Question

Which of the following compounds will exhibit geometrical isomerism?
(A) 1-phenyl-2-butene
(B) 3-phenyl-1-butene
(C) 2-phenyl-1-butene
(D) 1, 1-diphenylpropene

Answer 1

We first need to understand the concept of isomerism. In chemistry, isomerism is the phenomenon where molecules have the same molecular formula, i.e. same number of atoms but their arrangements in space are different and these molecules are known as isomers.

Complete step by step answer:
We have to remember that the geometrical isomerism is also known as cis-trans isomerism in which cis indicates that the functional group in compound present in the same side whereas Trans isomerism that the functional group of the compound present in opposite side.
To know whether a molecule exhibits geometrical isomerism or not, the molecule must have
1.Restricted rotation involving a carbon-carbon double bond.
2.There should be two different compounds on the left hand side and right hand side.
Let us consider each of the given molecules one by one.
(A) Now we can draw the possible structures of $1 - phenyl - 2 - butene$ as,

Option A isTrans isomer ,Cis isomer
This compound has a carbon double bond, has 2 different groups on the left and right hand sides of the double bond. Hence this compound shows geometrical isomerism.
(B) Now we can draw the structure of $3 - phenyl - 1 - butene$ as,

This compound has a carbon double bond but there is no group present on the left hand side of the double bond. Hence this compound does not show geometrical isomerism. Therefore, the option B is incorrect.
(C) We can draw the structure of $2 - phenyl - 1 - butene$ as,

This compound has a carbon double bond but there are no groups present on the left and right hand side of the double bond. Hence this compound does not show geometrical isomerism. Therefore, the option C is incorrect.
(D) Now we can draw the structure of $1,1 - diphenylpropene$ as,

This compound has 2 cyclic rings but no double bond in its side chain. Hence this compound does not show geometrical isomerism. Therefore, the option D is incorrect.

So, the correct answer is Option A.

Note: It must be noted that to predict whether the molecule shows geometric isomerism or not, one must know the fundamentals to draw the structure from its IUPAC name. Drawing the structure helps us to visibly locate the double bonds and whether different groups are present on either side of the bond. It is not possible to predict the type of isomerism by its IUPAC name alone.