Question

Write the reaction involved in: Cannizzaro's reaction

Hint:

Cannizzaro = No $\alpha$-H + conc. Base $\rightarrow$ Alcohol (reduction) + Acid salt (oxidation).

Answer 1

Step 1: Concept
Disproportionation (self-oxidation-reduction) of specific aldehydes.

Step 2: Meaning
Only aldehydes lacking $\alpha$-hydrogens undergo this reaction.

Step 3: Analysis
When an aldehyde such as formaldehyde ($HCHO$) or benzaldehyde ($C_6H_5CHO$) is heated with concentrated aqueous or alcoholic alkali, one molecule is oxidized to the corresponding carboxylate salt while another molecule is simultaneously reduced to the corresponding primary alcohol.

• This reaction is known as the \[ \boxed{\text{Cannizzaro reaction}}. \]

• It is exhibited by aldehydes that do not possess any $\alpha$-hydrogen atoms.

• Typical examples include: \[ HCHO \quad\text{and}\quad C_6H_5CHO. \]

• The reaction is carried out using concentrated aqueous or alcoholic sodium hydroxide or potassium hydroxide.

• During the reaction, one molecule of aldehyde undergoes oxidation to form the salt of the corresponding carboxylic acid.

• Simultaneously, another molecule of aldehyde undergoes reduction to form the corresponding primary alcohol.

• For benzaldehyde, the reaction is: \[ 2C_6H_5CHO+NaOH \rightarrow C_6H_5COONa+C_6H_5CH_2OH. \]

• Since oxidation and reduction occur simultaneously between two molecules of the same compound, the Cannizzaro reaction is an example of a \[ \boxed{\text{disproportionation (self-redox) reaction}}. \]

• Thus, aldehydes lacking $\alpha$-hydrogen atoms undergo Cannizzaro reaction to produce an alcohol and a carboxylate salt.

Step 4: Conclusion
This yields two different functional groups from one starting reactant.

Final Answer:
$2 HCHO + \text{conc. } NaOH \xrightarrow{\Delta} CH_3OH \text{ (Methanol)} + HCOONa \text{ (Sodium formate)}$