If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Solubility of organic compounds

Organic compounds tend to dissolve well in solvents that have similar properties to themselves. This principle is often referred to as "like dissolves like," which means that polar molecules will generally dissolve well in polar solvents and non-polar molecules will generally dissolve in non-polar solvents.

Want to join the conversation?

  • mr pants teal style avatar for user Austin Kemper
    Yes but WHY does non-polar dissolve in non-polar. You drew such a helpful diagram for the polar-polar one, what about the non-polar - non-polar explanation?
    (48 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user Jagat Budhathoki
      It has to do with London Dispersion Force. Due to transient fluctuation of charge density, non polar molecule develops instantaneous dipole moment. When another non polar molecule comes close to this transient dipole, it responds by producing transient dipole moment of its own. So, the net effect is the interaction between instantaneous dipole moments between solvent and solute non-polar molecules which results in dissolving non-polar by non-polar.
      (39 votes)
  • female robot grace style avatar for user Anna
    How do you know if a nonpolar compound will dissolve in water or not? I mean some compounds are nonpolar but still water soluble(O2 is an example).
    (25 votes)
    Default Khan Academy avatar avatar for user
    • piceratops tree style avatar for user Yasmeen.Mufti
      O2 dissolves in water, but 'dissolving' is a physical property not a chemical one. It just gets trapped there.
      Some 'non polar' compounds do chemically interact with water, because only a part of their molecule is non polar whilst another part is polar. Ethanol is a good example: The 'hydrocarbon' section doesn't mix with water, by the OH section forms hydrogen bonds with the water, which is why vodka can exist.
      (45 votes)
  • piceratops ultimate style avatar for user Gözde Polat
    The part I don't understand is, if ionic bonds are stronger than partial polar bonds (dipole dipole attraction) why would NaCl be dissolved in H2O isnt the attraction between Na and Cl stronger then the attraction between the Na and partially negative Oxygen and likewise between the Cl and the partially positive hydrogen... It felt counterintuitive if the fact that "ionic bonds are really stronger" then the other type of polar bonds - I thought it made more sense for NaCl to stay together
    (22 votes)
    Default Khan Academy avatar avatar for user
    • mr pink red style avatar for user Anthony Scaletti
      I had the same thought when I first encountered this topic. It is not a formal explanation, but I think about it in terms of concentration. There is so much more water than there is salt. And the partial charges of the water are attracted to the strongest opposite charge around, which are the respective ions of the table salt. Since there are so many water molecules available, by sheer volume the partial charges are able to overcome the ionic bond. Add too much salt and you run out of water molecules available to break the ionic bonds and the salt sinks to the bottom (saturated solution).
      However, if you add a non-polar compound to the water. The partial charges of water still attract to the strongest opposite charge around, which in this case would be other water molecules.
      A long-winded way of saying 'like dissolves like'
      (31 votes)
  • aqualine ultimate style avatar for user Nikhil Naidu
    How many hydro carbons(Hydrophobic parts) are necessary to over come the hydrophilic part of the compound like in ethanol the OH group made the compound soluble in water and in 1-octanol the OH group didn't had an effect on the solubility of the compound, like how many hydrocarbons are needed to net out the effect ?
    (9 votes)
    Default Khan Academy avatar avatar for user
    • spunky sam blue style avatar for user Ernest Zinck
      The dividing line is four carbons.
      Methanol, ethanol, and propan-1-ol are infinitely soluble in water.
      Butan-1-ol is partially soluble at 9 g/100 mL.
      The solubility of pentan-1-ol is 2.7 g/100 mL.
      Many people call this "insoluble".
      The solubility of octan-1-ol is 0.054 g/100 mL. That's definitely insoluble!
      (12 votes)
  • male robot johnny style avatar for user siddharth.bhatia17
    Polar solvent interacts with ions and because of dipole interaction it dissolves. But what kind of interaction is taking place in case of non polar solvent dissolving non polar molecules?
    (6 votes)
    Default Khan Academy avatar avatar for user
    • leaf red style avatar for user Zaznaow
      The main factor that makes nonpolar solvents dissolve in nonpolar solutes is not energetic favorability but entropic favorability; a solution of two neutral compounds is more random than two separate nonpolar compounds, with very little difference in the energetic favorability of the system.
      (8 votes)
  • blobby green style avatar for user ramya kommuri
    mostly alcohols are soluble in water then why isn't 1-octanol soluble ?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • aqualine sapling style avatar for user gizelle.gepigon
      Although octanol has a hydrophilic OH group, it also has a long chain of hydrophobic hydrocarbons (CH3 and CH2) in its structure. Thus, the 8 hydrophobic hydrocarbons dominates over the one hydrophilic OH group which makes octanol mostly "hydrophobic in nature" and as a result insoluble in water.

