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VALUE INVESTING SUMMIT MALAYSIA PHARMACEUTICAL
The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalysis.
The denticity of the polyether influences the affinity of the crown ether for various cations. For example, crown-6 has high affinity for potassium cation, crown-5 for sodium cation, and crown-4 for lithium cation. The high affinity of crown-6 for potassium ions contributes to its toxicity. The smallest crown ether still capable of binding cations is 8-crown-4, [1] with the largest experimentally confirmed crown ether being crown Let's look at dimethyl ether and see why it does not exhibit hydrogen bonding.
So if I were to draw one molecule of dimethyl ether here. And think about the polarization between the oxygen and this carbon right here. Oxygen is more electronegative. This carbon will be partially positive like that. If I think about the interaction of that molecule of dimethyl ether with another molecule of dimethyl ether like that, you might be tempted to say, well there could be some hydrogen bonding because I know that this carbon right here has some hydrogens attached to it.
And so some students will say, oh there must be hydrogen bonding between this oxygen down here and this hydrogen. But that is not the case because this hydrogen right here, while it is interacting with an oxygen, this hydrogen is bonded to a carbon which is not very electronegative. And so there's no large differences in electronegativity in the bond between carbon and hydrogen. Even the carbon's a little bit more electronegative. There's not enough to make this a true hydrogen bond.
And so really there's only a small amount of dipole-dipole interaction between two molecules of dimethyl ether. So somewhere on this second molecule, there is a partial negative, partial positive. And so there will be a little bit of dipole-dipole interaction. But it's not very strong. And certainly nowhere near as strong as the hydrogen bonding exhibited on the left.
Hydrogen bonding being just the super strong form of dipole-dipole interaction. And so dimethyl ether does not have as high of a boiling point as ethanol. Again, the answer is hydrogen bonding. Let's see what happens to the boiling point of ethers as we increase the number of carbons in the alkyl groups.
So if we're going to look at that dimethyl ether again, and let's compare that to an ether that has more carbons than the alkyl group, so diethyl ether. We've already seen the boiling point of dimethyl ether as approximately negative 25 degrees Celsius. Whereas diethyl ether is about 35 degrees Celsius. And so there's a large difference in boiling points diethyl ethers boiling point is just higher than room temperature. So it is still a liquid at room temperature and pressure.
So let's see if we can look at why diethyl ether has a higher boiling point. We know that ether molecules can't hydrogen bond with each other. So that cannot be the intermolecular force responsible for this increase in boiling point. So if we look at two molecules of diethyl ether interacting, one of the other intermolecular forces that we discussed was London dispersion forces. So London dispersion forces, you watched earlier video for more details.
But when you have these large alkyl groups, provides more surface area for a form of attraction called London dispersion. And so that increased attraction between alkyl groups means that it's harder to pull those molecules apart. It requires more energy to pull those molecules apart, requires more heat in order to do so.
And so that's the reason for the increase in boiling point that we see for diethyl ether, up to 35 degrees Celsius. And even though London dispersion forces are the weakest intermolecular forces, they're additive. So the effect is added when you have lots and lots of molecules. And that's the reason for the large difference between dimethyl ether and for diethyl ether. And so the increase of the number of carbons in the alkyl groups increases the boiling point just above room temperature but not much above room temperature.
So this makes diethyl ether an excellent solvent for extraction. The other thing the alkyl groups do, is they increase the nonpolar part of the molecule. So it's a little bit more nonpolar due to these alkyl groups right here which means that diethyl ether is very good for dissolving a lot of nonpolar organic compounds.
And so if you can dissolve a lot of nonpolar organic compounds and the boiling point is just above room temperature, it's an excellent solvent for extraction because you can dissolved are nonpolar organic molecules. And then you can just boil off the ether. And you're left with your organic product. So you'll use diethyl ether a lot for extractions.
Let's look at another type of ether which is a kind of an interesting one. And we call these ethers, crown ethers. So if we look at that gigantic either there, it's called a crown ether. This was discovered by a guy named Charles Peterson who won the Nobel Prize for this. And the system of nomenclature for crown ethers would be to first count up how many atoms comprise your a ring here, your crown. So if we go 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, and
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