Relationship between Structure and Thermodynamic Stability for Hydrocarbons

Relationship between Structure and Thermodynamic Stability for Hydrocarbons 

Extensive thermodynamic data are available for the major classes of hydrocarbons. Table 3.1 gives data for the C4-C6 and Cj alkanes and the C4-C6 alkenes, and several general relationships become apparent. One is that chain branching increases the stability of alkanes. This relationship is clear, for example, in the data for the Cg alkanes, with a total enthalpy difference of nearly 4kcal/mol between the straight-chain hexane and the tetra-substituted 2,2-dimethylbutane. There is a similar range of 4.5kcal/mol between the least stable (octane) and most stable 

2-methyl-2-pentene 2,3-dimethyl-2-butene AHf° = -23.5 kcal/mol AH,° = -24.3 kcal/mol 

These relationships are a result of the stabilizing effect of branching and double-bond substitution and hold quite generally, except when branching or substitution results in van der Waals repulsions (see Section 2.3), which have a destabilizing effect. 

---