What is Cannizzaro Reaction Mechanism Equation and Examples
Named after Stanislao Cannizzaro, it is a disproportionation reaction. When two molecules of an aldehyde are disproportionated by a base, leading to the formation of a carboxylic acid and primary alcohol, the reaction is termed the Cannizzaro reaction.
It is only possible in aldehydes that lack a Hydrogen atom on their $\alpha$ Carbon. When such aldehydes are treated with concentrated bases like $50% \ NaOH, \ KOH, \ Na_2CO_3$, they are disproportionated, one molecule is reduced to an alcohol, and one is oxidised to carboxylic acid.
Since the rate-determining step of the Cannizzaro reaction is hydride transfer towards the Carbonyl, a concentrated base is necessary because a dilute base cannot make hydride transfer.
Carbonyl Bond
The double bond between carbon and oxygen is known as the carbonyl bond, which can be represented as $(C=O)$. Multiple groups, like aldehydes, ketones, carboxylic acids, etc., are some groups containing the carbonyl bond. Usually, a weak nucleophile is attached to the carbonyl carbon, which can easily be removed. Since oxygen is more electronegative than carbon, the bond is polar.
The carbon, thus, has a partial positive charge on it. This makes the carbonyl carbon a very good centre for nucleophiles to attack. As nucleophiles, $Nu^-$ attack, the stronger one replaces the weaker one through an addition-elimination mechanism.
Strong nucleophile replaces the weaker nucleophile
Nucleophiles
Nucleophiles are positive charge-seeking species. These may be negatively charged species with a super-octet or species with lone pairs. These attack the positive centres and bond with them. In the presence of a weak nucleophile, these perform substitution elimination reactions, wherein the elimination of a weaker nucleophile follows the addition of a stronger nucleophile. Examples of nucleophiles include $R-S^- , CN^- , OH^- ,$ etc.
Cannizzaro Reaction Mechanism
To understand a reaction- estimate the possible products, the factors that affect it and how they affect it, we need to understand how a chemical reaction occurs. This is done by understanding its mechanism. The Cannizzaro reaction occurs in three steps, wherein the second step is the slowest and, thus, the rate-determining step. It is overall a substitution reaction, i.e., one group replaces the other in the broad sense. However, when understood in steps, it is an addition reaction followed by elimination. But, by addition-elimination only carboxylic acid is prepared. Nucleophilic addition whereas prepares the alcohol.
Step 1: Attack of the nucleophile.
A base ionises and disintegrates to give a cation and $OH^-$ . The hydroxide ion acts as a nucleophile and attacks the carbonyl carbon, which has positive polarity. Due to this, one bond between carbon and oxygen breaks and a negative charge appears on the oxygen atom. Here, disproportionation takes place.
Attack of Nucleophile $(OH^-)$ on a carbonyl Carbon
Step 2: Addition elimination to form carboxylic acid.
Although hydroxide ion is a better leaving group than hydride ion, the bond between Hydrogen and carbonyl carbon breaks, giving out a hydride. This is because the solution already has an excess of hydroxide ions, which makes it easier for a hydride ion to be added to the solution. This leads to the formation of carboxylic acid. This step is the slowest step and the rate-determining step of the reaction.
Release of Hydride ion and formation of carboxylic acid
Step 3: Attack of hydride and formation of alcohol.
Hydride acts as a nucleophile and attacks the other carbonyl carbon, forming alkoxide ion.
Attack of Hydride ion on another Carbonyl carbon to give alcohol
Since alkoxide ion is a strong base, it makes the carboxylic acid lose its Hydrogen, finally forming carboxylate ion and alcohol. This exchange occurs rather quickly.
Formation of salt of carboxylic acid
Thus, from two moles of aldehyde, one mole of carboxylic acid and one mole of alcohol are produced.
Types of Cannizzaro reaction
Cross Cannizzaro Reaction
Under ideal circumstances, just half of the necessary amounts of alcohol and carboxylic acid are produced by the crossed Cannizzaro reaction. The needed product can be made entirely from 2 distinct carbonyl compounds, increasing the output of the important molecule. The more reactive aldehyde, i.e., the one with more positive polarity on its carbonyl carbon, oxidises to acid, whereas the one with the less reactive carbonyl carbon converts to alcohol. The cross-Cannizzaro reaction may help increase desirable output.
Cross Cannizzaro Reaction
Intramolecular Cannizzaro Reaction
It is possible in such chemical substances with at least two carbonyl carbons lacking an $\alpha$ hydrogen.
