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I've seen their properties in my textbook and multiple online sources, but I can't seem to find a straight answer as to why. Excluding propane, the trend is increased melting point as the molecule gets bigger, but propane is the weird exception here.

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Ok, let's first look at the trend...

$$ \begin{array}{c|c|c|c}\hline \text{Chemical} & n \text{ in }\ce{C_nH_{2n+2}} & \text{m.p. / K} & \text{m.p.}(n) - \text{m.p.}(n-1)\text{ / K} \\ \hline \text{Methane} & 1 & 90.7 & \\ \text{Ethane} & 2 & 90.4 & -0.3 \\ \text{Propane} & 3 & 85.5 & -4.9 \\ \text{Butane} & 4 & 133\mathrm{-}139 & +50.5 \\ \text{Pentane} & 5 & 144.1 & +8.0 \\ \text{Hexane} & 6 & 177\mathrm{-}179 & +36.9 \\ \text{Heptane} & 7 & 182.2\mathrm{-}183.0 & +4.6 \\ \text{Octane} & 8 & 216.0\mathrm{-}216.6 & +33.7 \\ \text{Nonane} & 9 & 219.0\mathrm{-}220.0 & +3.2 \\ \text{Decane} & 10 & 242.7\mathrm{-}243.9 & +23.8 \\ \hline \end{array} $$

Using the mean for the alkanes with melting point ranges given (e.g. 136 for n-butane and 178 for n-hexane), it isn't clear there is any "great" linear trend. In fact it seems that the even number n-alkanes are on one curve and the odd n-alkanes are on another.

There is a "general trend in that with increasing MW the MP increases. However the exceptions would seem to be both methane and ethane, not propane. Also it is obvious that a simple correlation with MW alone is not sufficient to characterize the trends. All in all I think one must be careful not to read too much into such trends without a very careful analysis.

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  • $\begingroup$ In general even number of $\ce{C}$ atoms in a chain are much more accommodating to a lattice than odd number of carbons. This is why the increase in boiling point from odd $\implies$ even is much more higher than even $\implies$ odd. $\endgroup$ Jul 14 at 9:58
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The phase transition towards solid state involves crystalization, i.e. organization of the molecules into the long-range repetitive patterns. The formation of crystal lattice is reasonably simple for balls (methane) and rigid sticks (ethane), but becomes problematic for flexible triangle (propane), which needs to be properly aligned. Propane is the first of alkanes, which have important internal degrees of freedom, which have to adjust during the solidification.

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