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Current time:0:00Total duration:10:11

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if we look at the boiling points of ethanol and dimethyl ether we can see there's a large difference between them ethanol has a much higher boiling point 78 degrees Celsius whereas dimethyl ether is negative 25 degrees and this explains the state of matter of these molecules ethanol since its boiling point is higher than room temperature is of course a liquid at room temperature and pressure whereas dimethyl ether with a much lower boiling point has already turned into a gas and so we can explain the states of matter by looking at the intermolecular forces that are present in these molecules so if I think about one molecule of ethanol I know that the bond between oxygen hydrogen is polarized I know that oxygen is more electronegative so it will be partially negative and the hydrogen is partially positive as it loses some electron density if that molecule of ethanol interacts with another molecule of ethanol the second molecule of ethanol is also polarized right the oxygen is partially negative and the hydrogen is partially positive and we know that opposite charges attract so the partially positively charged hydrogen is attracted to the partially negatively charged oxygen like that and there's going to be attraction between those two molecules and we call this intermolecular force hydrogen bonding right the strongest type of intermolecular force so hydrogen bonding is present between molecules of ethanol and this accounts for its large boiling point let's look at more details about hydrogen bonding here so hydrogen bonding exists when you have hydrogen bonded to an electronegative atom like that this oxygen but students forget that you also need another electronegative atom over here to give you to give you more of a difference in charge and to make that hydrogen more partially positive so it's really it's really three atoms involved in hydrogen bonding there 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 so be partially negative this this carbon 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 right because I know that this carbon right here has some hydrogen's attached to it and so some students will say oh there must be hydrogen bonding between you know 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 hydrogen even though carbon is 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 the second molecule there's a partial negative partial positive and so there will be a little bit of dipole-dipole interaction a little bit of dipole-dipole interaction here but it's not very strong and it's certainly nowhere near as strong as the hydrogen bonding exhibited on the left right 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 all right so if we if we're going to look at dimethyl ether again all right and let's compare that to an ether that has more carbons in the alkyl groups so diethyl ether we've already seen the the boiling point of dimethyl ether is approximately negative 25 degrees Celsius whereas diethyl ether is about 35 degrees Celsius and so there's a large difference in boiling points diethyl ether 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 could look at why diethyl ether has a higher boiling point all right 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 okay so London dispersion forces all right you can watch the 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's Persian forces are the weakest intermolecular forces they're additive so the effect is added is added as you add you know we have lots and lots of molecules and 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 an excellent solvent for extraction it's the other thing the alkyl groups do is they increase the the nonpolar part of the molecule right 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 dissolve your nonpolar organic molecules and then you can just boil off the ether and you're left with your organic product okay 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 we call these ethers crown ethers alright so if you look at that gigantic ether there it's called a crown ether this is 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 how many how many atoms comprise your your ring here your crown so if we go one two three four five six seven eight nine 10 11 12 13 14 15 16 17 and 18 so there there 18 18 parts of this crown so we would write the right and 18 right here like that followed by the name crown followed by the number of oxygens in here so we have we have 1 oxygen 2 3 4 5 and 6 so the nomenclature will be 18 crown 6 ether and that just tells you what sort of crown ether that you are dealing with so why is it called a crown ether well the interesting thing about crown ethers are that they can interact with different ions for example the size of the potassium ion so k+ happens to fit right in the center of this so the spacing is just right for a potassium ion to fit in there and since all of these oxygens right have lone pairs of electrons on them right so negatively charged there's an attraction between the positively charged potassium ion and the negatively charged electrons or the partially negative charge oxygen atoms right so there's there's attraction opposite charges attract and those negative charges are going to hold that potassium ion in here like that so it looks like a crown if you think about you know the potassium line as being you know someone's head and then and then there's wearing this this ether crown on someone's head like that and crown ethers have proved to be very useful very useful things for example if you had if you had some potassium fluoride right so some k plus F minus right well normally potassium fluoride would not dissolve in a nonpolar organic solvent but if you use a crown ether right the oxygens can take care of the potassium right and the outside of the crown either is is nonpolar right so this portion and this portion the outside of the crown ether is nonpolar which will dissolve in an organic solvent and a non floor organic solvent like benzene's like that so like dissolves like so this portion would dissolve in benzene all right and then what that would do is that would free up your fluoride anion right that would increase the nucleophilic strength of your fluoride anion which could participate in an sn2 reaction so that's one of the uses of crown ethers is to is to go ahead and take the pick D cation leaving the anion to function as a better nucleophile because the potassium ion is solvated by the crown ether and of course since different ions have different sizes you can you can get different sized crown ethers to take care of those ions so crown ethers I just think are very interesting interesting molecules and if you can look at a three-dimensional representation of a crown ether it's much easier to see that the outside is is very nonpolar so interesting interesting molecules