Question

Which of the following is correct stability order of alkenes?

• A. Trans-2-butene $>$ Cis-2-Butene $>$ Isobutene $>$ but-1-ene
• B. Cis-2-Butene $>$ Trans-2-butene $>$ Isobutene $>$ but-1-ene
• C. Isobutene $>$ but-1-ene $>$ Cis-2-Butene $>$ Trans-2-butene
• D. Isobutene $>$ Trans-2-butene $>$ Cis-2-Butene $>$ but-1-ene

Hint:

In alkene stability questions, always rank based on: (1) degree of substitution, (2) trans $>$ cis (due to less steric hindrance), and (3) hyperconjugation effects.

Answer 1

The Correct Option is D

The stability of alkenes is influenced primarily by:
Hyperconjugation
Alkyl substitution (more substituted alkenes are more stable)
Steric hindrance (cis isomers are usually less stable than trans) Let’s analyze each alkene: 1. Isobutene (2-methylpropene): A highly substituted (trisubstituted) alkene. It benefits from both +I effect and extensive hyperconjugation.
⇒ Highest stability 2. Trans-2-butene: Disubstituted alkene with groups on opposite sides — less steric hindrance than cis. More stable than cis-2-butene. 3. Cis-2-butene: Also disubstituted but with steric repulsion due to same-side substituents. Less stable than trans. 4. But-1-ene: Monosubstituted alkene, minimal hyperconjugation and substitution.
⇒ Least stable among the given Correct Stability Order: \[ \text{Isobutene} > \text{Trans-2-butene} > \text{Cis-2-butene} > \text{But-1-ene} \]

Answer 2

The question asks for the correct order of stability among isobutene, trans-2-butene, cis-2-butene, and but-1-ene. Alkene stability rises with more alkyl substitution on the double bond carbons through hyperconjugation, and among similarly substituted alkenes, the trans form beats the cis form because the cis form has more steric clash between the two alkyl groups sitting on the same side. Using these two rules, each answer option can be checked directly.

1. Trans-2-butene > Cis-2-Butene > Isobutene > but-1-ene: This places isobutene, the most substituted (trisubstituted) alkene of the four, below two disubstituted alkenes. Since more substitution always adds more stability through hyperconjugation, isobutene cannot rank below trans or cis-2-butene, so this order is wrong.
2. Cis-2-Butene > Trans-2-butene > Isobutene > but-1-ene: This has the same problem of placing isobutene too low, and it also puts cis-2-butene above trans-2-butene, which goes against the rule that trans forms are more stable than cis forms of the same substitution level because of lower steric strain. Both parts of this order are incorrect.
3. Isobutene > but-1-ene > Cis-2-Butene > Trans-2-butene: This correctly puts isobutene first, but then ranks but-1-ene, a monosubstituted alkene, above both disubstituted 2-butenes. A monosubstituted alkene has less hyperconjugation than a disubstituted one, so but-1-ene should rank below cis and trans-2-butene, not above them.
4. Isobutene > Trans-2-butene > Cis-2-Butene > but-1-ene: Isobutene is trisubstituted and most stable. Trans-2-butene and cis-2-butene are both disubstituted, but trans has the two methyl groups on opposite sides with less steric strain, so it beats cis. But-1-ene is only monosubstituted, giving it the least hyperconjugation and placing it last. This order respects both the substitution rule and the trans-over-cis rule.

Checking every option against the substitution and steric strain rules leaves only one order that is consistent all the way through.

So the correct answer is Isobutene > Trans-2-butene > Cis-2-Butene > but-1-ene.