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Current time:0:00Total duration:11:07

Boiling points of organic compounds

Video transcript

liquid boils when it's molecules have enough energy to break free of the attractions that exist between those molecules and those attractions between the molecules are called the intermolecular forces let's compare two molecules pentane on the left and hexane on the right these are both hydrocarbons which means they contain only hydrogen and carbon pentane has five carbons one two three four five so five carbons four pentane and pentane has a boiling point of 36 degrees Celsius hexane has six carbons one two three four five and six so six carbons and a higher boiling point of 69 degrees C let's draw on another molecule of pentane right here so there's five carbons let's think about the intermolecular forces that exist between those two molecules of pentane pentane is a non-polar molecule and we know the only intermolecular force that exists between two nonpolar molecules that of course be the London dispersion forces so London dispersion forces exist between these two molecules of pentane London dispersion forces are the weakest of our intermolecular forces their attractions between molecules that only exist for a short period of time so I could represent the London dispersion forces like this so I'm showing the brief the transient attractive forces between these two molecules of pentane if I draw in another molecule of hexane so over here I'll draw on another one hexane is a larger hydrocarbon with more surface area and more surface area means we have more opportunity for London dispersion forces so I can show even more attraction between these two molecules of hexane so the two molecules of hexane attracting attract each other more than the two molecules of pentane that increased attraction means it takes more energy for those molecules to pull apart from each other more energy means an increased boiling points so hexane has a higher boiling point in pentane so as you increase the number of carbons in your carbon chain you get an increase in the boiling point of your compound so this is an example comparing two molecules that have straight chains let's let's compare let's compare a straight chain to a branched hydrocarbons so on the left down here once again we have pentane alright with the boiling point of 36 degrees C let's write down its molecular formula we already know there are five carbons and if we count up our hydrogen's one two three four five six seven eight nine 10 11 and 12 so there are 12 hydrogen's so H 12 c5h12 is the molecular formula for pentane what about neopentane on the right well there's 1 2 3 4 5 carbons so 5 carbons and 1 2 3 4 5 6 7 8 9 10 11 and 12 hydrogen's so c5h12 so these two compounds have the same molecular formula right so the same molecular formula c5h12 the difference is neopentane has some branching right so neopentane has branching whereas pentane doesn't it's a straight chain all right let's think about the boiling points pentanes boiling point is 36 degrees C neopentane drops down to 10 degrees C now let's try to figure out why if I draw in another molecule of pentane alright we just talked about the fact that London dispersion forces exist between these two molecules of pentane so let me draw in that those those transient attractive forces between those two molecules neopentane is also a hydrocarbon it's nonpolar so if I draw in another another molecule of neopentane and I think about the attractive forces between these two molecules of neopentane a must once again be London dispersion forces because of this branching the shape of neopentane in three dimensions resembles a sphere so it's just an approximation but if you could have imagine this molecule of neopentane on the left as being a sphere so spherical and just try to imagine this molecule of neopentane on the right as being roughly spherical and if you think about the surface area right for for an attraction between these two molecules it's a much smaller surface area than for the two molecules of pentane right we kind of stacked these two molecules of pentane on top of each other and get increased surface area and increase attractive forces but these two neopentane molecules because of their shape because of this branching right we don't get as much we don't get as much surface area and that means that there's decreased attractive forces between molecules of neopentane and because there's decrease attractive forces right that lowers the boiling point and so the boiling point is down to 10 degrees C all right I always think of room temperatures being pretty close to 25 degrees C so most most the time you see it listed as being between 20 and 25 but if room temperature is pretty close to 25 degrees C think about the state of matter of neopentane right we're we're already higher than the boiling point of neopentane so at room temperature and room pressure neopentane is a gas right the molecules have enough energy already to break free of each other and so neopentane is a gas at room temperature and pressure whereas if you look at pentane pentane has a boiling point of 36 degrees C which is higher than room temperature so we haven't reached the boiling point of pentane which means at room temperature and pressure pentane is still a liquid so pentane is a liquid and let's think about the trend for branching here so we're the same number of carbons right same number carbon same number of hydrogen's but we have different boiling points neopentane has more branching and a decreased boiling point so we can say for our trend here as you increase as you increase the branching alright so not talking about number of carbons here we're just talking about branching as you increase the branching you decrease the boiling point because you decrease surface area for the attractive forces let's do let's compare three more molecules here to finish this off let's look at these three molecules let's see if we can explain these different boiling points so once again we've talked about hexane already with a boiling point of 69 degrees C if we draw in another molecule of hexane our only intermolecular force our only intermolecular force is of course a London dispersion forces so I'll just write London here so London dispersion forces which exist between these two nonpolar hexane molecules next let's look at three hexa known alright hexane has six carbons and so does three hexon own one two three four five and six so don't worry about the names of these molecules at this point if you're just getting started with organic chemistry just try to think about what intermolecular forces are present in this video so three hex known also has six carbons and let me draw on another molecule of three hexa known so there's our other molecule let's think about electronegativity and we'll compare this oxygen to this carbon right here oxygen is more electronegative than carbon so oxygen withdraws some electron density and oxygen becomes partially negative this carbon here this carbon would therefore become partially positive and so this is a dipole alright so we have a dipole for this molecule and we have the same same dipole for this molecule three hexagons down here partially negative oxygen partially positive carbon and since opposites attract the partially negative oxygen is attracted to the partially positive carbon on the other molecule of three hexan ohm and so what intermolecular force is that we have dipoles interacting with dipoles so this would be a dipole dipole interaction so let me write that down here so we're talking about a dipole dipole interaction obviously London dispersion forces would also be present right so if you think about this area over here you can think about London dispersion forces but dipole dipole is a stronger intermolecular force compared to London dispersion forces and therefore the two molecules here of three hexa known are attracted to each other more the two molecules of hexane and so therefore it would take more energy for these molecules to pull apart from each other and that's why you see the higher temperature for the boiling point three hex known as a much higher boiling point than hexane and that's because dipole-dipole interactions are stronger intermolecular force compared to London dispersion forces and finally we have three hexanol over here on the right which also has six carbons one two three four five six so we're still dealing with six carbons if I draw in another molecule of three hexanol let me do that up here so sketch in the six carbons and then we have our our oxygen here and then hydrogen like that we know that there's opportunity for hydrogen bonding oxygen is more electronegative than hydrogen so the oxygen is partially negative and the hydrogen is partially positive the same set up over here on this other molecule of three hexanol so partially negative oxygen partially positive hydrogen and so hydrogen bonding is possible let me draw that in so we have a hydrogen bond right here so there's opportunities for hydrogen bonding between two molecules of three hex and also let me use let means deep-blue for that so now we're talking about hydrogen bonding and we know the hydrogen bonding you know the hydrogen bonding is really just a stronger type of dipole-dipole interaction so hydrogen bonding is our strongest intermolecular force and so we have an increased attractive force holding holding these two molecules of three hex and all together and so therefore it takes even more energy for these molecules to pull apart from each other and that's reflected in the higher boiling point four three hex and alright three hexanol has a higher boiling point than three hexa known and also more than hexane so when you're trying to figure out boiling points think about the intermolecular forces that are present between two molecules and that will allow you to figure out which compound has the higher boiling point