Sigmatropic
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Boiling point of organic carbonates
My understanding of physics is somewhat lacking, so forgive me putting forward so simple a question.
My question is why the boiling point of propylene carbonate (240 C) is so much higher than that of dimethyl carbonate (90 C) and diethyl carbonate
(126 C)?
My intuintion tells me that ring strain is somehow involved in much the same way that diethyl ether and tetrahydrofuran have a large difference in
boiling point and water miscibility. I feel like there must be a fundamental explanation but cannot find one. Does someone here care to elaborate?
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unionised
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The other cyclic ones seem more aligned, so I guess that's the thing that matters.
https://en.wikipedia.org/wiki/Ethylene_carbonate
gives 243
Trimethylene carbonate is apparently 256
I guess it's a bit like THF being better coordinating than diethyl ether because there's less steric hinderance to approaching the lone pairs.
That would also increase polar interactions which would raise the boiling point.
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CharlieA
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In general, boiling point can be related to intermolecular forces (van der Waals; dipole-dipole; and hydrogen bonding) for compounds of comparable
molecular weight. For compounds with similar functional groups bp can be related to increasing surface area, and polarizability. For the compounds
cited by the first 2 posters, I would estimate that the biggest single factor determining their bp's, would be the increasing surface area of those
molecules. I would judge the bp's to increase in the order: dimethyl carbonate < diethyl carbonate < propylene carbonate </= ethylene
carbonate < trimethyl carbonate.
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theAngryLittleBunny
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Quote: Originally posted by CharlieA  | | In general, boiling point can be related to intermolecular forces (van der Waals; dipole-dipole; and hydrogen bonding) for compounds of comparable
molecular weight. For compounds with similar functional groups bp can be related to increasing surface area, and polarizability. For the compounds
cited by the first 2 posters, I would estimate that the biggest single factor determining their bp's, would be the increasing surface area of those
molecules. I would judge the bp's to increase in the order: dimethyl carbonate < diethyl carbonate < propylene carbonate </= ethylene
carbonate < trimethyl carbonate. |
Surface area isn't the biggest factor, THF has a higher boiling point then diethyl ether even though it has less surface area.
I think the reason THF has a higher boiling point is because being bent into a ring exposes more surface of the oxygen, which has a partial negative
charge and thereby is attracted to the partial positive part of other THF molecules.
In diethyl ether the two ethyl grous aren't strained into a ring and therefore shield more of the oxygen. Something interesting I found out is that at
room temperature phenyllithium reacts rapidly with THF and everything turns into tar, while in diethyl ether it's stabile at room temperature.
I'm sure the same applies to dimethyl carbonate vs propylene carbonate, the oxygen in the two C-O-C bridges are more exposed in the latter one. In
dimethyl carbonate the methyl groups can probably also shield the carbonyl oxygen to some extent.
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CharlieA
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The difference between THF and Et2O is just 2 H atoms. I don't think that these 2 (smallest possible atoms) make for a very large difference in
surface area between diethyl ether and THF.
As for shielding: my understanding of what you are saying is that in THF, the rigidity of the molecule makes for a greater dipole moment than in Et2O
because due to the rotation of the C-O bonds, at times the two polar C-O bonds opposed to each other for a significant amount of time thus decreasing
the dipole moment. of Et2O compared to THF. With greater dipole moment I would expect THF to have a higher bp than Et2O. I would reason similarly for
the cyclic carbonate esters compared to the acyclic carbonate esters, and expect the cyclic esters to have higher bp's than the acyclic esters.
Regardless of these forces, I still expect the surface area is the predominant factor determining the bp's of the carbonate esters.
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clearly_not_atara
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It's the spontaneous molecular dipole moment.
Me2CO3: 0.18 D
Ethylene carbonate: 4.9 D
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Lionel Spanner
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The fundamental difference is that ethylene and propylene carbonate are cyclic, while dimethyl and diethyl carbonate are not. The cyclic carbonates
have less freedom of movement, so more energy is required to vaporise them.
Dipropyl carbonate boils at 167-168 °C, which is more in line with the boiling points of dimethyl and diethyl carbonates.
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macckone
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As CharlieA and clearly_not_atara have stated, it is the dipole moment. Dipole moment has a huge effect and is directly related to surface tension.
The other effect is molecular weight. steric bulk is more of an issue in melting point than boiling point. Ring strain doesn't really play into
either one except to the extent it increases dipole moment.
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