Aldol Condensation & Cannizaro Reaction – Crack Chemistry

Reaction Mechanisms of Carbonyl Compounds :-

Before seeing about Aldol condensation & cannizaro reaction
I hope that you’re familiar with
What is Carbonyl Group ?
And its Characterstic nature ?
Before Seeing Reaction Mechanism,
You must know about Alpha-Hydrogen :
The Hydrogen present in carbon which is nearby to the Corresponding Functional groups
Let’s Move to the Reaction Mechanisms

Aldol Condensation :-

•   Aldehyde containing Alpha-Hydrogen undergoes Self-addition in the presence of a base to form products called Aldols.
•   The corresponding Reaction is known as Aldol Condensation.
•    The Word ‘Aldol‘ is derived from the combination of the terms Aldehyde and Alcohol,the two functional groups present in the product.

General Reaction :-

Aldol Condensation, crackchemistry
Aldol Condensation
Two Molecules of acetaldehyde combine with each other in presence of dilute NaOH to form 3-Hydroxybutanal (Aldol)

Reaction Mechanism :-

This reaction is reversible reaction and it follows several steps,
Step 1 :

Formation of Nucleophile : ( Enolate ion )

Aldol Condensation, crackchemistry
Nucleophile Formation
In this above step, The OH⁻ ion in Sodium Hydroxide obstruct the Alpha-Hydrogen from the Carbon near to the Carbonyl Group, results in formation of Nucleophile.
Step 2 :

Attack of Nucleophile to carbonyl Carbon :

Aldol Condensation, crackchemistry

in this step, The formed Nucleophile is attack the another aldehyde’s Carbonyl carbon and the Oxygen gets the negativity charge.

Step 3 :

Formation of Aldol :

Cannizaro Reaction :

In this step, The negatively charged Oxygen gets and proton from the Water Molecule and it results in formation of 3-Hydroxybutanal (Aldol)

Cannizaro Reaction :

•    Aldehydes which do not have Alpha-Hydrogen undergoes Cannizaro Reaction,when heated with Concentrated NaOH.
•    It is a Disproportionation Reaction.
•    One half of the aldehyde molecule are oxidized to a Carboxylic acid and one half are reduced to an alcohol.
•    This Reaction is known as Cannizaro Reaction.
Note :  Ketones do not give this reactions.

General Reaction :-

Cannizaro Reaction, Crackchemistry
Cannizaro Reaction
Two Molecules of formaldehyde heated with Concentrated NaOH,it gives Methanol and Sodium Formate

Reaction Mechanism :-

Step 1 :

Attack of HO⁻ on the Carbonyl Group :

In this step, The OH⁻ of the Sodium Hydroxide is Attack the Carbonyl Group and Oxygen gets negatively charge

Step 2 :

Hydride transfer :

Cannizaro Reaction, Crackchemistry
Hydride transfer

then, the another formaldehyde present in the reaction is get attacked by the previously formed species.

Further Hydride transfer is taking place.
It resulting in formation of Respective Carboxylic Acid and Alcohol.
Note : You can use Benzaldehyde also in this reaction because Benzaldehyde also does not have any Alpha Hydrogen

Workout Test :-

Refference : Advanced Organic Chemistry by Arun Bahl & B.S.Bahl
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Originally posted 2022-10-30 07:16:00.