Question

How many geometrical isomers are possible for the given compound?
$C{H_3} - CH = CH - CH = CH - {C_2}{H_5}$
A.Four
B.Three
C.Two
D.Five

Answer 1

Hint:To answer this question, you must be familiar with the types of isomerism shown by organic compounds. Organic compounds show broadly two types of isomerism, namely structural or constitutional isomers or stereoisomers.

Complete answer:
Stereoisomerism can be further divided into configurational isomerism, conformational isomerism and geometrical isomerism. Conformational isomers are interconvertible forms of the same molecule derived from the rotation about the carbon- carbon $\sigma $ bond. Configurational isomers are those stereoisomers which cannot be converted to each other by rotation about carbon- carbon $\sigma $ bond.
Geometrical isomers are non- interconvertible stereoisomers due to the restricted rotation about carbon- carbon $\pi $ bond. Since free rotation around the carbon- carbon pi bond is restricted, the molecule exists in two non- interconvertible and different geometries.
The geometrical isomer in which the similar groups are present in the same direction or on the same side of the double bond is known as the cis isomer. The geometrical isomer, in which the similar groups are present on the opposite sides of the double bond is known as the trans isomer. Hence, geometrical isomerism is also commonly known as cis- trans isomerism.
In the given question, the given compound contains two double bonds and the groups at each end are different, i.e., methyl and ethyl groups. Therefore, the total number of geometrical isomers for each double bond will be two. Hence there are 4 geometrical isomers for the given compound.

Thus, the correct answer is A.
Note:
Among the two forms, cis form is less stable due to the mutual repulsion between the two similar groups on the same side. The trans form on the other hand has a less dipole moment or even is non- polar. Being polar, cis isomer has a higher boiling point and also a higher heat of hydrogenation, heat of combustion, refractive index and density.