Question

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R Assertion A: Alkenes and alkynes undergo addition reaction readily. Reason R: Alkenes and alkynes undergo addition reaction because all the bonds present in them are strong \( \sigma \) (sigma) bonds. In the light of the above statements, choose the most appropriate answer from the options given below
• Both A and R are correct and R is the correct explanation of A
• Both A and R are correct but R is NOT the correct explanation of A
• A is correct but R is not correct
• A is not correct but R is correct

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Addition reactions occur mainly in:
• Alkenes
• Alkynes because their weak \( \pi \)-bonds break easily and allow new atoms to add across the multiple bond.

Answer 1

The Correct Option is C

Concept: Organic compounds containing double or triple bonds are called unsaturated compounds.
• Alkenes contain carbon-carbon double bonds.
• Alkynes contain carbon-carbon triple bonds. These multiple bonds contain:
• One \( \sigma \)-bond
• One or more \( \pi \)-bonds Important characteristics of \( \pi \)-bonds:
• \( \pi \)-bonds are weaker than \( \sigma \)-bonds.
• They are more exposed outside the internuclear axis.
• They break more easily during reactions. Because of weak \( \pi \)-bonds, alkenes and alkynes readily undergo addition reactions.

Step 1: Examine Assertion A carefully. Assertion A states: “Alkenes and alkynes undergo addition reaction readily.” This statement is correct. Let us understand why. Alkenes and alkynes contain multiple bonds: \[ \mathrm{C=C} \] or \[ \mathrm{C\equiv C} \] These bonds possess \( \pi \)-bonds in addition to \( \sigma \)-bonds. During addition reactions:
• The weaker \( \pi \)-bond breaks.
• New atoms or groups add across the multiple bond. For example: Hydrogenation of ethene: \[ \mathrm{CH_2=CH_2 + H_2 \rightarrow CH_3-CH_3} \] Here, the double bond opens and hydrogen atoms add across it. Therefore, alkenes and alkynes indeed undergo addition reactions readily. Hence, Assertion A is correct.

Step 2: Examine Reason R carefully. Reason R states: “Alkenes and alkynes undergo addition reaction because all the bonds present in them are strong \( \sigma \)-bonds.” This statement is incorrect. The major reason behind addition reactions is not the presence of strong \( \sigma \)-bonds. In reality:
• Alkenes and alkynes contain weak \( \pi \)-bonds.
• \( \pi \)-bonds are more reactive and break easily. A double bond contains: \[ 1\sigma + 1\pi \] A triple bond contains: \[ 1\sigma + 2\pi \] Thus, the reactivity is mainly due to: \[ \boxed{\text{Weak } \pi \text{-bonds}} \] not due to strong \( \sigma \)-bonds. Hence, Reason R is incorrect.

Step 3: Determine the correct relationship between A and R. We observed:
• Assertion A is true.
• Reason R is false. Therefore: \[ \boxed{ \text{A is correct but R is not correct} } \] Additional Explanation: Why are \( \pi \)-bonds weaker? \( \sigma \)-bonds form by head-on overlap of orbitals and are very strong. \( \pi \)-bonds form by sideways overlap, which is less effective. Therefore: \[ \sigma \text{-bond strength} > \pi \text{-bond strength} \] Hence, \( \pi \)-bonds break easily during addition reactions. Final Conclusion: Assertion A is correct, but Reason R is incorrect. Hence, the correct answer is: \[ \boxed{(3)} \]