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

Step 1: Count the alkyl substituents on each double bond.
Isobutene, or 2-methylpropene, has two methyl groups on one of its double bond carbons, making it a trisubstituted alkene.
Cis-2-butene and trans-2-butene each have one methyl group on each double bond carbon, making them disubstituted alkenes.
But-1-ene has only one alkyl group across the whole double bond, making it monosubstituted.

Step 2: Apply the substitution rule for stability.
More alkyl groups on the double bond carbons means more hyperconjugation, since more carbon-hydrogen bonds on the neighboring carbons can donate electron density into the pi system, and this lowers the energy of the alkene and raises its stability.
By this rule alone, isobutene ranks above both 2-butenes, and both 2-butenes rank above but-1-ene.

Step 3: Break the tie between cis and trans-2-butene.
Cis-2-butene and trans-2-butene have the same substitution level, so hyperconjugation alone cannot separate them. In the cis form, both methyl groups sit on the same side of the double bond and crowd into each other, adding steric strain, while in the trans form the methyl groups sit on opposite sides with no such crowding.
This extra strain makes cis-2-butene less stable than trans-2-butene.

Step 4: Put the full order together.
Combining the substitution ranking with the cis and trans comparison gives isobutene as the most stable, followed by trans-2-butene, then cis-2-butene, and but-1-ene as the least stable.
\[ \boxed{\text{Isobutene} > \text{Trans-2-butene} > \text{Cis-2-butene} > \text{But-1-ene}} \]