Checking out the molecular structure and the melting points of various hydrocarbons I concluded:

1)branching increases the melting point of a hydrocarbon molecule

2)symmetry increases the melting point of a hydrocarbon molecule

  • symmetry increases it way more than branching does.

ordering in terms of melting points:

2-methyl butane < pentane < 2,2-dimethyl propane

2-methyl butane is branched but has a bad symmetry, pentane has a good symmetry although not being branched, 2,2-dimethyl propane has a good symmetry and is branched.

2-methyl pentane < 2,3 dimethyl butane < 2,2 dimethyl-butane < hexane

so 2-methyl pentane has a bad symmetry

2,3 dimethyl butane has a bad symmetry as well but more branched compared to 2-methyl pentane

2,2 dimethyl butane has a bad symmetry but it is more branched than the previous two.

Then hexane has the highest melting point. it is not branched at all and not symmetrical. so why does it have the highest melting point?

I kept the text a bit long before asking the question because I wasn't sure if I understood the concept correctly. Can you please answer my question about hexane and correct me if I have some mistake in my interpretation of the whole thing?

  • $\begingroup$ Shouldn't that last example be hexane instead of pentane, since it is compared with 3 other $\ce{C6H14}$ compounds? $\endgroup$ – Mathew Mahindaratne Mar 21 '19 at 16:57
  • $\begingroup$ yes exactly, sorry. I just edited it. $\endgroup$ – Taylan Mar 21 '19 at 17:08
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    $\begingroup$ For pentanes, the order is correct. But, I want to point out that explanation is not that simple. Basically, symmetry is not the only one playing a role here. For example, how about two hexane isomers (3-methylpentane and 2,3-dimethylbutane), you forgot to mention? In your standards, 3-Methylpentane should have better symmetry than 2-methylpentane, but look at their melting points. Same can tell about 2,3-dimethylbutane and 2,2-dimethylbutane, but 2,3-dimethylbutane has even lower melting point (Wikipedia). $\endgroup$ – Mathew Mahindaratne Mar 21 '19 at 17:24
  • $\begingroup$ yes you're right, my points seem to fail in those examples. Is there a simple way to understand which one has a higher melting point? $\endgroup$ – Taylan Mar 21 '19 at 17:33
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    $\begingroup$ I think @MathewMahindaratne's point is that you can create simplistic rules for small hydrocarbons that don't work for larger ones. So such rules are really an oversimplification. $\endgroup$ – MaxW Mar 21 '19 at 20:36

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