What is Cannizzaro Reaction?
Cannizzaro reaction is a chemical reaction named after Stanislao Cannizzaro that involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to yield a carboxylic acid and a primary alcohol.
Cannizzaro Reaction Mechanism details the method to get one molecule of alcohol and one molecule of carboxylic acid from two molecules of a given aldehyde. Scientist Stanislao Cannizzaro, in 1853 succeeded in obtaining benzyl alcohol and potassium benzoate from benzaldehyde. The reaction is executed by a nucleophilic acyl substitution on an aldehyde where the leaving group attacks another aldehyde. A tetrahedral intermediate results from the attack of hydroxide on a carbonyl. This tetrahedral intermediate collapses, thereby reforming the carbonyl and transferring a hydride which attacks another colony.
Now, a proton is exchanged by acid and alkoxide ions. When a base of high concentration is introduced, the aldehyde forms an anion which has a charge of 2. From this, a hydride ion is transferred to a second molecule of the aldehyde, forming carboxylate and alkoxide ions. The alkoxide ion also obtains a proton from the solvent for the reaction.
Mechanism of Cannizzaro Reaction
Mechanism 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.
Mechanism 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.
Mechanism Step 3
In this final step, water offers a proton to the alkoxide anion which gives rise to the final alcohol product. The reaction can proceed since the alkoxide is more basic than water. Now, the carboxylate ion gives rise to the final carboxylic acid product when acid workup is used (the acid workup is required since carboxylate is less basic than water and therefore cannot obtain a proton from water).
Crossed Cannizzaro Reaction
Aldehydes with alpha hydrogen atoms undergo deprotonation due to the strongly alkaline conditions of the reaction, leading to enolates and/or aldol reactions of these enolates where beta hydroxy aldehydes or ketones are obtained. Therefore, it is not surprising that the reaction produces only 50% of the required alcohol and carboxylic acid at ideal conditions. This is why the crossed Cannizzaro reaction is more commonly used. A sacrificial aldehyde is combined with a more valuable chemical and formaldehyde is used as a reductant, oxidizing it to sodium formate. The required alcohol is obtained from the reduction of the other aldehyde chemical. Since 2 different aldehydes can be completely converted into the required product, the yield of the valuable chemical is increased.
To conclude, the Cannizzaro reaction can be used to influence a disproportionation of a non-enolizable aldehyde. The cross Cannizzaro reaction is employed to increase the yield of the valuable chemical.
Frequently Asked Questions
Q.1: What are the products formed in a Cannizzaro reaction?
Ans: The products that are formed in a Cannizzaro reaction include primary alcohols and carboxylic acids. The carboxylate anion is protonated to afford the carboxylic acid whereas the alkoxide anion is protonated by water to yield alcohol.
Q.2: What are the advantages of the crossed Cannizzaro reaction?
Ans: This variation of the reaction improves the yield of the desired product. Both the aldehydes used are entirely converted to products and wastage of the valuable reactant chemicals is avoided. The atom economy of the process is also low.
Q.3: Why doesn’t acetaldehyde participate in a Cannizzaro reaction?
Ans: In this, the alpha-hydrogens are deprotonated due to the alkaline environment. Since acetaldehyde has 3 alpha hydrogens, it readily forms enolate ions upon deprotonation and therefore cannot participate in the reaction.
Q.4: What products are obtained when KOH or NaOH is used?
Ans: When potassium hydroxide or sodium hydroxide is used in the base-induced disproportionation process, the potassium carboxylate salt or the sodium carboxylate salt of the corresponding carboxylic acid is obtained.
Q.5: Why is the Cannizzaro reaction a redox process?
Ans: In this reaction, one aldehyde is oxidized to give a carboxylic acid whereas the other aldehyde undergoes reduction to yield the alcohol. Since both oxidation and reduction occurs in the hydride transfer, the reaction is said to be a redox process.