JEE Main 2022 · 24 June · evening · Single correct · +4 / −1
Given below are two statements :
Statement I : The presence of weaker π-bonds make alkenes less stable than alkanes.
Statement II : The strength of the double bond is greater than that of carbon-carbon single bond.
In the light of the above statements, choose the correct answer from the options given below :
- (a) Both Statement I and Statement II are correct.
- (b) Both Statement I and Statement II are incorrect.
- (c) Statement I is correct but Statement II is incorrect.
- (d) Statement I is incorrect but Statement II is correct.
Answer and solution
Answer (a) Both Statement I and Statement II are correct.
Solution
Chemistry ma’am · 4 steps
Look at the two bonds inside C=C separately: the whole double bond is strong, but its π part is weak.
- Inside a double bond. A C=C bond is one σ bond plus one π bond. The σ bond comes from head-on overlap and is strong. The π bond comes from sideways overlap of p orbitals and is weaker.
Bond Bond enthalpy (approx.) σ part of C=C 397 kJ mol⁻¹ π part of C=C 284 kJ mol⁻¹, the weak link Whole C=C 397 + 284 = 681 kJ mol⁻¹ C–C single (ethane) 348 kJ mol⁻¹ - Statement I. The loosely held π electrons are easy for electrophiles to attack, and the π bond breaks easily. That is why alkenes react readily and are less stable than alkanes. Correct.
- Statement II. Taken as a whole, C=C needs 681 kJ mol⁻¹ to break, while C–C needs only 348 kJ mol⁻¹. So the double bond is stronger than the single bond. Correct.
- So. Both statements are true and they do not contradict each other, so the answer is (a).
The idea to keep. A double bond is stronger than a single bond overall, yet alkenes are more reactive. The reason is the weak π bond on top, which breaks first in electrophilic addition.
Answer from the NTA final answer key. Solution by PrepWiser.