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?

Branched-chain alkanes are generally more stable than their straight-chain counterparts due to a few main factors:

1. Lower Steric Strain: In straight-chain alkanes, the carbon atoms and their attached hydrogens are more extended, which can cause increased steric interactions between the larger electron clouds of the bonds along the chain. Branching reduces this extended linear structure, thus lowering the overall steric strain.

2. Enhanced Hyperconjugation: Hyperconjugation is an effect where the electrons in C-H or C-C sigma bonds in alkyl groups can delocalize into adjacent empty or partially filled p-orbitals or π systems. In branched alkanes, there are more C-H bonds in close proximity to tertiary or quaternary carbons, increasing the potential for hyperconjugation, which provides additional stability.

3. More Compact Structure: The compact, spherical-like shape of branched alkanes tends to lower the surface area, which decreases the van der Waals interactions between molecules. This contributes to a lower boiling point and less intermolecular attraction, indirectly stabilizing the molecule by reducing intermolecular forces that would otherwise encourage the molecule to aggregate.

Together, these factors lead to a more stable structure for branched alkanes compared to non-branched ones.