The Cannizzaro reaction ( Cannizzaro reaction) is an organic disproportionation reaction in which an aldehyde having no alpha active hydrogen undergoes an intermolecular redox reaction under the action of a strong base to form a molecule of a carboxylic acid and a molecule of alcohol.
The Italian chemist Stanislau Cannizzaro obtained benzoic acid and benzyl alcohol by treating benzaldehyde with grass ash in 1895. The reaction was first discovered and thus called the Cannizzaro reaction.
A fatty aldehyde, an aromatic aldehyde or a heterocyclic aldehyde containing no α-hydrogen atom simultaneously undergoes oxidation and reduction reaction of the aldehyde molecule itself under the action of a concentrated alkali to form a corresponding carboxylic acid (formation of a carboxylate in an alkali solution) and alcohol. Organic disproportionation reaction.
Vanillin, p-hydroxybenzaldehyde, syringaldehyde, and formaldehyde are all aldehydes with no active hydrogen. Under the action of strong alkali, intramolecular and intermolecular redox reactions occur to form one molecule of carboxylic acid and one molecule of alcohol.
First, the nucleophilic addition of a base to a carbonyl group occurs, and the tetrahedral intermediate reacts with a strong base to lose a proton to become a double negative ion (Cannizzaro intermediate). Since the oxygen atom has a negative charge and is electrically charged, the ability of the ortho carbon atom to repel electrons is greatly enhanced.
Two intermediate anions with an aldehyde can effect of hydrogen on the carbon with a pair of electrons to the hydrogen ions transferred to the carbonyl carbon in the form of an aldehyde to form an alkoxide anion and a carboxylate anion of a.
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The water in the Cannizzaro reaction can participate in the reaction, generating hydrogen, and confirming the process of negative hydrogen transfer.
The reaction is a kinetic tertiary reaction with a secondary reaction for the aldehyde and a primary reaction for the base:
r= k[RCHO] 2 [OH – ]
In a high concentration alkaline environment, the second reaction course becomes dominant, at which point the base becomes a secondary reaction:
r = k[RCHO]2[OH –] + k'[RCHO]2[OH-]2
- An aldehyde without an alpha hydrogen atom: formaldehyde produces methanol and formic acid, and glyoxylic acid produces glycolic acid and oxalic acid.
- An aldehyde having an alpha hydrogen atom: the material is capable of forming aldol aldehydes under appropriate conditions. The butanol and the original aldehyde can cross the Cannizzaro reaction, as shown below:
- An aldehyde with an alpha hydrogen atom in the presence of formaldehyde:
The resulting beta-hydroxy aldehyde can continue to cross the Cannizzaro reaction:
1: Mono-substituted benzaldehyde:
2 double substituted benzaldehyde:
When the formyl bi-ortho position has at least one unsubstituted group, a normal disproportionation reaction is carried out.
When the 2 formyl ortho positions are halogen or nitro, the following reaction occurs:
3 tri- and tetra-substituted benzaldehydes.
4 produces a hydrolyzing enzyme and hydrogenated North American berberine.
1, the concentration of alkali, aromatic aldehyde. 50% sodium hydroxide or potassium hydroxide, nitrobenzaldehyde 15~35%.
Solvent: alkaline water solvent.
Temperature: The aldehyde and alkali are mixed and heated, and the temperature is lowered gently. It is not necessary to strictly control the temperature.
When the reaction is too slow, it can be heated in a water bath until the reaction is completed. For nitrobenzaldehyde reacted at 35% alkali concentration, the temperature is controlled at 45 ° C, otherwise, nitro azo benzoic acid and carboxylic acid are produced.
In below reaction, it is an example of Cannizzaro’s Reaction
HCHO + HCHO ———–> CH3OH + HCOOH
- Aldehydes do not have α (Alpha) hydrogen
- Using con Alkali (50% NaOH/KOH)
- Molecule undergo self-oxidation as well reduction
- Known disproportionating reaction
α (Alpha) hydrogen
- The Carbon directly connected to the functional groups
called α (Alpha) Carbon
- The Hydrogen associated with that carbon is called α Hydrogen
Let’s check with some chemical structures
Acetone | CH3COCH3
In Acetone, carbon group attached with Hydrogen through α Hydrogen
and carbon group attached with α Carbon
Acetaldehyde | C2H4O
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hydrogen and carbon connected though α Hydrogen with a main functional group. therefore in Acetaldehyde, there is one α carbon and three α hydrogen present.
Formaldehyde | CH2O
There is no α Hydrogen
Benzaldehyde | C7H6O
There is no α Hydrogen in benzaldehyde
Benzophenone | C13H10O
There is no α Hydrogen in Benzophenone but there are two α carbons
Cyclohexanone | C6H10O
cyclohexanone connected with two α carbons and four α hydrogen. so you can say α Hydrogen present in cyclohexanone.
Cyclohexanol | C6H12O
There is one α hydrogen and one α carbon present in cyclohexanol
now you must be understood, if there is no α Hydrogen in any aldehyde. it means it will give Cannizzaro’s Reaction.
Types of Cannizzaro’s Reaction:-
- Internal Cannizzaro’s Reaction
There is only one molecule but there are two groups of aldehyde which give Alcohol and carboxylic acid.
- Cross Cannizzaro’s Reaction
In Cross Cannizzaro’s reaction, there are two types of Aldehyde. the small molecule will change carboxylic acid and another will change into alcohol.
Mechanism of Cannizzaro’s Reaction
Step 1:- Attach of a nucleophile on carbonyl carbon for from an anion
Step 2:- The anion act as a hydride donor to the 2nd molecule of aldehyde.
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