Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Cis form of alkene is found to be more polar than the trans form
Reason (R): Dipole moment of trans isomer of 2-butene is zero. In the light of the above statements.
Choose the correct answer from the options given below :

• A. Both (A) and (R) are true but (R) is NOT the correct explanation of (A)
• B. (A) is true but (R) is false
• C. Both (A) and (R) are true and (R) is the correct explanation of (A)
• D. (A) is false but (R) is true

Answer 1

The Correct Option is C

 

The dipole moment is a vector quantity. In the trans isomer of 2-butene, the dipoles of substituents are equal in magnitude but opposite in direction, resulting in a net dipole moment of zero (\(\mu = 0\)). However, in the cis isomer, the dipoles do not cancel out, making the molecule polar with \(\mu > 0\). Thus, the dipole moment of the cis form is greater than the trans form.

Cis isomer: \(\mu > 0\),   Trans isomer: \(\mu = 0\).

Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Answer 2

Step 1: Recall the concept of polarity and dipole moment in alkenes
The polarity of a molecule depends on the vector sum of its individual bond dipoles. In alkenes with two substituents on the same or opposite sides of the double bond, the arrangement of these groups determines whether the dipole moments reinforce or cancel each other.

Step 2: Analyze the Assertion (A)
Assertion (A): “Cis form of alkene is found to be more polar than the trans form.” ✔ This is true because, in the cis isomer, both similar substituents are on the same side of the double bond, and their bond dipoles add up, producing a net dipole moment. Hence, the molecule is polar.

Step 3: Analyze the Reason (R)
Reason (R): “Dipole moment of trans isomer of 2-butene is zero.” ✔ This is also true. In trans-2-butene, the two methyl groups are on opposite sides of the double bond, and their bond dipoles cancel each other out due to symmetry. As a result, the molecule has no net dipole moment (μ = 0).

Step 4: Connect the reason with the assertion
Because the cis form has bond dipoles that reinforce each other and the trans form has dipoles that cancel, the cis isomer is more polar than the trans isomer. Thus, the Reason correctly explains the Assertion.

Final answer

Both (A) and (R) are true and (R) is the correct explanation of (A)