What Does the Heat of Hydrogenation Mean?

The hydrogen addition to a substrate is called hydrogenation. The heat released during this reaction is known as the heat of hydrogenation (ΔH). Alkenes in presence of a metal catalyst add hydrogen by breaking its double bond. The breaking of this double bond releases heat. If the substance undergoing hydrogenation is 1 mole, then this heat is known as the heat of hydrogenation (ΔH). The reaction is thus an exothermic one.

Mechanism of Hydrogenation of an Alkene

Hydrogenation reactions generally take place on alkene with the help of a metal catalyst. The hydrogen molecule first gets adsorbed on the surface of it and then gets attached to the alkene being provided in the reaction mixture. The metal catalyst is used to lower the activation energy barrier required to break the strong H-H bonds. Various metals like platinum, palladium, nickel and rhodium can be used. These metal catalysts are heterogeneous i.e. they are insoluble in the reactants and so easily recyclable.

As shown in the below-given image, the hydrogen molecule breaks into atoms first and then gets adsorbed on the metal catalyst. The alkene molecule approaches the metal surface, which then breaks its π-bond to add hydrogen to the carbons sharing the double bond.

Comparison of the Heat of Hydrogenation in Alkenes

Consider a molecule with the formula, C₆H₁₂. It can have three isomeric structures for this molecular formula, viz., (E)-2-hexene, (Z)-2-hexene, and 1-hexene (‘E’ is a notation given to structures similar to trans isomer while ‘Z’ is for cis kind of an isomer). Among the three, the least stable alkene generates the most heat. Therefore, in this case, 1-hexene has the highest value of heat of hydrogenation. On other hand, the most stable alkene will liberate less amount of heat.

When the big substituents are on the molecule’s same side, ‘Z-configuration’, their electron clouds interact and repel each other that causes strain in the molecule making it less stable than the ‘E-configuration’. Thus, in this case, it is (E)-2-hexene. All the three isomeric structure on hydrogenation gives n-hexane as the product with different values of heat of hydrogenation. The enthalpy change in hydrogenation of 1-hexene, (Z)-2-hexene and (E)-2-hexene to n-hexane is -29.7 kcal/mole, -28.3 Kcal/mole and -27.3 kcal/mole respectively.

Since the heat of hydrogenation is equal to the negative of the enthalpy change of a reaction, the order of heat of hydrogenation (ΔH)(kcal/mole) in this case is: 1-hexene [29.7] > (Z)-2-hexene [28.3] > (E)-2-hexene [27.3] as shown in the image given below.

Consider olefins with the molecular formula, C₅H₁₀. Three isomeric structures of this molecular formula could be drawn namely, 2-methyl-2-butene, 2-methyl-1-butene, and 3-methyl-1-butene. All three isomeric structures on hydrogenation will give rise to 2-methyl butane. According to the rule of Saytzeff, the most substituted alkene is the stable one. The alkyl groups on sp² hybridized carbon stabilize the substrate by increasing the electron density. Thus, in this case, 2-methyl-2-butene is the most stable isomer and so it will have the least heat of hydrogenation value among the three. 3-methyl-1-butene will be the least stable isomer generating a lot of heat with the highest value of heat of hydrogenation.

The enthalpy change in hydrogenation of 2-methyl-2-butene, 2-methyl-1-butene and 3-methyl-1-butene is -30.3 Kcal/mole, -28.5 Kcal/mole and -26.9 kcal/mole respectively. The order of heat of hydrogenation (kcal/mole) in this case is: 2-methyl-2-butene [30.3] > 2-methyl-1-butene [28.5] > 3-methyl-1-butene [26.9] as shown in the image given below.

From the above two examples, it is evident that the heat of hydrogenation is inversely proportional to the stability of the reactant alkene. In this way, heats of hydrogenation can become a measure in comparing the relative stabilities of alkenes. The general order of heats of hydrogenation in a simple alkene (ethylene) is as follows:

Tetra-substituted > tri-substituted > di-substituted > mono-substituted >ethylene

R₂C=CR₂ > RHC=CR₂ > RHC=CHR > H₂C=CHR > H₂C=CH₂

Heat of Hydrogenation of Dienes

Consider dienes with the molecular formula, C₅H₈, giving rise to two isomeric diene one is an isolated diene, 1,4-pentadiene, and another isomer is a conjugated diene, 1,3-pentadiene. On hydrogenation of the former high amount of heat was liberated than the latter but both result in the same product as n-pentane. 1,4-pentadiene. On hydrogenation, first gives 1-pentene liberating 60.2 kcal/mole of heat and then on further hydrogenation gives n-pentane liberating 30.1 kcal/mole of heat. This shows that the molecule 1,4-pentadiene behaves like two separate alkene units because the heat of hydrogenation in the first step (1,4-pentadiene to pentene) is exactly twice the second step (1-pentene to n-pentane).

H₂C=CH-CH₂-CH=CH₂        H₂C=CH-CH₂-CH₂-CH₃          H₃C-CH₂-CH₂-CH₂-CH₃  

1,4-pentadiene                           1-pentene                                    n-pentane

1,5-pentadiene first gives 2-pentene liberating 54.1 kcal/mole then results in n-pentane liberating 27.6 kcal/mole on hydrogenation.

H₂C=CH-CH=CH-CH₃      H₃C-CH₂-CH=CH-CH₃             H₃C-CH₂-CH₂-CH₂-CH₃                 

1,3-pentadiene                           2-pentene                               n-pentane

On comparing the above two examples, you could compare the hydrogenation of the terminal alkene (the first step in both the reaction) and find that the value of heat of hydrogenation of 1,4-pentadiene is ~3.6 kcal/mole higher than that of 1,3-pentadiene. This indicates that dienes that are conjugated are stable than the isolated ones.

The Heat of Hydrogenation of an Alkyne

The heat of hydrogenation values of alkynes also varies in the same way as in the alkenes, the more substituted alkyne has the least value. This is because alkyl groups attached to sp hybridized carbon release electrons to stabilize the substrate. For example, consider 1-Butyne and 2-Butyne.

CH₃-CH₂-C≡ CH, the heat of hydrogenation (-ΔH )= 69.9 kcal/mole

CH3-C ≡ C-CH3, heat of hydrogenation (-ΔH )= 65.6 kcal/mole

Common Mistakes

Do not confuse the heat of hydrogenation with enthalpy. Enthalpy gives information about both generation and absorption of heat whereas heat of hydrogenation is the amount of heat liberated on hydrogenation reaction.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry. 

Practice Problems

 Give the correct increasing order of heat of hydrogenation (ΔH)

1. tetra-substituted > tri-substituted > di-substituted > mono-substituted >unsubstituted
   
   
2. di-substituted > mono-substituted >tetra-substituted >unsubstituted> tri-substituted
   
   
3. tri-substituted> di-substituted > mono-substituted>tetra-substituted>unsubstituted
   
   
4.  unsubstituted> mono-substituted> di-substituted> tri-substituted>tetra-substituted
   
   

Solution:

Depending on the stability of the alkene bond, the correct order for the increasing heat of hydrogenation will be,

1. tetra-substituted > tri-substituted > di-substituted > mono-substituted >unsubstituted