Question

Which of the following compounds can exhibit geometrical isomerism?
A. 2-pentyne
B. 2-pentene
C. 2-methyl propene
D. 2-methyl-2-butene

Answer 1

Hint: We know that the compounds that have the same number of and same type of atoms but differ from each other in their spatial arrangement of atoms are the geometrical isomers of each other. To solve this we have to draw the structures of all the given compounds.

Complete answer:
We know that the compounds that have the same number of and same type of atoms but differ from each other in their spatial arrangement of atoms are the geometrical isomers of each other.

The conditions for geometrical isomers are as follows:
1. The molecule of the given compound must have a double bond.
2. Different atoms or groups of atoms must be attached on each side of the carbon-carbon double bond.
3. Based on the arrangement of atoms or group of atoms, the geometrical isomers are classified as cis-isomer and trans-isomer.
4. In the cis-isomer, the two similar atoms or group of atoms are on the same side of the carbon-carbon double bond.
5. In the trans-isomer, the two similar atoms or groups of atoms are on the opposite side of the carbon-carbon double bond.
We are given four compounds 2-pentyne, 2-pentene, 2-methyl propene and 2-methyl-2-butene. Now, we will draw the structures of all the four compounds.
The structures of 2-pentyne, 2-pentene, 2-methyl propene and 2-methyl-2-butene are as follows:

From the structures, we can see that 2-pentene has different atoms or groups of atoms attached on each side of the double bond. Thus, 2-pentene will exhibit cis-isomer and trans-isomer. The structures of cis-2-pentene and trans-2-pentene are as follows:

Thus, the compound that can exhibit geometrical isomerism is 2-pentene.

Thus, the correct option is (B) 2-pentene.

Note:
When the same atoms or group of atoms are present on the same carbon atom of the double bond then the molecule does not show geometrical isomerism. When cis-isomer is converted to trans-isomer, the double bond which is planar is rotated by ${180^ \circ }$.