[Vorld]

I hope this is the right place to ask, but I was recently doing some data processing using the data on NIST WebBook and noticed an interesting trend about alkanes and their enthalpies of formation. I noticed that branched alkanes tend to have more negative enthalpies of formation as compared to linear alkanes, leading to a conclusion that they are generally more stable than linear alkanes.

I did some reading, and found that this is supposed to be a commonly known relationship, but I can't figure out why this is the case. Why are branched alkanes generally more stable than linear alkanes? Are there some steric interactions at play?

Even if we just accept that relationship to be true, why are there instances where this relationship does not hold? Specifically, when I was exploring the data on NIST WebBook and I noticed that ΔfH⦵ is more negative for 2,2-dimethylpentane than 2,2,3-trimethylbutane even though the latter is more branched than the former.

Thank you so much!

[Corribus]

Nice catch. Not a question that has an easy answer, since papers are still being published on this topic not too long ago.

See, e.g. https://pubmed.ncbi.nlm.nih.gov/21086970/

When something doesn't feel right, blame quantum mechanics.