Cannizzaro reaction

Cannizzaro Reaction

The Cannizzaro reaction is a chemical reaction that involves the base-induced disproportionation of non-enolizable aldehydes into a mixture of alcohol and carboxylic acid. This reaction is significant in organic chemistry for the transformation of aldehydes lacking hydrogen atoms in the alpha position. The Cannizzaro reaction was first reported in 1853 by the Italian chemist Stanislao Cannizzaro, after whom the reaction is named.

Mechanism

The Cannizzaro reaction proceeds through a nucleophilic addition of a hydroxide ion (OH−) to the carbonyl carbon of the aldehyde, forming a hydroxyl intermediate. This intermediate then undergoes a redox process, where one molecule is reduced to an alcohol while another is oxidized to a carboxylic acid. The reaction mechanism involves several key steps: nucleophilic attack, hydride transfer, and proton transfer. The overall process can be summarized as follows:

1. Nucleophilic attack: A hydroxide ion attacks the carbonyl carbon of the aldehyde, leading to the formation of an alkoxide ion. 2. Hydride transfer: The alkoxide ion formed in the first step transfers a hydride ion (H−) to another aldehyde molecule, reducing it to an alcohol and oxidizing the alkoxide ion to a carboxylate ion. 3. Proton transfer: Finally, a proton (H+) is transferred from the alcohol to the carboxylate ion, yielding the carboxylic acid and completing the reaction.

Applications

The Cannizzaro reaction is utilized in organic synthesis for the preparation of alcohols and carboxylic acids from aldehydes. It is particularly useful for synthesizing these compounds from aldehydes that are resistant to direct oxidation or reduction. The reaction is also employed in the production of certain pharmaceuticals and fine chemicals.

Limitations

One of the limitations of the Cannizzaro reaction is its requirement for non-enolizable aldehydes, as aldehydes capable of enolization typically undergo the Aldol condensation instead. Additionally, the reaction requires a strong base, such as concentrated sodium hydroxide, which can be incompatible with sensitive functional groups.

Variants

Several variants of the Cannizzaro reaction have been developed to overcome its limitations and expand its applicability. These include the Tishchenko reaction, which is a related disproportionation reaction catalyzed by aluminum alkoxides, and the use of modified conditions or catalysts that allow for the reaction to proceed under milder conditions or with a broader range of substrates.

See Also

• Aldol condensation
• Redox reaction
• Organic synthesis
• Stanislao Cannizzaro