Intramolecular Cannizzaro Reaction
Summary
A quick and effective method for performing the Cannizzaro reaction was developed. A non-enolizable aldehyde can be made disproportionate using the Cannizzaro reaction. The cross-Cannizzaro reaction is used to boost the yield of the valuable chemical. The advantages of the current approach over earlier ones are its environmental friendliness, superior yield to those of earlier methods discussed, lack of need for specialised equipment, ease of operation, non-hazardous nature, and simplicity.
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FAQs on Cannizzaro Reaction in Organic Chemistry
1. What is the Cannizzaro reaction?
The Cannizzaro reaction is a base-induced disproportionation reaction in which a non-enolizable aldehyde is converted into an alcohol and a carboxylate salt. It occurs when an aldehyde without α-hydrogen reacts with a concentrated base such as NaOH or KOH.
- Two molecules of the same aldehyde react.
- One molecule is reduced to a primary alcohol.
- The other is oxidized to a carboxylate ion.
2. What is the mechanism of the Cannizzaro reaction?
The mechanism of the Cannizzaro reaction involves nucleophilic addition of hydroxide followed by hydride transfer between two aldehyde molecules. It proceeds in three main steps:
- Step 1: OH- attacks the carbonyl carbon forming a tetrahedral alkoxide intermediate.
- Step 2: A hydride ion (H-) transfers from this intermediate to another aldehyde molecule.
- Step 3: The resulting alkoxide ions are protonated to give alcohol and carboxylate salt.
3. Why do only aldehydes without alpha hydrogen undergo Cannizzaro reaction?
Only aldehydes without α-hydrogen undergo the Cannizzaro reaction because aldehydes with α-hydrogen prefer aldol condensation under basic conditions. In the presence of base:
- Aldehydes with α-H form an enolate ion.
- The enolate undergoes aldol reaction instead of disproportionation.
- Non-enolizable aldehydes (e.g., benzaldehyde, formaldehyde) cannot form enolates, so they undergo Cannizzaro reaction.
4. What are the conditions required for the Cannizzaro reaction?
The Cannizzaro reaction requires a non-enolizable aldehyde and a concentrated strong base such as NaOH or KOH. The essential conditions are:
- Concentrated aqueous base (usually 40–50% NaOH).
- Aldehyde lacking α-hydrogen.
- Room temperature or mild heating.
5. Can you give an example of the Cannizzaro reaction with a balanced equation?
A classic example of the Cannizzaro reaction is the reaction of formaldehyde with sodium hydroxide to form methanol and sodium formate. The balanced equation is:
2HCHO(aq) + NaOH(aq) → CH3OH(aq) + HCOONa(aq)
Here, one molecule of formaldehyde is reduced to methanol (CH3OH), and the other is oxidized to sodium formate (HCOONa).
6. What is crossed Cannizzaro reaction?
The crossed Cannizzaro reaction occurs when two different non-enolizable aldehydes react in the presence of a strong base. In practice:
- Formaldehyde is commonly used with another aldehyde.
- Formaldehyde is preferentially oxidized to formate ion.
- The other aldehyde is reduced to the corresponding alcohol.
7. What are the products of the Cannizzaro reaction?
The products of the Cannizzaro reaction are a primary alcohol and a carboxylate salt formed from the same aldehyde. Specifically:
- One aldehyde molecule is reduced to a primary alcohol.
- The other is oxidized to a carboxylate ion (which forms a salt with Na+ or K+).
8. What is the difference between Cannizzaro reaction and aldol reaction?
The key difference between Cannizzaro reaction and aldol reaction is the presence or absence of α-hydrogen in the aldehyde. The main differences are:
- Cannizzaro reaction: Occurs with aldehydes lacking α-H; gives alcohol + carboxylate salt.
- Aldol reaction: Occurs with aldehydes/ketones having α-H; gives β-hydroxy aldehyde or ketone.
- Cannizzaro involves redox (disproportionation), while aldol involves carbon–carbon bond formation.
9. Is Cannizzaro reaction a redox reaction?
Yes, the Cannizzaro reaction is a redox (disproportionation) reaction because one aldehyde molecule is oxidized while another is reduced. In this process:
- Oxidation: Aldehyde → carboxylate ion.
- Reduction: Aldehyde → primary alcohol.
10. What is the role of base in the Cannizzaro reaction?
The strong base in the Cannizzaro reaction initiates nucleophilic attack and facilitates hydride transfer between aldehyde molecules. Specifically:
- OH- attacks the carbonyl carbon to form an intermediate.
- It promotes the hydride ion transfer step.
- It stabilizes the formed carboxylate ion as its sodium or potassium salt.