      Also, the alcohols that are mostly soluble in water are smaller alcohols with lesser than 3 or 4 carbons. I hope my answer helped your question. May you have a nice day.
      (11 votes)
  • duskpin ultimate style avatar for user Jack Yao
    How do you determine if a compound(Or something) is nonpolar or polar.
    (4 votes)
    Default Khan Academy avatar avatar for user
    • orange juice squid orange style avatar for user Vivek Anand
      Usually one dissolves the given compound in a known polar solvent and a non polar solvent.
      For example let's take water and CS2( Carbon disulphide). Water is a polar solvent while the latter is a nonpolar solvent.
      If the substance dissolves easily in the water If it dissolves and sparingly or not at all in CS2 it it is usually polar.
      If the substance dissolves in CS2 but does not dissolve in water it is usually nonpolar.
      To determine if a substance is non polar or polar we have to check for the presence of polar groups attached to the compound, the spatial arrangement of the molecule, lines of symmetry,hybridisation etc. Then we have to analyze each of these factors to weigh in their contribution. This can be quite challenging sometimes and is usually used to explain phenomena not to predict them.
      (4 votes)
  • leaf orange style avatar for user Berkamin
    I learned a while ago that soaps and detergents and surfactants are hydrophilic at one end, and hydrophobic on the other. In light of this, and in light of some of these molecules you described being partially hydrophilic and partially hydrophobic, could it be said that alcohol is 1) a surfactant or a soap, and 2) would work at dissolving grease? How about the others? What exactly is the difference between a surfactant and a soap or a detergent?

    Specifically, I heard that saponified fats have a long hydrophobic tail and a hydrophilic head. But if the hydrophobic portion is large, wouldn't this overwhelm the hydrophilic portion? Soap was historically rendered from animal fats; doesn't fat have a long carbon + hydrogen tail? Why would it be water soluble at all?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • spunky sam blue style avatar for user Ernest Zinck
      Ethanol is not a surfactant. Neither is it a good solvent for grease, because it is too polar.

      A surfactant is a substance that reduces the surface tension of water.
      Both soaps and detergents are surfactants.

      Soaps are the salts of fatty acids. A typical soap is sodium stearate, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂COO⁻, Na⁺.
      Most detergents are synthetic surfactants. They often have a sulfonate group (SO₃⁻) at one end. A typical detergent is sodium dodecylbenzenesulfonate, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂C₆H₄SO₃⁻, Na⁺.
      As you can see, both saponified fats (soaps) and detergents have a hydrophilic (ionic) head and a hydrophobic (nonpolar) tail.

      Soaps are slightly soluble at low concentrations.
      As the concentration increases, the molecules band together to form *"micelles* — spherical globules in which the nonpolar tails point to the inside of the micelle, and the water-loving ionic heads present a united front to the water molecules.
      Thus, the soaps don’t really dissolve. Rather, they form colloidal suspensions.
      (5 votes)
  • male robot donald style avatar for user vanaparthisuhas
    so all hydrocarbons are non polar and don't dissolve in water[non polar] else there are any exceptions?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user btremelling
      Correct, to my knowledge, there are no stable hydrocarbons which will dissolve in polar solvents. Now, there are intermediates to reactions where carbanions are formed (carbon's with a negative charge) and carbocations (carbons with a positive charge) are formed, but as stated, these are only intermediates. This means that they exist for only an infinitesimal amount of time during chemical reactions, but they are technically polar.
      (2 votes)
  • winston default style avatar for user J.R. Foster
    How do you know when the hydrophobic portion of a molecule will overcome the hydrophilic portion of a molecule and vice versa?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • eggleston blue style avatar for user Aarushi
      It usually depends on the lengths of the hydrophobic and hydrophilic portion of a molecule. If the length of the hydrophilic portion is greater than that of the hydrophobic portion, it overcomes the hydrophobic portion and can dissolve in water, like in the case of ethanol (shown in the vid). Whereas, in the case of 1- octanol, the hydrophobic portion is much larger than that of the hydrophilic portion and thus it cannot dissolve in water
      (1 vote)

Video transcript

- [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.