An alkene is a hydrocarbon molecule that contains at least one carbon-carbon double bond. In chemistry, stability refers to a molecule’s relative energy state: a more stable molecule exists at a lower energy level and is therefore less reactive. This concept is similar to a physical object, like a ball, being more stable when it rests lower on a hill. The energy difference between various alkene structures determines their stability, which in turn influences their chemical behavior.
Measuring Relative Stability
Chemists quantify the relative stability of different alkenes by measuring the heat released during hydrogenation. This is an exothermic reaction where a hydrogen molecule (\(\text{H}_2\)) is added across the double bond, converting the alkene into a single-bonded alkane. The heat released is known as the heat of hydrogenation (\(\Delta H_\text{hydrog}\)).
Since all alkene isomers that produce the same alkane product end up at the same final energy state, the amount of heat released is directly related to the starting energy of the alkene. A less stable alkene starts at a higher energy level and must release more heat. Conversely, a more stable alkene starts at a lower energy level and releases less heat upon hydrogenation. Therefore, a lower heat of hydrogenation corresponds to greater alkene stability.
The Rule of Alkene Substitution
The most significant factor determining an alkene’s stability is the number of non-hydrogen groups, known as substituents, attached directly to the double bond carbons. Stability increases proportionally to the number of substituents present. The stability trend is classified based on how many of the four possible positions on the double-bonded carbons are occupied by non-hydrogen atoms.
An unsubstituted alkene, such as ethene, has only hydrogen atoms attached, making it the least stable. Stability increases through monosubstituted (one alkyl group), disubstituted (two alkyl groups), and trisubstituted (three alkyl groups) alkenes. The most stable arrangement is a tetrasubstituted alkene, where all four positions are occupied by alkyl groups.
This hierarchy establishes a clear rule: Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted > Unsubstituted. For example, 2,3-dimethyl-2-butene (tetrasubstituted) is significantly more stable than 1-butene (monosubstituted).
Stability Differences in Geometric Isomers
The overall substitution count is not the only factor, as stability differences also exist within substituted classes depending on the spatial arrangement of the groups. Alkenes can exist as geometric isomers, known as cis or trans forms. The cis isomer has the two larger substituents positioned on the same side of the double bond, while the trans isomer has them on opposite sides.
The trans isomer is generally more stable than its cis counterpart. This difference is primarily due to steric hindrance, which is the repulsion that occurs when bulky groups are forced too close together in space. In a cis alkene, the larger substituents crowd each other on the same side, creating repulsive forces that raise the molecule’s internal energy. By contrast, the trans configuration allows the substituents to be farther apart, minimizing this steric strain and resulting in a lower-energy, more stable molecule.
The Electronic Reason for Increased Stability
The underlying electronic reason for the stability conferred by alkyl substitution is a stabilizing interaction called hyperconjugation. This effect involves the electron-sharing interaction between the sigma (\(\sigma\)) bonds of the adjacent alkyl groups and the pi (\(\pi\)) system of the double bond. Specifically, the electrons in the \(\text{C}-\text{H}\) or \(\text{C}-\text{C}\) \(\sigma\) bonds of the alkyl substituents overlap with the vacant or partially filled antibonding \(\pi^\) orbital of the double bond.
This overlap allows for a slight delocalization of electron density from the alkyl groups onto the double bond system. This spreading out of electron density lowers the overall energy of the molecule, effectively strengthening the double bond and increasing its stability. Since each alkyl group provides multiple \(\sigma\) bonds that can participate in this interaction, a greater number of substituents leads to more extensive hyperconjugation.
The result is that tetrasubstituted alkenes, having the maximum number of adjacent \(\sigma\) bonds, experience the greatest degree of electron delocalization and are therefore the most stable. Stability in alkenes is thus a combination of two factors: electronic stabilization through hyperconjugation, which favors more substituted alkenes, and spatial factors like steric hindrance, which favors trans geometric isomers over cis.