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In which case the product is a mixture of geometrical isomers :
A. $PhCHO\xrightarrow{{HCN/NaOH}}$
B. $PhCHO\xrightarrow{{AgN{O_3}/N{H_4}OH}}$
C. $PhCHO\xrightarrow{{A{c_2}O/AcONa}}$
D. $PhCHO\xrightarrow{{HCHO/O{H^ - }}}$

Last updated date: 19th Jun 2024
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Hint: Geometrical isomerism is a type of stereoisomerism . Isomers which have the same structural formula but have different relative arrangement of atoms or groups in space are called geometrical isomers .

Complete step by step answer:
Let us first understand what are products are formed in each of the following reactions :
$PhCHO\xrightarrow{{HCN/NaOH}}Ph - CH(OH)(CN)$
In this reaction two isomeric products are formed ( nitrile and isonitrile ).
$PhCHO\xrightarrow{{AgN{O_3}/N{H_4}OH}}PhCO{O^ - }$
The above reaction is an example of the Tollens test . Carboxylic acid of equal carbon atoms is obtained in this reaction .
$PhCHO\xrightarrow{{A{c_2}O/AcONa}}Ph - CH = CH - COOH$
This reaction is known as perkin's reaction . The product formed can havecis and trans
isomers around the double bond and hence shows geometrical isomerism. Due to
$\Pi - bonding$ between the two carbon atoms , the rotation around carbon-carbon
double bond is prohibited and hence the geometry of the atoms or groups attached to the
carbon atoms get fixed in space . This is the reason why this type of stereoisomerism is
known as geometrical isomerism .
$PhCHO\xrightarrow{{HCHO/O{H^ - }}}PhC{H_2}OH + HCOONa$
The products formed in this reaction do not show geometrical isomerism .
Hence geometrical isomerism is only shown when benzaldehyde reacts with acetic anhydride in the presence of sodium acetate .

So, the correct answer is Option C .

Note : All compounds containing carbon-carbon double bonds dont show geometrical isomerism. There are certain conditions which should be satisfied by a compound to show geometrical isomerism , they are :
The molecule must contain a double bond between carbon atoms
Each of the carbon atoms of the double bond must have different substituents which may be the same or different .