cannizzaro Reaction with 4-Chlorobenzaldehyde Name: Saoirse Treacy | Date: 11/11/14 | I.D. Number: 13379026 | | Abstract: This experiment illustrates the simultaneous oxidation and reduction of an aromatic aldehyde to form the corresponding benzoic acid and benzyl alcohol [1]. The aromatic aldehyde, 4-chlorobenzldehyde, is refluxed so that the Cannizzaro reaction takes place in order to synthesise an alcohol, 4-chlorobenzyl alcohol. These products are then isolated by extraction methods and purified by re-crystallisation. Introduction: This reaction is the redox disproportionation of an aldehyde to an equimolar mixture of primary alcohol and carboxylic salt when treated with a concentrated strong base [2]. 4-chlorobenzaldehyde is refluxed with methanol and aqueous potassium hydroxide solution. The two products are isolated by adding dichloromethane to the reflux product, which separates the alcohol from the acid, as the alcohol dissolves in the polar dichloromethane. Both the acid and alcohol products are crystallised. To purify the alcohol, the crude product is recrystallized using a 4% acetone in hexane mix, and then dried by vacuum filtration. Similarly, the acid is recrystallized using a small portion of hot methanol. Mechanism: The potassium hydroxide base provides the reaction with OH- ions, which attack the C=O in the 4-chlorobenzadehyde, due to their attraction to the slightly positive charge on the alpha carbon. This leaves a negative charge on the single bond oxygen. The negatively charged oxygen pushes back into a double bond and forces the single bonded hydrogen to leave as a H- ion which attacks the C=O of another 4-chlorobenzaldehyde molecule. This forms the 4-chlorobenzoic acid and leaves a negatively charged, single bonded oxygen on the other molecule. This negatively charged oxygen picks up H+ ion from solution and forms

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