Solubility of organic compounds

- [Voiceover] You often hear the phrase like dissolves like when you're talking about solubility and even though this idea isn't perfect, it does allow you to predict the solubility of compounds. For example, a polar solvent will dissolve a polar compound in general, so like dissolves like. I also have here a polar solvent will dissolve in ionic solute because you don't usually describe ionic compounds as being polar. Next, a nonpolar solvent will dissolve a nonpolar compound, so like dissolves like, but a polar solvent will not dissolve a nonpolar compound, so this would be like and unlike here. An example of a polar solvent is water. An example of a nonpolar compound could be something like oil. We know that water will not dissolve oil. Let's go back to this first idea of a polar solvent being able to dissolve a polar compound or a polar solvent dissolving an ionic compound like sodium chloride. We know from experience that sodium chloride, or salt, is soluble in water. Over here on the left we have part of a salt crystal. We know that crystals are held together by attractive forces, the positively charged sodium cation is attracted to the negatively charged chloride anion. Opposite charges attract and our crystal is held together by these attracted forces. If we get some water molecules to come along, we know that water is a polar solvent, water is a polar molecule. The oxygen is more electronegative than this hydrogen, so the oxygen pulls some of the electron density in this bond closer to it giving it a partial negative charge. If we are withdrawing electron density from this hydrogen, this hydrogen gets a partial positive charge. Since opposite charges attract, the partially positive hydrogen in water is attracted to the negatively charged chloride anion, so there's an interaction here. If we get a bunch of water molecules, here's another one right here, so partially negative oxygen, partially positive hydrogen, so there's another attractive force. We can pull off these chloride anions from the solid and bring the anion into solution. On the right here we have our chloride anion in solution surrounded by a bunch of water molecules and we have all these partially positive hydrogens interacting with our negatively charged chloride anion. For the sodium cations let's go back to our solid on the left. Since the sodium cation is positively charged, that's going to interact with the partially negatively charged oxygen in the water molecule, so opposite charges attract and if you get enough water molecules you can pull off these sodium cations and bring the sodium cations into a solution. We have the partially negative oxygens on water interacting with our positively charged sodium cations in our solution. Our polar solvent, water, needs to be able to interact with our solutes and in this case the polar solvent attacks the solid over here on the left and it replaces these ion interactions of our crystal with ion-dipole interactions in our solution. By ion-dipole, I mean we have a cation right here, so that's our ion and then our di-pole would be water, water's a polar molecule, it has di-pole moment, so we have all of these ion di-pole interactions. Ionic solutes that are able to participate in these interactions will dissolve in water. If you have a polar compound, right, a similar idea, you have attractive forces that allow the polar compounds to be dissolved in a polar solvent like water. Let's move on to a nonpolar compound, so a nonpolar compound, something like this molecule on the left here and this molecule's called naphthalene. Naphthalene is a solid with a very distinctive smell to it. The first time I smelled naphthalene in the lab it reminded me of my grandparents' house because my grandparents, when I was a kid, had mothballs that were made of naphthalene, so it's a very distinctive smell. Now naphthalene is nonpolar because it's composed of only carbons and hydrogens, it's a hydrocarbon, so naphthalene is nonpolar and you would need a nonpolar solvent to get it to dissolve. Toluene is a nonpolar solvent, again, this is a hydrocarbon, so if you take solid naphthalene and liquid toluene, naphthalene will dissolve in toluene, so like dissolves like, our nonpolar solvent will dissolve our nonpolar compound. Finally, let's look at this last idea here, so a polar solvent, something like water, should not dissolve a nonpolar compound, something like naphthalene, and that's true, naphthalene will not dissolve in water, so water doesn't interact well enough with the naphthalene molecules to get them to dissolve and form a solution. This concept of like dissolves like is important because it allows you to predict whether or not a compound will be soluble in water. Let's look at several organic compounds and determine whether or not those compounds are soluble in water. We'll start with ethanol. Ethanol has a polar oxygen-hydrogen bond, the oxygen is more electronegative than hydrogen, so the oxygen withdraws some electron density making the oxygen partially negative and leaving the hydrogen partially positive. If water comes along, I'll draw in a water molecule here, and we know that water is a polar solvent, water is a polar molecule, the oxygen has a partial negative and the hydrogens have partial positive charges. We can see that there's an opportunity for an attractive force, opposite charges attract, so the partially positive hydrogen on ethanol is attracted to the partially negatively