Intermolecular Forces - Hydrogen Bonding, Dipole Dipole Interactions - Boiling Point & Solubili

In this video I want to talk about

intermolecular

forces

, particularly

dipole

-

dipole

interactions

and

hydrogen

bonds, so let's start with dipole-dipole

interactions

, this usually occurs between polar molecules, for example consider the molecule acetone, it is a molecule polar and the central carbon has a double bond. an oxygen atom that 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 will happen if that acetone molecule is placed near another molecule of acetone? If that happens, Being a dipole-dipole interaction, the carbon in the second molecule will be attracted to the oxygen in the first molecule because opposites attract, so this negatively charged oxygen atom will be attracted to this carbon atom. partially positively charged and this creates the dipole dipole interaction, so keep in mind that when it comes to

intermolecular

forces

, these are forces or interactions between molecules and not necessarily within a molecule.

Let's consider another example, carbon monoxide, which looks like this: Carbon has a partial positive. charge and oxygen has a partial negative charge and let's draw another carbon monoxide molecule next to it so that each of these molecules has 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 is an interaction between two separate molecules and this is the dipole-dipole interaction and those two molecules attract each other, as you know, opposites attract, so they feel a force that will bring them together now, the

hydrogen

bond It is a special type of dipole dipole interaction that occurs when hydrogen bonds with one of these three elements nitrogen, oxygen or fluorine, so let's use water as an example so the oxygen in water has a partial negative charge and the hydrogen has a negative charge. partial positive and therefore the oxygen of one water molecule is attracted to the hydrogen of another and therefore these hydrogen bonds or this special type of dipole-dipole reactions keep the water molecules together and therefore Therefore, this is an h bond that is made between two separate water molecules, rather than within one water molecule. molecule now, whenever you have hydrogen bonds, what it does to a molecule is it increases the

boiling

point

of the molecule and it also increases the water

solubili

ty of a molecule, for example, ammonia has hydrogen bonds, so what ammonia is polar and is going to be highly soluble in water, methanol has hydrogen bonds, 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 bonds of hydrogen and also the

solubili

ty of methanol in water is very high. methanol mixtures completely with water, now let's compare ethanol and dimethyl ether, which of these two molecules has a higher boiling point and which has a higher solubility in water.

We could see that ethanol has hydrogen bonds, so it is highly polar. Dimethyl ether is also polar due to the oxygen and the fact that it has a curved shape, so it has a similar molecular geometry to water. However, note that hydrogen is not directly bonded to oxygen and therefore does not have any hydrogen bonds, because hydrogen bonds have more intermolecular forces or rather stronger intermolecular forces than regular dipole dipole reactions, ethanol It will have a higher boiling point and as a result it will also have a higher solubility in water due to age bonds, the boiling point of ethanol is about 78 degrees Celsius and for dimethyl ether. is negative 23 degrees Celsius, so you can see that it is much higher than that number, so hydrogen

bonding

will greatly increase the boiling point and water solubility of a molecule.

What happens if we compare ethanol with one in butanol, which one has a greater? boiling point and which has greater solubility in water now both molecules contain hydrogen bonds, however, notice that the size of the hydrocarbon chain is not the same for butanol, there are more atoms in that molecule and as a result, in addition of the hydrogen bonds, you will have more London dispersion forces, as a result you will 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 so.

The same, in this case 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 oh bond is polar but the ch bond is nonpolar and water is a polar molecule. dissolves like this, molecules with the o h group will be highly soluble in water, while those with a ch group will not be soluble in water. Now ethanol is soluble in water and a butanol is still soluble in water; However, due to this non-polar chain, the solubility will be less than that of ethanol, so ethanol will have a higher solubility in water because the non-polar region is smaller than this bulky non-polar region.

In fact, the solubility of butanol is small in water, it can dissolve a little, but not much. Another example is an octanol where we have a total of eight carbon atoms, now this molecule still has hydrogen bonds, however, because it has such a large and bulky non-polar region, it is not soluble in water, in fact, it is It mixes with other non-polar molecules like oil and things like that, but it doesn't. It doesn't mix in water because the oh part is soluble in water, but the ch part is not. Ch bonds are non-polar so they don't want to mix with water, so as long as you have small chain alcohols like ethanol or even methanol it is highly soluble in water, but when you start adding a lot of ch or ch2 groups, the solubility will decrease but the boiling point will increase, so an octanol has the highest boiling point of these three molecules, but methanol has the highest solubility.

Because it has the fewest nonpolar bonds, let's now consider two other molecules, pentane, which has five carbons, and neopentane, which also has five carbons. They are now considered constitutional isomers because they have the same chemical formula c5h12 but a different structure, they are connected differently. the molecular formula is the same but the connectivity differs then which one will have a higher boiling point since they have the same number of carbon atoms, if the number of carbon atoms is the same then what you could do is look at the way it is Straight chain structured alkanes have higher boiling points than branched alkanes, so pentane will have a higher boiling point than neopentane.

The neopentane on the right will have a lower boiling point because it has less surface area when it is removed, but if you have a straight chain alkane it will have more surface area and as a result will have more London dispersion forces and each Once you increase the intermolecular forces, the boiling point of that molecule will increase, you will increase the number of interactions between separate molecules, now none of These are soluble in water because they don't have oh groups. A molecule containing only carbon hydrogen bonds such as methane, ethane, or propane. All of these molecules are non-polar and therefore do not mix with water.