Q.32 - Organic Chemistry

Question

$\mathbf{\text{∆H°}}$values obtained for a series of similar reactions are one set of experimental data used to determine the relative stability of alkenes. Explain how the following data suggest that cis-but-2-ene is more stable than but-1-ene (Section 12.3A).

Step-by-Step Solution

Answer

The less negative value of the heat of hydrogenation indicates that the alkene is stable as more amount of heat is released. Since cis-but-2-ene has less negative value than but-1-ene, it is more stable.

Heat of hydrogenation

The heat of hydrogenation is defined as the energy’s amount released by hydrogenating a $\mathbf{\text{π}}$ bond.

 

The more the heat amount released, the more is the stability of the alkene.

Identification of the double bond

The double-bonded carbons are identified first.

Representation of the double bond

Stability based on substitutions

But-1-ene is a monosubstituted alkene whereas cis but-2-ene is a disubstituted alkene. As the count of substitutions of alkyl groups increases the stability of the system due to more impact of +I effect is observed. 

Due to this stability more amount of heat is released during hydrogenation. The release of more amount of heat increases the stability of the system.

 

The less negative value of the heat of hydrogenation indicates that the alkene is stable as more amount of heat is released. Since cis but-2-ene has less negative value than but-1-ene, it is more stable.