in this video i want to talk about intermolecular forces particularly dipole-dipole interactions and hydrogen bonding so let's start with dipole-dipole interactions this occurs usually between polar molecules for example consider the molecule acetone it's a polar molecule and the central carbon is double bonded to an oxygen atom which has two lone pairs now the oxygen atom has a partial negative charge and the carbon atom has a partial positive charge now what do you think is going to happen if that acetone molecule is placed nearby another acetone molecule if that happens there's going to be a dipole-dipole interaction the carbon from the second molecule is going to be attracted to the oxygen of the first molecule because opposites attract and so this negatively charged oxygen atom will be attracted to this partially positively charged carbon atom and so this creates the dipole dipole interaction so keep in mind when you're dealing with intermolecular forces you're dealing with forces or interactions between molecules and not necessarily within a molecule let's consider another example and that is carbon monoxide which looks like this the carbon bears a partial positive charge and the oxygen bears a partial negative charge and let's draw another carbon monoxide molecule next to it so each of these molecules possess a dipole moment and so you can call it a dipole and so the interaction between one dipole with another is known as a dipole dipole interaction so that's an interaction between two separate molecules and so this is the dipole-dipole interaction and so those two molecules are attracted to each other as you know opposites attract so they feel a force that will pull them together now hydrogen bonding is a special type of dipole dipole interaction it occurs when hydrogen is attached to one of these three elements nitrogen oxygen or fluorine so let's use water as an example so the oxygen of water bears a partial negative charge and the hydrogen bears a partial positive charge and so the oxygen of one water molecule is attracted to the hydrogen of another and so these hydrogen bonds or these special type of dipole dipole reactions they keep the water molecules held together and so this is an h bond it's between two separate water molecules as opposed to within a water molecule now whenever you have hydrogen bonding what it does to a molecule is it increases the boiling point of the molecule and also it increases the water solubility of a molecule for example ammonia has hydrogen bonds in it and so ammonia is polar and it's going to be highly soluble in water methanol has hydrogen bonds in it you can see the oh bond and as a result it has a relatively higher boiling point compared to other molecules that do not have hydrogen bonds and also the solubility of methanol in water is very high methanol mixes completely with water now let's compare ethanol and dimethyl ether which of these two molecules has a higher boiling point and which one has a higher solubility in water ethanol we could see has hydrogen bonds and so it's highly polar dimethyl ether is polar as well because of the oxygen and the fact that it has a bent shape so it has a similar molecular geometry to that of water however notice that hydrogen is not directly attached to oxygen and so it doesn't have any hydrogen bonds therefore because hydrogen bonds have more intermolecular forces or rather stronger intermolecular forces than regular dipole dipole reactions ethanol is going to have a higher boiling point and as a result it will also have a higher solubility in water due to the age bonds the boiling point of ethanol is about 78 degrees celsius and for dimethyl ether it's negative 23 celsius so you can see that this is a lot higher than that number and so hydrogen bonds will greatly increase the boiling point and the solubility in water of a molecule now what if we compare ethanol with one in butanol which one has a higher boiling point and which one has a greater solubility in water now both of these molecules they contain the hydrogen bonds however notice that the size of the hydrocarbon chain is not the same for butanol there's more atoms in that molecule and as a result in addition to hydrogen bonds it's going to have more london dispersion forces as a result it's going to have a higher boiling point so molecules with a large number of carbon atoms tend to have a higher boiling point than molecules with only a small number of carbon atoms assuming everything else is the same in this case they both have the same o-h functional group now what about water solubility and which one is more soluble in water now you need to know that the o h bond is polar but the c h bond is nonpolar and water is a polar molecule light dissolves like so molecules with the o h group will be highly soluble in water whereas those with a ch group will not be soluble in water now ethanol is soluble in water and one butanol is still soluble in water however because of this nonpolar chain the solubility will be less than that of ethanol so ethanol is going to have a greater solubility in water because the nonpolar region is smaller than this bulky nonpolar region in fact the solubility of butanol it's it's small in water you can dissolve some of it but not much another example is one octanol where we have a total of eight carbon atoms now this molecule still has hydrogen bonds however because it has such a huge bulky nonpolar region it's not soluble in water in fact it mixes with other nonpolar molecules like oil and things like that but it doesn't mix in water because the oh part is soluble in water but the the ch part is not the ch bonds are non-polar so they don't want to mix with water and so whenever you have like small chain alcohols like ethanol or even methanol these are highly soluble in water but when you start adding a lot of ch groups or ch2 groups then the solubility will decrease but the boiling point will go up so one octanol has the highest boiling point of these three molecules but methanol has the highest solubility because it has the least number of nonpolar bonds now let's consider two other molecules pentane which has five carbons and neopentane which also has five carbons now these are considered constitutional isomers because they have the same chemical formula c5h12 but a different structure they're connected differently the molecular formula is the same but the connectivity differs so which one is going to have a higher boiling point since they have the same number of carbon atoms if the number of carbon atoms are the same then what you could do is look at the way it's structured straight chained alkanes have higher boiling points than branched alkanes so pentane is going to have a higher boiling point than neopentane neopentane on the right is going to have a lower boiling point because it has less surface area when it's brashed out but if you have a straight chain alkane it's going to have more surface area and as a result it's going to have more london dispersion forces and any time you increase the intermolecular forces the boiling point of that molecule will increase you increase the number of interactions between separate molecules now none of these are soluble in water because they have no oh groups a molecule that contains only carbon hydrogen bonds like methane ethane or propane all of these molecules they're nonpolar and so they do not mix with water you