No, ethyne is less stable than ethene.
While ethyne contains a stronger carbon-carbon triple bond compared to ethene's double bond, its overall molecular energy content is higher, making it inherently less stable. This lower stability also directly correlates with its higher reactivity.
Understanding Hydrocarbon Stability
The stability of hydrocarbons, especially unsaturated ones like ethene and ethyne, can be assessed by their heat of hydrogenation. This measurement indicates the amount of energy released when a molecule is fully hydrogenated to its saturated form (e.g., ethane). A higher heat of hydrogenation suggests a higher energy content in the initial molecule, thus indicating lower stability.
Why Ethyne is Less Stable Than Ethene
Several factors contribute to ethyne's lower stability and higher reactivity:
- Hybridization and Bond Energy:
- Ethyne (C₂H₂): Each carbon atom is sp hybridized, forming a carbon-carbon triple bond (one sigma, two pi bonds). This arrangement results in a linear geometry with bond angles of 180°.
- Ethene (C₂H₄): Each carbon atom is sp² hybridized, forming a carbon-carbon double bond (one sigma, one pi bond). This leads to a trigonal planar geometry with bond angles around 120°.
- Despite the C≡C triple bond being stronger in terms of bond dissociation energy than a C=C double bond, the presence of two highly reactive pi bonds in ethyne makes it more susceptible to addition reactions.
- Electron Density and Reactivity: The pi bonds, particularly in the triple bond, contain electron density that is more exposed and accessible for electrophilic attack. This higher electron density per carbon-carbon bond in ethyne contributes to its increased reactivity compared to ethene.
- Molecular Strain: While not as pronounced as in cyclic compounds, the compact nature of the triple bond, especially when considering the overall energy landscape of the molecule, contributes to its higher potential energy.
Ultimately, the higher energy state of ethyne means it readily undergoes reactions to achieve a more stable, lower-energy state. For example, ethyne (acetylene) is extremely reactive and requires careful handling, often stored dissolved in acetone, due to its propensity for decomposition.
Key Differences Between Ethene and Ethyne
To summarize the distinctions that influence their relative stabilities:
| Feature | Ethene (C₂H₄) | Ethyne (C₂H₂) |
|---|---|---|
| Bond Type | Carbon-carbon double bond (C=C) | Carbon-carbon triple bond (C≡C) |
| Hybridization | sp² hybridization | sp hybridization |
| Geometry | Trigonal Planar | Linear |
| Bond Angle | ~120° | 180° |
| Pi Bonds | One pi (π) bond | Two pi (π) bonds |
| Relative Stability | More stable (lower energy content) | Less stable (higher energy content) |
| Relative Reactivity | Less reactive | More reactive |
| s-Character | 33% s-character | 50% s-character |
Practical Implications
The difference in stability and reactivity is crucial in industrial applications and organic synthesis:
- Ethyne (Acetylene): Its high reactivity makes it valuable for specialized welding and cutting applications (oxy-acetylene torches) due to the intense heat produced during its combustion. In organic synthesis, it's a versatile building block for creating more complex molecules because its triple bond can be selectively modified.
- Ethene (Ethylene): Less reactive than ethyne, ethene is still a fundamental building block in the petrochemical industry. It's primarily used for polymerization to produce polyethylene, a widely used plastic, and as a precursor for ethanol, ethylene oxide, and other organic compounds.
By understanding the unique structural and electronic properties of ethene and ethyne, we can appreciate why ethyne, despite its stronger individual bond, possesses a higher inherent energy and is therefore less stable than ethene.
