The stability order of different alkene substitution patterns is measured by comparing their heat of hydrogenations (denoted as ∆Ho) in kJ/mole or kcal/mol.
Understanding the Hydrogenation Reaction
In a hydrogenation reaction, the alkene is reacted with H2 gas in the presence of a platinum (Pt) catalyst. The alkenes' pie bond breaks so that the hydrogen adds to the double bond by forming two new sigma bonds. The alkene, by the addition reaction, changes to form an alkane; therefore, the hydrogenation reaction is a bond-forming reaction.
For example, ethene forms ethane in reaction with H2 gas and Pt catalyst.
Role of Hydrogenation Reaction in measuring alkene stability
The bond forming Hydrogenation reaction releases energy by giving out 80-120 kJ of heat per mole of the H2 gas consumed and is, therefore, exothermic. This energy release comes from breaking the pie bond.
The hydrogenation reaction measures the ease of breaking the pie bond so that the energy released is proportionally compared to the stored potential energy in the double bonds.
If the pie bonds’ potential energy is lower, it correlates to less instability in the bond. Such an alkene will be stable and less reactive. Correspondingly, the stable alkene will have lower heat of hydrogenation (∆ H0) value.
For example, comparing monosubstituted but-1-ene with disubstituted but-2-ene, the ∆ H0 value of but-2-ene is lower than but-1-ene, indicating its greater stability.
Structural feature in alkenes responsible for stability
The structural features that contribute to alkene stabilization/destabilization can be understood by comparing the heat of hydrogenation of different substituted alkenes.
The popular observation is that more substituted double bonds are the most stable. Such an observation is derived from two facts-
a) In alkene-forming chemical reactions, if there is a choice between more substituted and less substituted alkene, the major product formed is always a more stable, substituted alkene.
b) Suppose an alkene formed is a less substituted one; the molecule will then try and undergo rearrangement (reorganization) to form the most substituted form of the alkene. However, there are exceptions to this rule.
Factors contributing to the stability in substituted alkenes
A substituted alkene has the greatest number of alkyl groups across the double bond.
Therefore, the two main factors responsible for the stabilizing effect of the alkyl groups on the double bonds are- the +I effect (positive inductive effect) and steric crowding.
a) Alkyl groups are alkene’s allies. They are electron donors by the positive inductive (+I) effect, increasing the electron density of the alkene pie bond. This makes the pie bond more willing to act as a nucleophile in the addition reactions with the electrophiles. Addition reactions create more bonds (two new covalent bonds by losing one pie bond), bringing more stability.
Alkyl groups also support by stabilizing carbocation intermediates formed during the reaction course. Carbocations are electron-deficient, and alkyl groups offer their electron density and stabilize it. Therefore, alkyl groups help from the start to the end of the reaction stages.
b) The steric crowding of the alkyl groups is minimal when they spread far apart. The alkyl groups are separated with a bond angle of 120o to minimize the inter-bond repulsions. The maximum spreading is seen for tetrasubstituted alkenes making it the most stable form.
Therefore, the stability order in alkenes is-
Tetrasubstituted > trisubstituted> disubstituted> monosubstituted> unsubstituted
The heat of hydrogenation value (∆ H0) echoes the stability order.
The ∆ H0 is highest for unsubstituted alkene implying higher potential energy of the alkene pie bond and, therefore, lower bond stability.
The Special case of disubstituted alkene and their stability
The disubstituted alkene can have a cis, trans, and geminal arrangement of the alkene.
The trans variant has the maximum separation of the alkyl groups lowering the potential energy; hence, it is the most stable form.
Between geminal and cis alkenes, the geminal alkene is more stable than the cis due to multiple factors.
The first contributing factor is the wider angle between the alkyl substituents in geminal alkene versus the cis form of the alkene, lessening the inter-bond steric repulsion.
The other factor supporting geminal alkene is the presence of two alkyl groups on the same carbon atom that can effectively stabilize the carbocation reaction intermediate by hyperconjugation.
In hyperconjugation, the sigma bond delocalizes temporarily to stabilize the carbocation; therefore, the hyperconjugation is also called a sigma bond resonance. A higher number of hyperconjugative structures implies higher carbocation stability.
(Note: The carbon attached to the carbocation is the alpha carbon. The hydrogens attached to the alpha carbon are the alpha hydrogens.)
Carbocation formed from geminal alkene has the greatest number of hyperconjugative structures supporting its formation and stability.
The carbocation formed of trans alkene would have only five hyperconjugative structures.
Therefore, when compared with trans alkene, a geminal alkene is marginally less stable. The trans alkene’s higher stability is due to the minimum steric hindrance between the alkyl substituents.
Therefore, the final stability order for the disubstituted alkene is:
trans> geminal > cis
Complete list of the relative energies of typical π bond in substituted alkene versus the unsubstituted alkene (ethene)
Unsubstituted> monosubstituted> disub cis > disub geminal> disub trans> trisubstituted> tetrasubstituted
137 kJ > 126 kJ > 120 kJ > 117 kJ > 116 kJ > 113 kJ > 111 kJ
Difference: 11kJ 17kJ 20kJ 21kJ 24kJ 26kJ
The monosubstituted alkene is 11 kJ, disubstituted trans alkene is 21 kJ, and tetrasubstituted alkene is 26 kJ more stable than unsubstituted alkene (137kJ).
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