Question

Arrange the following alkenes in order of decreasing stability.

1. 1-pentene 2. cis-2-pentene 3. trans-2-pentene 4. 2-methyl-2-butene

Answer 1

There are 3 factors that influence alkene stability:
1. Degree of substitution, i.e. how many alkyl groups are attached to the C=C ? More highly alkylated alkenes are more stable, so tetra > tri > di > mono-substituted.
2. Stereochemistry: trans > cis due to reduced steric interactions when R groups are on opposite sides of the double bond.
3. Conjugated alkenes are more stable than isolated alkenes.

From above data 2-methyl-2-butene is tri substituted alkene . So that will be more stable alkene.

cis-2-pentene and trans-2-pentene are di substituted alkenes. So these will be less stable than tri substituted alkene and in that trans isomer will be more stable than cis isomer.

Finally 1-pentene is mono substituted alkene from the above group and will be the least stable alkene.

So the decreasing order of the stabilty will be

2-methyl-2-butene > trans-2-pentene > cis-2-pentene > 1-pentene