Relative Stability of Acyclic Alkanes

Heat of combustion as measure of stability.

The stability of alkanes can be assessed by examining their heats of combustion. Generally, branched alkanes have lower heats of combustion compared to their straight-chain isomers, indicating higher stability. This is because a lower heat of combustion corresponds to a lower energy content in the molecule, reflecting greater stability.

For example, consider the isomers of pentane (C₅H₁₂):

• n-Pentane: Heat of combustion = −3272 kJ/mol
  
• 2,2-Dimethylpropane (neopentane): Heat of combustion = −3251 kJ/mol
  

Here, neopentane, a branched isomer, has a slightly lower heat of combustion than n-pentane, indicating it is more stable.

Similarly, for hexane (C₆H₁₄) isomers:

• n-Hexane: Heat of combustion = –4163 kJ/mol
  
• 2-Methylpentane: Heat of combustion = –4158 kJ/mol
  
• 2,2-Dimethylbutane: Heat of combustion = –4154 kJ/mol
  

In this case, 2,2-dimethylbutane, the most branched isomer, has the lowest heat of combustion, indicating the highest stability among the three.

Why?

William McKee and Paul von Ragué Schleyer from the University of Georgia, Athens, GA, USA, have expanded on their earlier protobranching model to explain the greater stability of branched alkanes compared to their straight-chain isomers. Their theory focuses on the electronic correlation between interactions of 1,3-alkyl groups with other alkyl groups within an alkane. The increased branching results in a more compact electronic structure, which reduces the molecular surface area per atom. This decrease in surface area lowers the energy, enhancing the overall stability of the molecule.