charged oxygen on water. This is an example of hydrogen bond density, remember hydrogen bonding from earlier videos. Here is a good example of that. We can even have some more hydrogen bonding, I could draw in another water molecule down here, so let me go ahead and do that, we know that the oxygen is partially negative, hydrogens are partially positive, so here's another opportunity for hydrogen bonding between partially negative oxygen on ethanol and the partially positive hydrogen on water. This portion of the ethanol molecule is polar and loves water, so this is the polar region and this portion loves water, we call this hydrophilic, so let me write that down here so this portion of the molecule is hydrophilic, or water loving. Let's look at the other portion of the ethanol molecule, so this portion on the left. We have a CH2 here and a CH3 here, so carbons and hydrogens which we know are nonpolar, so this region is nonpolar, this region doesn't like water, it's scared of water, we call this hydrophobic, or water fearing. We know that ethanol is soluble in water just by experience, so that must mean this hydrophobic region doesn't overcome the hydrophilic region, so the hydrophilic region is polar region of the ethanol molecule, it's enough to make ethanol soluble in water. If you think about that same concept and look at a different molecule, so on the right here's 1-octanol. 1-octanol has an opportunity for hydrogen bonding we have this OH here, so it's the same situation as the ethanol on the left, so we have a polar or hydrophilic region of the molecule. However, the difference is this time we have extremely large nonpolar hydrophobic portion of the molecule. This nonpolar region overcomes the slightly polar region making the 1-octanol molecule nonpolar overall, so 1-octanol will not dissolve in water. This one is a no and this one over here was a yes, ethanol is a yes. Next, let's look at cinnamaldehyde, so down here on the left is cinnamaldehyde, let's focus in on, let's focus in on this carbon oxygen double bond first. We know that oxygen is more electronegative than this carbon here, so the oxygen withdraws from the electron density making it partially negative and this carbon would be there for partially positive. This very small portion of the molecule is polar, this small portion could interact with water. However, we have an extremely large, nonpolar region of the molecule, all of these carbons and hydrogens over here on the left. This very hydrophobic region, or nonpolar region, overcomes the small polar region making cinnamaldehyde overall nonpolar. Since it's overall nonpolar, cinnamaldehyde will not dissolve in water. If it's nonpolar, you would need a more nonpolar solvent to get cinnamaldehyde to dissolve and there are several examples of nonpolar organic solvents that will do that. Next let's look at sucrose, so over here on the right is sucrose or one way to draw or represent the sucrose compound. Now we see lots of carbons and hydrogens, so all of these right here, let me just go ahead and highlight all these carbons in this ring and so all these carbons in these rings, all these hydrogens, so at first you might think okay, there's lots of carbons and hydrogens, this might be nonpolar, but of course we have lots of these OH groups, so I'm gonna go ahead and circle a few of them, right, we have all of these OH groups in the sucrose molecules, so lots of them. That means opportunities for hydrogen bonding. Because of all these opportunities for hydrogen bonding, sucrose is soluble in water which we know from experience. Of course sucrose, or sugar, sugar will dissolve in water, so the opportunity for hydrogen bonding is the reason for that. Benzoic acid is a solid at room temperature. If you take some benzoic acid crystals and you put them in some room temperature water, the crystals won't dissolve. We can explain that by looking at the structure for benzoic acid. While we do have this portion of the compound which we know is polar and hydrophilic due to the presence of the electronegative oxygens, we also have this portion of the compound on the left which is nonpolar and hydrophobic due to the presence of all the carbons and hydrogens. Since the benzoic acid crystals don't dissolve at room temperature water, the hydrophobic portion of the compound must overcome the hydrophilic portion of the compound. You actually can get benzoic acid crystals to dissolve in water if you heat up the water, if you increase the solubility of the compound by increasing the temperature of the solvent. Let's think about benzoic acid crystals in room temperature water and let's add a base, let's add sodium hydroxide. The sodium hydroxide's going to react with the most acidic proton on benzoic acid, so benzoic acid is acidic, it will donate this proton right here. That means the electrons in red in this bond are left behind on the oxygen, so I'll show those electrons in red over here. That's gives this oxygen a negative charge and we form sodium benzoate. I won't get too much into acid base chemistry, but we took the most acidic proton off of benzoic acid to give us the conjugate base sodium benzoate. Sodium benzoate is highly soluble in room temperature water. That must mean we increase this hydrophilic portion because now we have a negative charge, so the hydrophilic portion now is able to overcome the hydrophobic portion. Sodium benzoate is soluble, this negative charge is better able to interact with our solvent which is water.