The enthalpy of hydrogenation of butane is 125.8 kJ/mol but that of but-2-ene is 120kJ/mol. Why?

To understand why the enthalpy of hydrogenation of butane is higher than that of but-2-ene, we need to delve into the concepts of molecular structure and stability. The enthalpy of hydrogenation refers to the heat released when an alkene or alkane reacts with hydrogen to form a saturated compound. In this case, we are comparing butane, which is a saturated hydrocarbon, with but-2-ene, an unsaturated hydrocarbon.

Understanding Molecular Stability

The key to this difference lies in the stability of the molecules involved. Butane (C4H10) is a fully saturated alkane, meaning it has single bonds only, while but-2-ene (C4H8) contains a double bond between two carbon atoms. The presence of this double bond in but-2-ene makes it less stable compared to butane.

Energy Considerations

When we talk about enthalpy changes, we are essentially discussing energy. The hydrogenation process involves breaking the double bond in but-2-ene and forming new C-H bonds. The energy required to break the double bond is less than the energy released when new C-H bonds are formed. This results in a lower enthalpy change for but-2-ene compared to butane.

• Butane: Fully saturated, no double bonds, higher enthalpy of hydrogenation (125.8 kJ/mol).
• But-2-ene: Contains a double bond, more stable due to resonance, lower enthalpy of hydrogenation (120 kJ/mol).

Resonance and Stability

But-2-ene can also exhibit some degree of resonance stabilization due to the presence of the double bond. This resonance allows for some distribution of electron density, which contributes to its stability. In contrast, butane does not have this feature, making it less stable and requiring more energy to hydrogenate.

Comparative Analysis

To summarize, the difference in enthalpy of hydrogenation between butane and but-2-ene can be attributed to the following factors:

• The presence of a double bond in but-2-ene makes it more stable than butane.
• Breaking the double bond in but-2-ene requires less energy compared to the energy released when forming new bonds during hydrogenation.
• Butane, being fully saturated, has a higher enthalpy of hydrogenation due to its lack of stability compared to the alkene.

In essence, the stability of but-2-ene due to its double bond leads to a lower enthalpy of hydrogenation compared to butane, which is fully saturated and less stable. This concept is crucial in organic chemistry, as it helps us understand the energy dynamics involved in reactions of hydrocarbons.