7.5
Relative Stabilities of Alkenes
The relative stability of alkenes can be determined by comparing their heats of hydrogenation. The lower heat of hydrogenation indicates the more stab…
The energy released when an alkene reacts with hydrogen is called the heat of hydrogenation.
The relative stability of alkenes is determined by comparing their heats of hydrogenation. The smaller the heat of hydrogenation, the higher the stability of the alkene.
Increasing the substitution across the double bond is found to decrease the heat of hydrogenation, revealing that the tetrasubstituted alkene is more stable than the tri-, di-, and monosubstituted alkenes.
The reason is that the more-substituted alkenes have a higher ratio of sp2–sp3 bonds than sp3–sp3 bonds. The sp2–sp3 bond is lower in energy and stronger than the sp3–sp3 bond since it has a higher contribution of s electrons, which are lower in energy.
Another factor responsible for the stability of substituted alkenes is the stabilizing interaction, called hyperconjugation, between the delocalized electron density of the pi bond and the carbon–hydrogen sigma bonds on the substituents.
Alkyl substituents provide additional hyperconjugation and lead to more stability. Therefore, 2,3-dimethyl-2-butene, with four substituents across the double bond, is more stable than propene, which has just one substituent.
Further, the stereochemistry of substituents around the double bonds also contributes to the stability of alkenes. trans alkenes typically have a lower heat of hydrogenation and are more stable than the cis isomers.
The reason lies in the steric strain produced by the interaction between the substituents present on the same side of the double bond in the cis isomer.
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Q1: How is the relative stability of alkenes determined?
Relative alkene stability is determined by comparing their heats of hydrogenation, which is the energy released when an alkene reacts with hydrogen. The smaller the heat of hydrogenation, the higher the stability of the alkene. This measurement allows chemists to rank alkenes by their thermodynamic stability and predict their reactivity in introduction to electrophilic addition reactions of alkenes.
Q2: Why are tetrasubstituted alkenes more stable than monosubstituted ones?
Tetrasubstituted alkenes are more stable because they have a higher ratio of sp2–sp3 bonds compared to sp3–sp3 bonds. The sp2–sp3 bond is lower in energy and stronger due to higher s-electron contribution. Additionally, more alkyl substituents provide greater hyperconjugation, a stabilizing interaction between the delocalized pi-bond electron density and adjacent carbon–hydrogen sigma bonds on substituents.
Q3: What role does hyperconjugation play in alkene stability?
Hyperconjugation is a stabilizing interaction between the delocalized electron density of the pi bond and carbon–hydrogen sigma bonds on alkyl substituents. Alkenes with more alkyl substituents experience greater hyperconjugation, leading to increased stability. For example, 2,3-dimethyl-2-butene with four substituents is more stable than propene with just one substituent due to enhanced hyperconjugation effects.
Q4: How does stereochemistry affect the stability of alkene isomers?
Trans alkenes are more stable than their cis isomers because cis isomers experience steric strain from substituent crowding on the same side of the double bond. Trans alkenes typically have lower heats of hydrogenation, reflecting their greater thermodynamic stability. The spatial arrangement of substituents significantly influences overall alkene stability.
Q5: Why are sp2–sp3 bonds stronger than sp3–sp3 bonds in alkenes?
The sp2–sp3 bond is stronger and lower in energy than the sp3–sp3 bond because it has a higher contribution of s electrons, which are inherently lower in energy. This orbital composition difference makes sp2–sp3 bonds more favorable energetically. Highly substituted alkenes benefit from this bond strength advantage, contributing to their increased stability compared to less-substituted isomers.
Q6: How do you compare the stability of 2-butene and 1-butene?
2-butene is more stable than 1-butene because it has two smaller substituents across the double bond, while 1-butene has only one substituent. The additional substitution in 2-butene increases the ratio of sp2–sp3 bonds and provides greater hyperconjugation. This makes 2-butene release less energy during hydrogenation, indicating superior thermodynamic stability.
Q7: What factors determine whether a cis or trans alkene is more stable?
The primary factor is steric strain caused by substituent interactions. In cis alkenes, substituents on the same side of the double bond experience crowding and repulsion, destabilizing the molecule. Trans alkenes avoid this steric strain by positioning substituents on opposite sides, resulting in lower heats of hydrogenation and greater overall stability.