7.5: Stability of Alkenes

Although the cis–trans interconversion of alkene isomers does not occur spontaneously, it can often be made to happen by treating the alkene with a strong acid catalyst. If we interconvert cis-2-butene with trans-2-butene and allow them to reach equilibrium, we find that they aren’t of equal stability. The trans isomer is more stable than the cis isomer by 2.8 kJ/mol (0.66 kcal/mol) at room temperature, corresponding to a 76 : 24 ratio.

Cis alkenes are less stable than their trans isomers because of steric strain between the two larger substituents on the same side of the double bond. This is the same kind of steric interference that we saw previously in the axial conformation of methylcyclohexane (Section 4.7).

Hydrogenation Reaction

Although it’s sometimes possible to find relative stabilities of alkene isomers by establishing a cis–trans equilibrium through treatment with strong acid, a more general method is to take advantage of the fact that alkenes undergo a hydrogenation reaction to give the corresponding alkane when treated with H₂ gas in the presence of a catalyst such as palladium or platinum.

Alkene hydrogenation is the syn-addition of hydrogen to an alkene, saturating the bond. The alkene reacts with hydrogen gas in the presence of a metal catalyst which allows the reaction to occur quickly. The energy released in this process, called the heat of hydrogenation, indicates the relative stability of the double bond in the molecule.

Although the catalyst is not consumed in the reaction, it is required to accelerate the reaction sufficiently to be observed in a reasonable amount of time. Catalysts commonly used in alkene hydrogenation are: platinum, palladium, and nickel. With this catalyst present, the sigma bond of H₂ breaks, and the two hydrogen atoms bind to the metal (see #2 in the figure below). The π bond of the alkene weakens as it also interacts with the metal (see #3 below).

Since both reactants are bound to the metal catalyst, the hydrogen atoms can easily add, one at a time, to the previously double-bonded carbons (see #4 and #5 below). The position of both reactants bound to the catalyst makes it so the hydrogen atoms are only exposed to one side of the alkene. This explains why the hydrogen atoms add to same side of the molecule, called syn-addition.

Figure \(\PageIndex{1}\): Hydrogenation takes place in the presence of a metal catalyst.

Energy diagrams for the hydrogenation reactions of cis- and trans-2-butene are shown in Figure \(\PageIndex{1}\). Because cis-2-butene is less stable than trans-2-butene by 2.8 kJ/mol, the energy diagram shows the cis alkene at a higher energy level. After reaction, however, both curves are at the same energy level (butane). It therefore follows that ΔG° for reaction of the cis isomer must be larger than ΔG° for reaction of the trans isomer by 2.8 kJ/mol. In other words, more energy is released in the hydrogenation of the cis isomer than the trans isomer because the cis isomer has more energy to begin with.

Figure \(\PageIndex{2}\): Energy diagrams for hydrogenation of cis- and trans-2-butene. The cis isomer is higher in energy than the trans isomer by about 2.8 kJ/mol and therefore releases more energy when hydrogenated.

Alkene stability can be determined by measuring the amount of energy associated with the hydrogenation of the molecule. Since the double bond is breaking in this reaction, the energy released in hydrogenation is proportional to the energy in the double bond of the molecule. This is a useful tool because heats of hydrogenation (ΔH°_hydrog) can be measured very accurately.

Figure \(\PageIndex{3}\): Trans-2-butene is the most stable because it has the lowest heat of hydrogenation.

The (ΔH°_hydrog) is usually around -30 kcal/mol for alkenes. Stability is simply a measure of energy. Lower energy molecules are more stable than higher energy molecules. More substituted alkenes are more stable than less substituted ones due to hyperconjugation. They have a lower heat of hydrogenation.

Example: cis-2-Butene, for instance, has ΔH°_hydrog = −28.6 kcal/mol, while trans-2-butene has ΔH°_hydrog = −27.6 kcal/mol —a difference of 4 kJ/mol. While 1-butene, the terminal alkene has ΔH°_hydrog = −30.3 kcal/mol, closer to the expected value.

The 4 kJ/mol energy difference between the 2-butene isomers calculated from heats of hydrogenation agrees reasonably well with the 2.8 kJ/mol energy difference calculated from equilibrium data, but the values aren’t exactly the same for two reasons. First, there is probably some experimental error, because heats of hydrogenation are difficult to measure accurately. Second, heats of reaction and equilibrium constants don’t measure exactly the same thing. Heats of reaction measure enthalpy changes, ΔH°, whereas equilibrium constants measure free-energy changes, ΔG°, so we might expect a slight difference between the two.

Table \(\PageIndex{1}\): Heats of Hydrogenation of Some Alkenes

Substitution	Alkene	ΔH°hydrog
		(kJ/mol)	(kcal/mol)
Ethylene	\(\ce{H2C═CH2}\)	−136	−32.6
Monosubstituted	\(\ce{CH3CH═CH2}\)	−125	−29.9
Disubstituted	\(\ce{CH3CH═CHCH3}\) (cis) \(\ce{CH3CH═CHCH3}\) (trans) \(\ce{(CH3)2C═CH2}\)	−119 −115 −118	−28.3 −27.4 −28.2
Trisubstituted	\(\ce{(CH3)2C═CHCH3}\)	−112	−26.7
Tetrasubstituted	\(\ce{(CH3)2C═C(CH3)2}\)	−110	−26.4

Table \(\PageIndex{1}\) lists some representative data for the hydrogenation of different alkenes and shows that alkenes become more stable with increasing substitution. That is, alkenes follow the stability order:

The stability order of substituted alkenes is due to a combination of two factors. One is a stabilizing interaction between the  \(\ce{C═C}\) \(\pi\) orbital and adjacent C−H σ bonds on substituents. In valence-bond language, the interaction is called hyperconjugation. In a molecular orbital description, there is a bonding MO that extends over the four-atom \(\ce{C═C−C−H}\) grouping, as shown in Figure \(\PageIndex{2}\). The more substituents present on the double bond, the more hyperconjugation occurs and the more stable the alkene.

Figure \(\PageIndex{4}\): Hyperconjugation is a stabilizing interaction between the $\mathbf{\text{C═C}}$ π orbital and adjacent C−H σ bonds on substituents. The more substituents there are, the greater the stabilization of the alkene.

A second factor that contributes to alkene stability involves bond strengths. A bond between a sp² carbon and a sp³ carbon is somewhat stronger than a bond between two sp³ carbons. Thus, in comparing 1-butene and 2-butene, the monosubstituted isomer has one sp³–sp³ bond and one sp³–sp² bond, while the disubstituted isomer has two sp³–sp² bonds. More highly substituted alkenes always have a higher ratio of sp³–sp² bonds to sp³–sp³ bonds than less highly substituted alkenes and are therefore more stable.

Exercise \(\PageIndex{1}\)

Name the following alkenes, and tell which compound in each pair is more stable:

1.  
2.  
3. 

Answer

1. 2-Methylpropene is more stable than 1-butene
2. trans-2-Hexene is more stable than cis-2-hexene
3. 1-Methylcyclohexene is more stable than 3-methylcyclohexene

Exercise \(\PageIndex{2}\)

Name the following alkenes, and indicate the order of stability, from the most stable to the least stable:

1.  
2.  
3. 

Answer

Most stables: 

a.    b.    c.