Cannizzaro reaction

Cannizzaro reaction

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  CANNIZZARO REACTION PRESENTED BY:MR K V NANDA KUMAR ASSOCIATE PROFESSOR KRISHNA TEJA PHARMACY COLLEGE, TIRUPATHI-517506
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  CONTENTS HISTORY INTRODUCTION REQUIREMENTS FORTHE REACTION MECHANISMOF REACTION EXAMPLES LIMITATIONS CROSSED CANNIZZARO REACTION INTRAMOLECULAR CANNIZZARO REACTION 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 2
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  HISTORY It is areaction of organic compounds having an aldehydic group (-CHO). It was given by StanislaoCannizzaro in 1853 and hence was named after him. Cannizzaro treated benzaldehyde with potassium carbonate (potash) to obtain benzyl alcohol and potassium benzoate. 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 3 Benzaldehyde Potassium benzoate Benzyl alcohol
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  CANNIZZARO REACTION The disproportionationreaction of aldehydes without α-hydrogens in presence of a strong base to furnish an alcohol and a carboxylic acid is called Cannizzaro reaction. One molecule of aldehyde is reduced to the corresponding alcohol, while a second one is oxidized to the carboxylic acid. 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 4
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  DISPROPORTIONATION The reaction thatinvolves simultaneous oxidation and reduction of atoms of same element from one oxidation state (OS) to two different oxidation states is known as disproportionation reaction. The minimum requirement for this reaction to occur is: the element undergoing disproportionation should exhibit minimum three different oxidation states. 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 5
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  REQUIREMENTS FORTHE REACTION Aromaticaldehydes which do not contain α-hydrogen, for example, C6H5CHO (benzaldehyde) Aliphatic aldehydes which do not contain α-hydrogen, for example, HCHO, (CH3)3CCHO Presence of concentrated aqueous alkali (KOH, NaOH) 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 6
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  MECHANISMOFCANNIZZARO REACTION STEP 1: Anucleophile such as a hydroxide ion is used to attack the carbonyl group of the given aldehyde, causing a disproportionation reaction and giving rise to an anion carrying 2 negative charges. 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 7
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  MECHANISMOFCANNIZZARO REACTION STEP 2: •Thisresulting intermediate can now function as a hydride reducing agent. •Due to its unstable nature, the intermediate releases a hydride anion. •This hydride anion proceeds to attack another aldehyde molecule. •Now, the doubly charged anion is converted into a carboxylate anion and the aldehyde is converted into an alkoxide anion. 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 8
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  MECHANISMOFCANNIZZARO REACTION STEP 3: •Inthis final step, water offers a proton to the alkoxide anion which gives rise to the final alcohol product. •Now, the carboxylate ion gives rise to the final carboxylic acid product when acid workup is used. 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 9
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  EXAMPLES FORCANNIZZARO REACTION 15-06-2020PHARMD YEAR I POC - MR K V NANDA KUMAR 10
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  LIMITATIONS The applicability ofCannizzaro reaction in organic synthesis is limited as the yield is not more than 50% for either acid or alcohol formed. In case of aldehydes that do have α-hydrogens, the aldol condensation reaction takes place preferentially. The α,α,α-Trihalo aldehydes undergo haloform reaction in strongly alkaline medium. E.g. Choral will give chloroform in presence of an alkali. 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 11
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  CROSSEDCANNIZZARO REACTION This reactionbasically includes a mixture of two aldehydes having no α-hydrogen to yield all the products which are possible from the reactants used. The reaction between benzaldehyde and sodium formate is given below to understand the reaction easily. 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 12 Benzaldehyde Formaldehyde Benzyl alcohol Formic acid
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  INTRAMOLECULARCANNIZZARO REACTION This reactionis within the same compound in which two aldehydic groups present which are devoid of α-hydrogen. α-keto aldehydes can be converted to α-hydroxy carboxylic acids by an intermolecular Cannizzaro reaction. Phthalaldehyde can undergo intramolecular Cannizzaro reaction by giving (o- hydroxymethyl) benzoic acid. 15-06-2020 PHARMD YEAR I POC - MR K V NANDA KUMAR 13
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