Question

In crossed cannizzaro reactions between formaldehyde and Benzaldehyde the alcohol formed is only benzyl alcohol and methanol. Give reason.

Answer 1

Hint: We know that if an aldehyde without any $\alpha$ -hydrogen is made to react with formaldehyde in presence of a base to give sodium formate and an alcohol, it is called as Cross Cannizzaro reaction.

Complete step by step answer:
Aldehydes with alpha hydrogen atoms undergo deprotonation due to the strongly alkaline conditions of the reaction, resulting in enolates and/or aldol reactions of those enolates where beta-hydroxy aldehydes or ketones are obtained. Therefore, it's not surprising that the reaction produces only $$50\mathrm{\%}$$ of the specified alcohol and acid at ideal conditions. This is often why the crossed Cannizzaro reaction is more commonly used. A sacrificial aldehyde is combined with a more valuable chemical and formaldehyde is employed as a reductant, oxidizing it to sodium formate. The specified alcohol is obtained from the reduction of the opposite aldehyde chemical. Since two different aldehydes are often completely converted into the specified product, the yield of the precious chemical is increased.
We can write the chemical equation for the Crossed Cannizzaro reaction between formaldehyde and Benzaldehyde as,
$$C_6{H}_5-CHO+\left(Formaldehyde\right)HCHO\stackrel{NaOH}{\to }HCOOH+C_6{H}_5-C{H}_2-OH$$
As there is no electron donating group on formaldehyde, the initial nucleophilic addition of hydroxide ion is faster on formaldehyde due to which formaldehyde oxidizes easily and forms formic acid. Thus, the alcohol formed is only benzyl alcohol and not methanol.

Note:
We must remember that the Cannizzaro reaction may be a reaction named after Stanislao Cannizzaro that involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to yield an acid and a primary alcohol.
Now we can discuss about the example of Cannizzaro reaction as,
When two molecules of benzaldehyde are treated with $NaOH$ to give benzyl alcohol and benzoic acid.
We can write the chemical equation for the above reaction as,