Question

Assertion: Cis- form of alkene is found to be more polar then the trans- form.
Reason : Since the groups are in opposite directions in the trans-form, the dipole moments of bond cancel each other making the trans-form almost non-polar.
(A)Both Assertion and Reason are correct and reason is the correct explanation for Assertion
(B) Both Assertion and Reason are correct and reason is the correct but Reason is not the correct explanation for Assertion
(C) Assertion is correct but Reason is incorrect
(D)Both Assertion and Reason are incorrect

Answer 1

Hint: Dipole moment is a vector quantity, and the dipole moment of a polar molecule depends on its geometry. A symmetrical molecule or having a regular geometry is non- polar. Only the molecule which has an asymmetrical shape has a dipole moment.

Complete answer:
Due to the restricted rotation about the double bond the alkene shows geometrical isomerism because of the relative position of atoms or groups attached to the carbon atom of double bond get fixed. In cis- form two similar groups are on the same side while in trans -form two similar groups or atoms are on opposite side.
In cis isomer two polar groups attached to the same side of a restricted system and form an unsymmetrical molecule. Each bond have dipole movement and both the bond produce a resultant dipole moment in one direction, so cis form of alkene are polar. However, trans form both the polar groups attached in opposite directions to each other in a restricted system and make molecules symmetrical. The dipole moment of each molecule cancels out to each other and produces zero dipole moment or makes alkene non-polar.

Note – bond polarity is determined by electronegativity of an atom while polarity of a molecule is determined by dipole moment. Cis- form of alkenes are more soluble then trans-form of alkene.