This seems consistent with intuitive expectations based on conformational models where, in the cyclic ethers where the molecular geometry is constrained by the ring, the lone pairs are pointing in the opposite direction from the carbon skeleton. Whereas the lone pairs and carbon chains in the conformationally more mobile acyclic isomers are less geometrically fixed and therefore less "directed" in space.
It seems likely that the higher dipole moments in the cyclic compounds would lead to greater alignment / ordering in the liquid phase, which in turn would lead to higher boiling points.
This seems consistent with intuitive expectations based on conformational models where, in the cyclic ethers where the molecular geometry is constrained by the ring, the lone pairs are pointing in the opposite direction from the carbon skeleton. Whereas the lone pairs and carbon chains in the conformationally more mobile acyclic isomers are less geometrically fixed and therefore less "directed" in space.
It seems likely that the higher dipole moments in the cyclic compounds would lead to greater alignment / ordering in the liquid phase, which in turn would lead to higher boiling points.
Interesting pair for comparison. I wonder what the explanation in that case would be. Polarity would seem to favour a higher boiling point for the ether, while molecular mass, dispersion interactions, vaporization entropy and liquid packing seem similar. This is complicated stuff to predict...More
In my mind, molecules in general are more conformationally restricted in liquids due to mutual proximity. Cyclic molecules naturally have fewer conformational degrees of freedom in both phases, and so would not gain quite as much entropy upon vaporization. In an acyclic molecule, the chain would be much more free in the gas phase. This could make it entropically more favourable for acyclic molecules to evaporate, thus decreasing their boiling point relative to cyclic ones. This effect has been suggested More
The data I could find suggests that cyclic ethers have higher dipole moments than their acyclic counterparts.
$$\mathbf{Four~carbons}$$ \begin{array}{c @{} c} \hline \text{THF} & \mathrm{1.63~ D ^1, 1.75~ D ^2} \\ \text{1-butene oxide} & \mathrm{1.89~ D^2} \\ \text{ethyl vinyl ether} & \mathrm{1.26~ D ^2} \\ \text{diethyl ether} & \mathrm{1.15 ~D ^3, 1.15 D ^2} \\ \text{methyl propyl ether} & \mathrm{1.11 ~D ^2} \\\hline \end{array}
$$\mathbf{Five~carbons}$$ \begin{array}{c @{} c} \hline \text{Tetrahydropyran} & \mathrm{1.58~D ^2, 1.87~D ^4} \\ \text{2-Methyltetrahydrofuran} & \mathrm{1.38~ D ^5} \\ \text{dipropyl ether} & \mathrm{1.21~D^2, 1.00~D^6} \\\hline \end{array}
This seems consistent with intuitive expectations based on conformational models where, in the cyclic ethers where the molecular geometry is constrained by the ring, the lone pairs are pointing in the opposite direction from the carbon skeleton. Whereas the lone pairs and carbon chains in the conformationally more mobile acyclic isomers are less geometrically fixed and therefore less "directed" in space.
It seems likely that the higher dipole moments in the cyclic compounds would lead to greater alignment / ordering in the liquid phase, which in turn would lead to higher boiling points.
References:
1: http://en.wikipedia.org/wiki/Tetrahydrofuran
2: https://physicalchemistryrosamonte.wordpress.com/material-balances/material-balances-on-a-crystallizer/physical-properties-of-pure-methanol/dipole-moment/
3: http://en.wikipedia.org/wiki/Diethyl_ether
4: http://www.drugfuture.com/chemdata/tetrahydropyran.html
5: http://www.stenutz.eu/chem/solv28.php?s=1&p=20
6: https://books.google.com/books?id=G6jaBwAAQBAJ&pg=PA50&lpg=PA50&dq=propyl%20ethyl%20ether%20dipole%20moment&source=bl&ots=VA52gqJ0kn&sig=GM8tua_QpXccFW3VXpy9qiNf7rk&hl=en&sa=X&ei=rzRvVZ__Cc32yQT9gIPACQ&ved=0CDsQ6AEwBQ#v=onepage&q=propyl%20ethyl%20ether%20dipole%20moment&f=false
The data I could find suggests that cyclic ethers have higher dipole moments than their acyclic counterparts.
$$\mathbf{Four~carbons}$$\begin{array}{c @{} c} \hline\text{THF} & \mathrm{1.63~ D ^1, 1.75~ D ^2} \\\text{1-butene oxide} & \mathrm{1.89~ D^2} \\\text{ethyl vinyl ether} & \mathrm{1.26~ D ^2} \\\text{diethyl ether} & \mathrm{1.15 ~D ^3, 1.15 D ^2} \\\text{methyl propyl ether} & \mathrm{1.11 ~D ^2} \\\hline\end{array}
$$\mathbf{Five~carbons}$$\begin{array}{c @{} c} \hline\text{Tetrahydropyran} & \mathrm{1.58~D ^2, 1.87~D ^4} \\\text{2-Methyltetrahydrofuran} & \mathrm{1.38~ D ^5} \\\text{dipropyl ether} & \mathrm{1.21~D^2, 1.00~D^6} \\\hline\end{array}
This seems consistent with intuitive expectations based on conformational models where, in the cyclic ethers where the molecular geometry is constrained by the ring, the lone pairs are pointing in the opposite direction from the carbon skeleton. Whereas the lone pairs and carbon chains in the conformationally more mobile acyclic isomers are less geometrically fixed and therefore less "directed" in space.
It seems likely that the higher dipole moments in the cyclic compounds would lead to greater alignment / ordering in the liquid phase, which in turn would lead to higher boiling points.
References:
1: http://en.wikipedia.org/wiki/Tetrahydrofuran
2: https://physicalchemistryrosamonte.wordpress.com/material-balances/material-balances-on-a-crystallizer/physical-properties-of-pure-methanol/dipole-moment/
3: http://en.wikipedia.org/wiki/Diethyl_ether
4: http://www.drugfuture.com/chemdata/tetrahydropyran.html
5: http://www.stenutz.eu/chem/solv28.php?s=1&p=20
6: https://books.google.com/books?id=G6jaBwAAQBAJ&pg=PA50&lpg=PA50&dq=propyl%20ethyl%20ether%20dipole%20moment&source=bl&ots=VA52gqJ0kn&sig=GM8tua_QpXccFW3VXpy9qiNf7rk&hl=en&sa=X&ei=rzRvVZ__Cc32yQT9gIPACQ&ved=0CDsQ6AEwBQ#v=onepage&q=propyl%20ethyl%20ether%20dipole%20moment&f=false
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