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Current time:0:00Total duration:9:24

Video transcript

let's think a little bit about some of the properties of alcohol so the general formula for an alcohol we saw is some type of group or chain of carbons bonded to an oxygen bonded to a hydrogen and of course the oxygen will have two lone pairs just like that let's compare this to water so water just looks like this you know hydrogen bonded to an oxygen bonded to another hydrogen with two lone pairs now in the case of water the oxygen is much more electronegative than the hydrogen so it hogs the electrons towards it so you have a partial you have a partial negative charge at the oxygen end and then you have partial positive charges you have partial positive charges at the hydrogen ends and that's what Oh that's what allows oxygen to chi or sorry that's what allows water to kind of bond to itself or to kind of have not a ridiculously low boiling point so let me show this so let me copy and paste this and we've seen all this before in regular chemistry so copy and paste so let me draw some more water molecules here and let me draw another water molecule here so you see water because the oxygen end has a partial negative charge and the hydrogen ends have partial positive charges the oxygen of one water molecule will be attracted to the hydrogen of another water molecule and we've seen this before this we call hydrogen bonding so that right there is hydrogen bonding hydrogen bonding the exact same thing can happen with alcohols although alcohols really only have the partial positive charge on the hydrogen we don't know exactly what's going on here we have probably have carbons bonded to the oxygen and with the carbons they're not as they're reasonably electronegative they're not going to have their electrons hogged as much as a hydrogen would so in the case of an alcohol actually let me draw instead of having this R 4 radical there let me make it a little bit more concrete let me draw an actual alcohol so an actual alcohol maybe we have methanol maybe we have methanol it would look like that it has a hydrogen right over here ox is much more electable electronegative than the hydrogen so you have a partial negative charge there and then you have a partial positive charge there so it - because of these hydrogen bonds it will have a reasonable boiling we won't just turn immediately into the gaseous state it would actually try to bond to each other let me copy and paste that so it can also form the hydrogen bonds although they won't be quite as strong as what you see in water and that's why something like methanol actually has a lower boiling point than water it's easy to make it boil it's easier to make these bonds break apart because you don't have as much of the hydrogen bonding so this is an example of hydrogen bonding with methanol now because methanol can have hydrogen bond hydrogen bonding and it has this slight polarity to it and water obviously has hydrogen bonding methanol is actually miscible in water and all that means is that it's soluble in water in any proportion no matter how much methanol or how much water you have it is soluble so if I were to draw some methanol molecules you may be actually maybe this is the water right here so if you drop a methanol molecule right there this would have that would have a hydrogen bond right over there or draw another methanol molecule maybe right over here you would have another hydrogen bond right over there and that's what allows methanol to be soluble in water now as this chain grows or if you have alcohols with longer radical chains then they become less and less soluble in water but their boiling points actually do go up and let's think about why that is so if I have something like let me do butanol so butanol is going to have 4 carbons so it's going to be a ch3 ch3 let me just draw it like a ch3 see ch2 ch2 CH let me do it like this H to see then that carbon that last carbon right there is going to be bonded to the oxygen it's going to be bonded to an oxygen which is bonded to a hydrogen now when you have a situation like this the oxygen will have a partial negative charge the hydrogen will still have a partial positive charge just like we saw up here with both the water and the methanol but now you have this big thing here that has no polarity so this part of the alcohol is not going to be soluble in water it's going to make it harder for this part to be soluble over here so this right here is less soluble this is less soluble it still be a little bit soluble so if you have some oxygen here you will still have a little bit of the hydrogen bonding you still will have a little bit of the hydrogen bonding going on but this part is kind of you can imagine it's almost you know it doesn't want to dissolve with the water it is non-polar and so this is you could actually for example butanol in particular it actually is soluble in water but not in any proportion so methanol is miscible let me write this and this is a new word I don't think I've ever used it before in the context of the organic chemistry videos so the methanol is let me write that in a brighter color since it's a new word methanol is miscible miscible which just means soluble in any proportion soluble soluble in any proportion so I don't care what what percent is is is methanol what percentage water they will the methanol will dissolve into the water in any proportion if you look at butanol it is soluble but not in any proportion if you had a ton of butanol some of it would not dissolve in the water so this is soluble so the butanol right here is soluble soluble but not not miscible not miscible in water if you have too much of the butanol all of a sudden some of it will not actually be able to be dissolved and if this chain if this was a deccan all or something with a really long carbon chain then of course it's going to be very non soluble I mean you might be able to get a couple of molecules in the water but most of them will not dissolve now the other reason I you know I hinted look you know the reason why the alcohols have well you know not to lovo boiling point is that they're able to do this hydrogen bonding but you would say well look you know these longer carbon chains these are going to have less of the hydrogen bonding going on maybe these would have lower boiling points but actually the longer the chain gets these actually have higher boiling points and that's because these chains can interact with each other so the longer the chain so longer longer are or the longer our chain I guess I could say we could say the higher the boiling point in an alcohol higher boiling point it's harder you have to put more heat into the system or the temperature has to be higher for the things to be rigged apart and that's because this is one Deccan all molecule here another Deccan all molecule might look like this maybe might look like this if an oxygen and a hydrogen and then you have your carbons so you have your CH your CH 2 CH 2 H 3 C so you have this other butanol here and what the interaction between these two chains are dependent you know these these are the Vander Waal forces so you know even though they have no products so these guys are going to have some polar interactions they're going to have the hydrogen bonding we've seen that multiple times already but these long chains they're going to have the London dispersion forces which are a subset of Vander Waal forces where even though they're neutral every now and then one of these might become slightly negative on one side so you might have a very temporary you might have a temporary partial negative charge and that's just because of the randomness of how electrons move on this side of the molecule all of a sudden you might have more electrons over there so you have a partial negative charge and and because of that you're going to have the electrons over here get what they're not going to want to be there so you're going to have a partial positive charge then you can have a very temporary interaction that's a very weak force much weaker than hydrogen bonds but as these chains get longer and longer is is they possibly even get intertwined with each other and get close to each other these London dispersion forces or Vander Waal forces are going to keep propagating so all of a sudden maybe these guys are going to be attracted to each other and that's going to disappear then these guys are going to be attracted to each other and then that's going to disappear and then these guys are going to be attracted to each other and then that's going to disappear and so you can imagine the longer the chain the more of these type of interactions you're going to have the more attracted they're going to be to each other and it's going to be harder to break them apart higher boiling point so those are just kind of the two big takeaways on the properties of alcohols they are the especially smaller chained alcohols are soluble in water the very small ones are completely miscible and the longer the chain you have the harder is to dissolve in water but also the higher the boiling point the heart the harder it is to break them apart because you have these London dispersion forces