Cannizzaro reaction

The Cannizzaro reaction is a chemical reaction named after its discoverer, Stanislao Cannizzaro, which involves the base-induced disproportionation of an aldehyde that lacks an alpha hydrogen. In this reaction, one molecule of the aldehyde is reduced to the corresponding alcohol, while another molecule is oxidized to the corresponding carboxylic acid or its salt[1][3][5].

Mechanism of the Cannizzaro Reaction

The mechanism of the Cannizzaro reaction involves several steps:

1. Nucleophilic Attack: A hydroxide ion (OH⁻) attacks the carbonyl carbon of the aldehyde, resulting in the formation of a tetrahedral intermediate[3][6].

2. Hydride Transfer: The tetrahedral intermediate then undergoes an intermolecular hydride (H⁻) transfer to another molecule of the aldehyde. This step is the key to the disproportionation process, as one aldehyde molecule is reduced to an alcohol, and the other is oxidized to a carboxylate ion[3][4][6].

3. Protonation: Finally, the carboxylate ion is protonated to form the carboxylic acid, and the alkoxide ion formed from the reduction step is also protonated to yield the alcohol[3][9].

Conditions for the Cannizzaro Reaction

• The reaction typically requires a strong base, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH)[3][11].
• It is essential that the aldehyde does not contain an alpha hydrogen, as the presence of alpha hydrogens would lead to competing reactions such as aldol condensation[3][11].

Examples of the Cannizzaro Reaction

• Formaldehyde (HCHO) can be disproportionated to formic acid (HCOOH) and methanol (CH₃OH) in the presence of a strong alkali[4].
• Benzaldehyde (C₆H₅CHO) can be converted to benzoic acid (C₆H₅COOH) and benzyl alcohol (C₆H₅CH₂OH) under similar conditions[4].

Crossed Cannizzaro Reaction

A variation of the Cannizzaro reaction is the crossed Cannizzaro reaction, where two different aldehydes are used. Typically, formaldehyde is used as the sacrificial aldehyde because it is more reactive due to the absence of electron-donating alkyl groups. In this scenario, formaldehyde is oxidized to sodium formate, while the other aldehyde is reduced to the corresponding alcohol[3][8].

Significance

The Cannizzaro reaction is significant in organic chemistry as it provides a method for the simultaneous oxidation and reduction of aldehydes to form valuable products such as alcohols and carboxylic acids. However, it is limited to aldehydes without alpha hydrogens and is not as commonly used in modern synthetic chemistry due to the availability of more efficient and selective reducing and oxidizing agents[1][3].

Citations:

[1] https://en.wikipedia.org/wiki/Cannizzaro_reaction

[2] https://www.youtube.com/watch?v=tEkOcmJR27o

[3] https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Reactions/Organic_Reactions/Cannizzaro_Reaction

[4] http://www.adichemistry.com/organic/namedreactions/cannizzaro/cannizzaro-1.html

[5] https://www.merriam-webster.com/dictionary/Cannizzaro%20reaction

[6] https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/19:_Aldehydes_and_Ketones-_Nucleophilic_Addition_Reactions/19.12:_Biological_Reductions

[7] https://www.rsc.org/suppdata/books/184973/9781849739634/bk9781849739634-chapter%2017.pdf

[8] https://www.chemistrysteps.com/cannizzaro-reaction/

[9] https://byjus.com/chemistry/cannizzaro-reaction-mechanism/

[10] https://www.youtube.com/watch?v=TG6fxnDqjx0

[11] https://byjus.com/jee/cannizzaro-reaction-for-iit-jee/

[12] https://study.com/academy/lesson/cannizzaro-reaction-mechanism-examples.html

[13] https://chemistrytalk.org/cannizzaro-reaction/

[14] https://pubs.acs.org/doi/10.1021/ol052506y

[15] https://www.sciencedirect.com/topics/chemistry/cannizzaro-reaction

[16] https://www.britannica.com/science/Cannizzaro-reaction

[17] https://www.thoughtco.com/illustrated-cannizzaro-reaction-608567

[18] https://www.pw.live/concepts-cannizzaro-reaction