Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Alkanes, Cycloalkanes, and Alkenes: Isomers, Conformations, and Stabilities
4.6 Stability of Alkanes

Since it is possible to have different structural isomers of alkanes, a question that chemists typically probe is the relative stability of different isomers of compounds. That is, of two isomers, which is the most stable? For various reasons, chemists are constantly concerned about the stability of compounds; stable compounds are typically not as reactive as compounds that are not stable. Chemists have developed a method to determine the relative stability of compounds, which is determined based on the heat liberated from specific reactions. A basic concept learned in general chemistry is that an exothermic reaction gives off heat and an endothermic reaction must be heated in order for the reaction to occur. The energy profile for an exothermic reaction is shown in Figure 4.23.

Compounds that are low on the energy profile diagram are more stable than compounds higher on an energy profile diagram. Thus, in the diagram, as shown in Figure 4.23, as a reaction proceeds from reactants to products, the reactants are less stable than the products and the difference in heat (ΔH) for the reaction gives an idea of the stability of the reactants, relative to the products. A comparison of two reactions that give the same products can be carried out to reveal the relative stabilities of the reactants, as shown in Figure 4.24.

From the energy profile shown in Figure 4.24, the reactants of the reaction with the larger ΔH are less stable than the reactants of the reaction with the smaller ΔH. As a result, this type of analysis will give us an idea of the relative stabilities of the reactants of two different reactions that give the same products.

The reaction of alkanes with oxygen to produce heat, also known as combustion, is an important type of exothermic reaction. As a result, alkanes make exceptionally good fuels for heating homes, cooking, and as a fuel for the heat engines of vehicles. The combustion reactions of two isomers of octane are shown in Figure 4.25, along with the amount of heat given off. The energy profile of the type shown in Figure 4.24 can be used to analyze the stabilities of these two isomers of octane. Note that both reactions give the same products, but the energy for each reaction is different since different isomers are used. As a result, it can be concluded that 2,2,3,3-tetramethylbutane is more stable than octane since the ΔH is less for its combustion, compared to octane.

Δ

Figure 4.23 Energy profile for an exothermic reaction.

ΔΔ

Figure 4.24 Reaction profile for the reactions of two different reactants that give the same products.

Figure 4.25 Combustion reactions of two isomers of octane showing the amounts of heat liberated.

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Figure 4.26 Reaction profile for the combustion reactions of octane (shown in black) and 2,2,3,3-tetramethylbutane (shown in red). Note that the more branched isomer is the most stable isomer.

Figure 4.26 gives the energy profile for both reactions. Note that the more branched isomer liberates the least amount of heat, hence the more stable isomer, compared to an isomer that is not as branched or to the straight chain isomer.

Figure 4.27 shows four different alkanes, along with the heat liberated upon combustion of each alkane.

Figure 4.27 Combustion of different alkanes, along with the amount of heat liberated.

You will notice that as the number of carbons and hydrogens increases, the amount of heat that is liberated also increases. Approximately 157 kcal mol-1 of heat is liberated for each CH₂ added to a straight chain alkane. Thus, it is possible to estimate the amount of heat liberated for a straight chain alkane. Based on knowledge of the heats of combustion, one can predict the relative stabilities of isomers of alkanes and also estimate the amount of heat liberated for the combustion of straight chain alkanes.