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Cannizzaro Reaction CANNIZZARO REACTION In the presence of concentrated alkali, aldehydes containing no α - Hydrogen undergo self oxidation & reduction (Redox) reaction to yield a mixture of an alcohol ( Carbinol ) & a salt of carboxylic acid. O O OH OH - 2 R C H R C O R C H concentrated H carboxylate anion alcohol Examples: MECHANISM OF THE REACTION 1st Mechanism: O O carbonyl-hydroxide R C H OH C anion Step I R OH aldehyde-hydroxide H adduct 1st aldehyde conc. molecule alkali Tetrahedral intermediate intermediate I O O O O R C H C C C R OH R H R OH H H Step II 2nd aldehyde intermediate I molecule OH O C C R H R O H salt of alcohol carboxylic acid 2nd Mechanism : Step I Generation of Intermediate I . Step II OH OH OH OH H R C H R C H R C R C O O O O H intermediate I intermediate I H O R C R C OH O H How can you prove that the H comes from another equivalent of aldehyde and not from the medium, ( i.e. Involvement of a hydride shift in the mechanism of Cannizzaro reaction) ? H.W. 3rd Mechanism If the oxidant alde. differs from the reductant alde. the reaction is Crossed Cannizzaro Reaction. If one of the aldehydes is formaldehyde , the reaction yields almost exclusively a salt of formic acid and the alcohol corresponding to the other aldehyde. O O O OH OH - R C H H C H H C O R C H formaldehyde formate H anion alcohol Aldehydes or ketones with an α – hydrogen undergo Aldol Condensation ( carbonyl condensation reaction ) faster under the influence of dilute base or acid. O H O OH H O - OH H3C C H H C C H H3C C C C H H H H Acetaldehyde Acetaldehyde 3- hydroxy-butanol - OH Propanal ? - OH Acetone ? Name of Experiment : Cannizzaro Reaction. Aim of Experiment :Synthesis of Benzoic acid & Benzyl alcohol Principals : Both benzoic acid and benzyl alcohol can be prepared in the laboratory by Cannizzaro Reaction , by the action of concentrated NaOH or KOH on benzaldehyde. O O O H KOH C H C H conce. C O K C OH H Benzaldehyde Benzaldehyde Potassium benzoate Benzyl alcohol Benzoic Acid: C7H6O2 , C6H5COOH Molar mass = 122.12 g / mol, Melting point = 121 – 123 oC. Colorless plate or needle like crystals. Sparingly soluble in water but soluble in hot or boiled water. Reacts with NaHCO3 to give CO2 gas. Used as preservative and antiseptic. Benzyl Alcohol : C7H8O , C6H5CH2OH . Molar mass = 108.14 g / mol , Boiling point = 204 – 207 oC Colorless oily liquid. Volatile with steam. Immiscible with water but completely miscible with alcohols & organic solvents. Has some local anesthetic properties so used in injectibles and in ointment as antipruretic & antiseptic. Part I SYNTHESIS : Part II Isolation of Benzoic acid & Benzyl Alcohol : 1- Add 30 ml of H2O to ensure complete dissolve of Pot. Benzoate. 2- Pour the liquid to a separatory funnel. 3- Add 20 ml ether to the reagent bottle & shake. 4- Add this ether to the liquid in the separatory funnel. 5- Shake well for 2 min. & stand. ? Give the reason for shaking the ethereal layer with saturated Sodium bisulfite solution in our experiment today ? To get rid of excess unreacted benzaldehyde which may be present. O O Na OH Na HSO3 C H C H C H SO SO3H 3 Na ? Why we add Na2CO3 to the ethereal layer after that? To get rid of any excess unreacted Sodium bisulfite. Na2CO3 + 2 NaHSO3 CO2 + H2O + 2 Na2SO3 excess Post Lab Exercises : ? How would propanal react with KOH under the condition of this experiment ? Answer the same question for the reaction of 2,2-dimethylpropanal. ? In the presence of alkali , Acetaldehyde undergoes a mixed aldol condensation with formaldehyde, followed by a crossed Cannizzaro reaction , to produce the tetra alcohol penta - erythritol, C(CH2OH)4 , write the equations . ? In the benzilic acid rearrangement , the diketone benzil is converted by NaOH into the salt of benzilic acid. If sodium methoxide (CH3O Na) is used instead of NaOH, the ester C6H5)2C(OH)COOCH3 is obtained. Suggest a possible mechanism for this rearrangement. ? 4- When o- phthalaldehyde is treated with base, o- (hydroxymethyl) benzoic acid is formed. Show the mechanism of this reaction. CHO CO H - 2 1. OH + 2. H3O CHO CH2OH o-Phthalaldehyde o-(Hydroxymethyl) benzoic acid.
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