Question

Choose the incorrect option assume that all products are major products. (A) \(\mathrm{HCHO} \frac{\text { conc. }}{\mathrm{NaOH}} \mathrm{HCOONa}+\mathrm{CH}_{3} \mathrm{OH}\) (B) \(\mathrm{Ph}-\mathrm{CHO}+\mathrm{HCHO} \frac{\mathrm{conc} .}{\mathrm{NaOH}}{\longrightarrow} \mathrm{PhCOONa}+\mathrm{CH}_{3} \mathrm{OH}\) (C) Cannizaro reaction is disproportionation reaction (D) \(\mathrm{PhCHO}+\mathrm{PhCOCH}_{3} \frac{\stackrel{\ominus}{\mathrm{O}} \mathrm{H}}{293 \mathrm{~K}}\) Benzalacetophenone

Short answer

The incorrect option is (D) because it describes a reaction between PhCHO and PhCOCH3 with OH- at 293 K, producing Benzalacetophenone. This reaction is not a Cannizzaro reaction, as it involves a mild base and does not represent a major product of the disproportionation reaction.

Step-by-step solution

Analyze reactions of HCHO with concentrated NaOH

In option (A), the reactants are HCHO and concentrated NaOH. This reaction is the Cannizzaro reaction, where an aldehyde gets oxidized and reduced, forming an alcohol and a salt of the carboxylic acid. In this case, the products are HCOONa and CH3OH. As it is a major product in this case, option (A) is a correct statement.

Analyze reactions of Ph-CHO and HCHO with concentrated NaOH

In option (B), reactants are Ph-CHO and HCHO with concentrated NaOH. This reaction is a crossed Cannizzaro reaction where the reagents are two different aldehydes, one with no alpha-hydrogen and the other with alpha-hydrogen (in this case, HCHO has alpha-hydrogen). The product formed is PhCOONa and CH3OH. So, option (B) is also a correct statement.

Analyze Cannizzaro reaction type

Option (C) states that Cannizzaro reaction is a disproportionation reaction, which is indeed true. A disproportionation reaction is one where a substrate gets both oxidized and reduced, and the Cannizzaro reaction follows this pattern. Thus, option (C) is also correct.

Analyze reaction of PhCHO and PhCOCH3 with OH- at 293 K

In option (D), the reactants are PhCHO and PhCOCH3 with OH- at 293 K. The reaction occurs between PhCHO (benzaldehyde) and PhCOCH3 (acetophenone). This reaction is not a Cannizzaro reaction because it is occurring in a mild base and does not involve the oxidation and reduction of an aldehyde. Therefore, option (D) is the incorrect statement as it does not represent a major product of the Cannizzaro reaction.

Key concepts

Disproportionation Reaction

A disproportionation reaction is a fascinating chemical process where a single substance is simultaneously oxidized and reduced. This unique property allows one compound to form two different products.
In the context of the Cannizzaro reaction, we see a classic example of disproportionation. An aldehyde, lacking an alpha-hydrogen, undergoes both oxidation and reduction in the presence of a strong base such as concentrated NaOH.
Here's how it works:

• One molecule of the aldehyde is reduced to form alcohol.
• Another molecule is oxidized to become a carboxylic acid or its salt form.

This dual transformation makes disproportionation reactions important in synthetic chemistry, as they facilitate the transformation of aldehydes into useful products without the need for additional oxidizing or reducing agents.

Aldehyde Oxidation

Aldehyde oxidation is a crucial concept within the broader context of organic reactions. It involves the transformation of the aldehyde functional group into a carboxylic acid by addition of an oxygen atom.
In the Cannizzaro reaction, one of the aldehyde molecules is oxidized. The oxygen in the reaction mixture turns it into a carboxylate ion, typically forming a salt of the carboxylic acid due to the basic reaction conditions.
Key points to remember about aldehyde oxidation in the Cannizzaro reaction include:

• It occurs under alkaline conditions, where no alpha-hydrogen is available.
• The basic environment facilitates the transfer of a hydride to another aldehyde, promoting the reduction step.

Overall, understanding how aldehyde oxidation functions within this reaction helps in grasping the critical roles of different reactants and conditions needed to successfully accomplish the Cannizzaro reaction.

Crossed Cannizzaro Reaction

The crossed Cannizzaro reaction is a variation of the standard Cannizzaro reaction. In this reaction, two different aldehydes are used as reactants.
One of these aldehydes lacks alpha-hydrogens and undergoes disproportionation. A common example includes using formaldehyde and benzaldehyde.
In this crossed version:

• The formaldehyde gets reduced, forming methanol.
• The benzaldehyde gets oxidized, forming benzoate.

A unique factor of this variation is the selective reactivity, where formaldehyde acts as both a reducing agent and the source of methanol, while the main aldehyde substrate provides the carboxylic acid salt.
This method is useful for selectively obtaining distinct products from mixed aldehydes using mild bases. It underscores the flexibility and utility of Cannizzaro chemistry in selective synthesis processes.