2. CONTENTS
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HISTORY INTRODUCTION
REQUIREMENTS
FORTHE REACTION
MECHANISMOF
REACTION
EXAMPLES LIMITATIONS
CROSSED
CANNIZZARO
REACTION
INTRAMOLECULAR
CANNIZZARO
REACTION
3. HISTORY
It is a reaction 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.
Benzaldehyde
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Potassium benzoate Benzylalcohol
4. CANNIZZARO REACTION
The disproportionation reaction of aldehydes without α-hydrogens in presence of a strong base
to furnish an alcohol and a carboxylic acid is calledCannizzaro reaction.
One molecule of aldehyde is reduced to the corresponding alcohol, while a second one is
oxidized to the carboxylic acid.
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5. DISPROPORTIONATION
The reaction that involves 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.
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6. REQUIREMENTS FORTHE REACTION
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Aromatic aldehydes 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)
7. MECHANISMOFCANNIZZARO REACTION
STEP 1:
A nucleophile 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.
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8. MECHANISMOFCANNIZZARO REACTION
STEP 2:
•This resulting 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.
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9. MECHANISMOFCANNIZZARO REACTION
STEP 3:
•In this 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.
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11. LIMITATIONS
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The applicability of Cannizzaro 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.
12. CROSSEDCANNIZZARO REACTION
This reaction basically 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.
Benzaldehyde
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Formaldehyde Benzylalcohol Formicacid
13. INTRAMOLECULAR CANNIZZARO REACTION
This reaction is 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.
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