Solubility of organic compounds

you often hear the phrase light 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 and ionic solute because you don't usually describe ionic compounds as being polar next a non-polar solvent will dissolve a non-polar compound so like dissolves like but a polar solvent will not dissolve a non-polar compound so this would be like an unlike here an example of a polar solvent is water an example of a non-polar compound could be something like oil and 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 so 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 so opposite charges attract and our crystal is held together by these attractive 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 and since opposite charges attract the partially positive hydrogen on 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's there's another attractive force we can pull off these chloride anions from the solid and bring the anion into solution so on the right here we have our chloride anion in saline solution surrounded by a bunch of water molecules and we have all these partially positive hydrogen's 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 so we have the partially negative oxygens on water interacting with our positively charged sodium cations in our solution so 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-ion interactions of our crystal with ion dipole interactions in our solution and by ion dipole I mean we have a cation right here so that's our ion then our dipole would be water water is a polar molecule it has a dipole moment so we have all of these ion dipole interactions so ionic solutes that are able to participate in these interactions and right 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 non-polar compound so a non-polar compound is something like this molecule on the left here and this molecule is called naphthalene naphthalene is a solid with a very distinctive smell to it so 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 naphthalene is non polar because it's composed of only carbons and hydrogen's it's a hydrocarbon so naphthalene is nonpolar and you would need a non-polar solvent to get it to dissolve toluene is a non-polar 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 non-polar solvent will dissolve our non-polar compound but finally let's look at this last idea here so a polar solvent something like water should not dissolve a non-polar 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 so 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 and 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 as a partial negative and the hydrogen's 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 bonding so if you remember hydrogen bonding from earlier videos here is a good example of that we could even have some more hydrogen bonding I could draw on another water molecule down here so let me go ahead and do that we know that the oxygen is partially negative the hydrogen's are partially positive and so here's another opportunity for hydrogen bonding Queen the partially negative oxygen on ethanol and the partially positive hydrogen on water so 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 and loving let's look at the other portion of the ethanol molecule so this portion on the Left we have a ch2 here at a ch3 here so carbons and hydrogen's which we know are non-polar so this region is nonpolar this region doesn't like water it's scared of water we call this hydrophobic or water theory so we know that ethanol is soluble in water just by experience so that must mean that this hydrophobic region doesn't overcome the hydrophilic region so the hydrophilic region this polar region of the ethanol molecule is enough to make ethanol soluble and water if we think about that same concept and we look at a different molecule so on the right here's one octenol when octanol has an opportunity for hydrogen bonding right we have this o H here so it's the same situation as the ethanol on the left so we have a a polar or hydrophilic region of the molecule however the difference is this time we have extremely large nonpolar hydrophobic portion of the molecule and this nonpolar region overcomes the slightly polar region making the 1-octanol molecule nonpolar overall so one octave 1-octanol will not dissolve in water so this one is a no and this one over here was 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 them on this carbon oxygen double bond first we know that oxygen is more electronegative than this carbon here so the oxygen withdraws some electron density making it partially negative and this carbon would be therefore partially positive so this very small portion of the molecule is polar this small portion could interact with water however we have extremely large nonpolar region of the molecule all these carbons and hydrogen's over here on the left and so this very hydrophobic region or nonpolar region overcomes the small polar region making cinnamaldehyde overall nonpolar since it's over all nonpolar it's animal head 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 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 hydrogen's so all of these right here let me just go ahead and highlight all these carbons in this ring alright so all there all these carbons in these rings they're all these hydrogen's so you at first you might think okay there's lots of carbons and hydrogen's this might be this might be nonpolar but of course we have lots of these Oh H groups so I'm going to go ahead and circle a few of them right we have all of these o H groups in the sucrose molecule so lots of them and that means opportunities for hydrogen bonding and because of all those 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 and 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 hydrogen's and 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 eat up the water if you increase the solubility of the compound by increasing the temperature of the solvent but let's think about benzoic acid crystals in room temperature water and let's add a base let's add sodium hydroxide so the sodium hydroxide is going to react with the most acidic proton on benzoic acid so benzoic acid is acidic it will donate this proton right here and that means the electrons in red and this bond are left behind on the oxygen so I'll show those electrons in red over here that gives this oxygen a negative charge and we form sodium benzoate so 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 and sodium benzoate is highly soluble in in room-temperature water and that must mean we increase this hydrophilic portion but because now we have a negative charge so the hydrophilic portion now is able to overcome the hydrophobic portion and sodium benzoate is soluble this negative charge is better able to interact with our with our solvent which is water