# Why do alkanes have higher boiling point than their ether counterparts?

Based on my understanding of inter-molecular forces, I expect dipole-dipole interactions to be significantly stronger than van der Waal's interactions. Hence, I expect ethers (which obviously have dipole-dipole interactions) to have higher boiling points (as boiling point $$\propto$$ inter-molecular forces) than alkanes (assuming approximately the same molecular mass). But, the experimental data suggests otherwise.

$$\begin{array}{|c|c|c|c|} \hline \textbf{Compound}&\textbf{Boiling point / °C} &\textbf{Compound}&\textbf{Boiling point / °C} \\ \hline \textit{n}\text{-pentane} & 36.1 & \text{ethoxyethane} & 34.6 \\ \hline \textit{n}\text{-heptane} & 98.42 & \text{1-propoxypropane} & 90 \\ \hline \textit{n}\text{-nonane} & 151 & \text{1-butoxybutane} & 141 \\ \hline \textit{n}\text{-undecane} & 196 & \text{1-pentoxypentane}& 184 \\ \hline \end{array}$$

As evident, experimental data suggests that alkanes are on par with ethers when it comes to inter-molecular forces, infact the former are slightly higher.

I tried to reason this discrepancy as follows. Ethers have weak dipole, so their dipole-dipole interactions will also be weak (though stronger than their van der Waal's interactions). Alkanes on, the other hand, have van der Waal's forces only but I suspect that size is playing a dominant role here. Ethers have a v-shaped bent structure, and therefore, have reduced surface area as compared to alkanes. Therefore, I believe that alkanes lead over ether in this case and this compensates the weak dipole of ethers. Though this is only a hypothesis from my part.

But there is another problem:

$$\begin{array}{|c|c|c|c|} \hline \textbf{Compound}&\textbf{Boiling point / °C} &\textbf{Compound}&\textbf{Boiling point / °C} \\ \hline \text{propane} & -42 & \text{methoxymethane} & -24 \\ \hline \end{array}$$

Now at this point I am out of ideas. From $$\ce{Et-O-Et}$$ to $$\ce{Pe-O-Pe}$$ we had ethers leading over alkanes which felt odd but when we compare di methyl ether and propane, everything just seems fine with ether having the dominating lead (as expected initially). What is real cause? Am I missing some important concept here? Or is there any other factor that I am not considering?

• I did not intentionally skip decane and octane, it's just that there is no symmetrical ether counterpart of these (I am avoiding asymmetrical ethers). – Sarthak123 Mar 11 '18 at 15:59
• Why did you avoid non-symmetrical ethers? That might give you important insight into why this trend is such. Also, polyethers. Someone actually studying this problem would probably have looked at different permutations. – Zhe May 16 '18 at 18:09
• I actually did look at non-symmetrical ethers, but the trend was way too irregular. I had to consider many possibilities like one carbon difference non-symmetrical ethers, 2 carbon difference, etc. It extensively messed up my data and no conclusion could be drawn. Their irregular dipole is also a problem both understanding and (i suppose) explaining. Under these consideration, I thought I would first keep my observations confined to the simplest case, once I get a strong theory I would expand it to other cases. – Sarthak123 May 19 '18 at 2:43