Cannizzaro Reaction: A Key Mechanism in Organic Chemistry
by Yuvi K - December 17, 2023
Cannizzaro Reaction: A Key Mechanism in Organic Chemistry
Organic chemistry can be a daunting and complicated subject to tackle. There are dozens of reactions and mechanisms to know, and to move forward in organic chemistry, having a solid understanding of these mechanisms is essential. One such reaction is the Cannizzaro reaction, a unique and powerful mechanism that is encountered in many organic synthesis methods.
What is a Cannizzaro Reaction?
A Cannizzaro reaction is a particular type of organic chemistry reaction in which aldehydes are reduced to primary alcohols. It is also referred to as an aldol or Cannizzaro- Aldol reaction. The mechanism of the reaction is based on the disproportionation or fragmentation of the aldehyde in presence of a base. The reaction is named after Giulio Cannizzaro, the Italian chemist who first reported this reaction in 1853.
Mechanism of Cannizzaro Reaction
The mechanism of the Cannizzaro reaction is a bit different and complex compared to other organic reactions. It involves some interesting concepts such as carbanion formation and enolate formation.
In the first step, an aldehyde with the presence of base is protonated to form a cation. This cation is then attacked by another molecule from the base to form a carbanion. This carbanion then undergoes fragmentation to form two separate fragments. The first fragment is an alkoxide anion and the second is a hydrogen molecule, which is further protonated to form a primary alcohol.
Formulaically, it looks like this:
| Reactants | Reaction | Products |
|---|---|---|
| Aldehyde + Base | Protonation → Carbanion Formation → Fragmentation | Alcohol + Alkoxide Anion |
Conditions of Cannizzaro Reaction
The Cannizzaro reaction generally proceeds under relatively mild conditions. It is usually carried out in aqueous solutions of a base at mild temperatures (30 to 60 degrees Celsius).
The base used in the reaction directly affects the reaction rate. Bases such as sodium hydroxide, lithium hydroxide, potassium hydroxide, sodium ethoxide, etc. are commonly used. Alcohols can also be used instead of bases.
The reactant, i.e. the aldehyde, should be a primary aldehyde or a derivative with an α-hydrogen atom. Secondary or tertiary aldehydes will not undergo this reaction.
Uses of Cannizzaro Reaction
The Cannizzaro reaction is an important and useful organic chemistry tool. It has several applications in organic synthesis. One of the most common uses of the reaction is in the production of primary alcohols from aldehydes. The reaction is also used for the oxidative cleavage of aromatic aldehydes, for the removal of acyl groups from ketones or cyano groups from nitriles.
The reaction has also been used as a method for the qualitative analysis of aldehydes. According to this method, when an unknown aldehyde is treated with a suitable base, the formation of an alcohol (primary) indicates the presence of primary aldehydes.
Limitations of Cannizzaro Reaction
The reaction has some limitations as well. As mentioned previously, it is limited to primary aldehydes and it cannot be used to reduce secondary or tertiary aldehydes. The reaction is also not suitable for aldehydes containing an alpha-hydrogen atom that can form an aldol condensation. In such cases, the Cannizzaro reaction will be suppressed by the aldol condensation.
Conclusion
Overall, the Cannizzaro reaction is a valuable method for organic synthesis that is sometimes utilized in qualitative analysis as well. While it is limited to certain types of aldehydes and situations, it can be invaluable in certain instances and will likely remain a key reaction in organic chemistry for many more years to come.
