Question

What is the name of the following reactions?
$HCHO\, + HCHO\xrightarrow[\Delta ]{{NaOH}}C{H_3}OH\, + HCOONa$
A.Clemmensen reaction
B.Cannizzaro reaction
C.None of the options
D.Hell-Volhard reaction

Answer 1

You need to be quite familiar with the name reactions in order to solve these questions.
Clemmensen reaction is the reduction of a carbonyl compound to a hydrocarbon or alkane with the use of zinc amalgam and concentrated hydrochloric acid. Cannizzaro reaction is a reaction of carbonyl compounds (on addition of a base) which can’t undergo aldol condensation reaction due to unavailability of alpha hydrogens. Hell-Volhard reaction or HVZ reaction is the conversion of carboxylic acids to alpha halogenated carboxylic acids.

Complete step-by-step answer: A general Clemmensen reduction reaction is shown below:
$RCHO\xrightarrow[{aq,HCl}]{{Zn/Hg}}RC{H_3}$
A general Cannizzaro reaction is shown below:
$2RCHO\xrightarrow{{NaOH}}RC{H_2}OH + RCOOH$
Where R can be phenyl etc. or any group not containing an alpha hydrogen.
This reaction is a redox reaction where one carbonyl group is oxidized to carboxylic acid while the other one is reduced to an alcohol.
The carbons adjacent to a carbonyl carbon are known as alpha carbons. And the hydrogens attached to these alpha carbons are known as alpha hydrogens.
One point to remember about the cannizzaro reaction is that this reaction is undergone by the carbonyl compounds lacking alpha hydrogens.
A general Hell-Volhard reaction is shown below:
$RC{H_2}COOH\xrightarrow[{{H_2}O}]{{B{r_2},red\,P}}RCH(Br)COOH$
In this reaction alpha carbon (carbon next to a carboxylic acid) gets halogenated on addition of red phosphorus and bromine gas.
Now, we can compare our given reaction with these reactions,
$HCHO\, + HCHO\xrightarrow[\Delta ]{{NaOH}}C{H_3}OH\, + HCOONa$
This matches with a Cannizzaro reaction.
Here, one molecule of formaldehyde gets oxidized and the other gets reduced. Formaldehyde is a carbonyl compound not containing an alpha hydrogen.

Hence, option B Cannizzaro reaction is the correct option.

Note: You must be quite careful while doing questions related to name reactions. Always practice them a lot. Read out aldol condensation, cross aldol and other reactions related to Cannizzaro reactions. Wolff-Kishner is another similar reaction to Clemmensen reduction reaction to convert carbonyl group to alkanes.