An alkene is a hydrocarbon that contains at least one carbon-carbon double bond. These molecules exhibit a consistent pattern of stability based on their structure: alkenes with a greater number of non-hydrogen groups, known as substituents, attached to the double-bond carbons are inherently more stable than those with fewer. This difference in stability is measurable and is a consequence of specific electronic and structural factors.
Measuring Alkene Stability
The scientific method for quantifying the stability difference between various alkenes is based on hydrogenation. Hydrogenation is an exothermic reaction where hydrogen gas is added across the double bond, converting the alkene into a saturated alkane, typically in the presence of a metal catalyst. The energy released during this conversion is known as the heat of hydrogenation (\(\Delta H_{hyd}\)).
This heat of hydrogenation serves as a precise, quantitative metric for the relative stability of the starting alkene. Since the product, the alkane, is the same for a set of alkene isomers, any differences in the heat released must reflect differences in the energy of the starting material. A more stable alkene possesses less internal energy, meaning less heat is released when it is converted to the lower-energy alkane product. Therefore, a lower, or less negative, heat of hydrogenation value corresponds directly to a more stable alkene.
Classifying Alkene Substitution
Alkene substitution refers to the number of non-hydrogen groups, or alkyl groups (designated as R groups), that are directly bonded to the two carbons of the double bond. The stability trend is classified based on counting these alkyl substituents, moving from unsubstituted (ethene) to a maximum of four substituents.
The stability generally follows the order: tetrasubstituted (four R groups) \(>\) trisubstituted (three R groups) \(>\) disubstituted (two R groups) \(>\) monosubstituted (one R group). For disubstituted alkenes, the spatial arrangement of the substituents also plays a role in overall stability. Trans isomers, where the two larger groups are on opposite sides of the double bond, are more stable than cis isomers. This difference arises because the cis arrangement forces the bulky alkyl groups closer together, creating steric hindrance, which slightly destabilizes the molecule.
Stabilization Through Hyperconjugation
The primary chemical reason for the increased stability of more substituted alkenes is a phenomenon called hyperconjugation. This effect involves the stabilizing interaction between the filled sigma (\(\sigma\)) bonding orbitals of the adjacent carbon-hydrogen (C-H) bonds on the alkyl substituents and the empty, antibonding pi (\(\pi^\)) orbital of the double bond. Hyperconjugation effectively delocalizes the electron density across the molecule.
The interaction involves the electrons in the \(\sigma\) bonds of the substituent groups slightly shifting into the nearby \(\pi^\) orbital of the double bond. This delocalization lowers the overall energy of the system, making the molecule more stable. More alkyl substituents mean a greater number of adjacent C-H bonds are available to participate in this electron-donating interaction. As the number of these participating bonds increases, the extent of electron delocalization increases, leading directly to greater thermodynamic stability. The more alpha-hydrogens (hydrogens on the carbon atom directly next to the double bond) an alkene possesses, the more opportunities there are for this stabilizing interaction to occur.
The Influence of Orbital Hybridization
A secondary factor contributing to alkene stability relates to the type of carbon-carbon bonds formed as substitution increases. The carbons involved in the double bond are \(sp^2\)-hybridized, while the carbons in the attached alkyl groups are typically \(sp^3\)-hybridized. Bonds formed between an \(sp^2\) carbon and an \(sp^3\) carbon are slightly shorter and stronger than bonds formed solely between two \(sp^3\) carbons.
This increase in bond strength is due to the concept of s-character. An \(sp^2\) orbital has 33% s-character, while an \(sp^3\) orbital has only 25% s-character. Orbitals with a higher s-character are held closer to the nucleus, making them more electronegative and allowing them to form stronger, more efficient overlaps.
The substitution of hydrogen atoms on the double bond with alkyl groups replaces weaker carbon-hydrogen bonds with stronger \(sp^2-sp^3\) carbon-carbon bonds. As the number of alkyl substituents increases, the number of these stronger \(sp^2-sp^3\) bonds also increases. This accumulation of stronger bonds contributes favorably to the molecule’s overall thermodynamic stability.