Related questions How do I determine the molecular shape of a molecule? What is the lewis structure for co2? What is the lewis structure for hcn?
How is vsepr used to classify molecules? What are the units used for the ideal gas law? When you place a non-polar molecule in a polar solvent like oil in water the molecules try to minimize surface contact between them. This is actually the basis for the cells in our bodies. The lipids oily fatty acids form our cell membranes so that their non-polar tails face inward away from the polar cytoplasm and the polar heads face towards the polar cytoplasm.
Although much of the explanation for why certain substances mix and form solutions and why others do not is beyond the scope of this class, we can get a glimpse at why solutions form by taking a look at the process by which ethanol, C 2 H 5 OH, dissolves in water. Ethanol is actually miscible in water, which means that the two liquids can be mixed in any proportion without any limit to their solubility.
Much of what we now know about the tendency of particles to become more dispersed can be used to understand this kind of change as well. Picture a layer of ethanol being carefully added to the top of some water Figure below. Because the particles of a liquid are moving constantly, some of the ethanol particles at the boundary between the two liquids will immediately move into the water, and some of the water molecules will move into the ethanol. In this process, water-water and ethanol-ethanol attractions are broken and ethanol-water attractions are formed.
The attractions that form between the ethanol and water molecules are also hydrogen bonds Figure below. Because the attractions between the particles are so similar, the freedom of movement of the ethanol molecules in the water solution is about the same as their freedom of movement in the pure ethanol. The same can be said for the water. Because of this freedom of movement, both liquids will spread out to fill the total volume of the combined liquids. In this way, they will shift to the most probable, most dispersed state available, the state of being completely mixed.
There are many more possible arrangements for this system when the ethanol and water molecules are dispersed throughout a solution than when they are restricted to separate layers. Figure below. We can now explain why automobile radiator coolants dissolve in water. These substances mix easily with water for the same reason that ethanol mixes easily with water. The attractions broken on mixing are hydrogen bonds, and the attractions formed are also hydrogen bonds.
There is no reason why the particles of each liquid cannot move somewhat freely from one liquid to another, and so they shift toward the most probable most dispersed , mixed state. We have a different situation when we try to mix hexane, C 6 H 14 , and water.
If we add hexane to water, the hexane will float on the top of the water with no apparent mixing. The reasons why hexane and water do not mix are complex, but the following gives you a glimpse at why hexane is insoluble in water.
There actually is a very slight mixing of hexane and water molecules. Carbon chain on the other hand as nonpolar is repelled. Solubility of alcohols is therefore determined by the stronger of the two forces. Because of the strength of the attraction of the OH group, first three alcohols methanol, ethanol and propanol are completely miscible.
At about four or five carbons, the hydrophobic effect begins to overcome the hydrophilic effect, and water solubility is lost. Now, try dissolving glucose in the water — even though it has six carbons just like hexanol, it also has five hydrogen-bonding, hydrophilic hydroxyl groups in addition to a sixth oxygen that is capable of being a hydrogen bond acceptor. We have tipped the scales to the hydrophilic side, and we find that glucose is quite soluble in water.
We saw that ethanol was very water-soluble if it were not, drinking beer or vodka would be rather inconvenient! How about dimethyl ether, which is a constitutional isomer of ethanol but with an ether rather than an alcohol functional group?
We find that diethyl ether is much less soluble in water. Is it capable of forming hydrogen bonds with water? Yes, in fact, it is —the ether oxygen can act as a hydrogen-bond acceptor. The difference between the ether group and the alcohol group, however, is that the alcohol group is both a hydrogen bond donor and acceptor. The result is that the alcohol is able to form more energetically favorable interactions with the solvent compared to the ether, and the alcohol is therefore more soluble.
Here is another easy experiment that can be done with proper supervision in an organic laboratory. Try dissolving benzoic acid crystals in room temperature water — you'll find that it is not soluble. As we will learn when we study acid-base chemistry in a later chapter, carboxylic acids such as benzoic acid are relatively weak acids, and thus exist mostly in the acidic protonated form when added to pure water. Acetic acid, however, is quite soluble. This is easy to explain using the small alcohol vs large alcohol argument: the hydrogen-bonding, hydrophilic effect of the carboxylic acid group is powerful enough to overcome the hydrophobic effect of a single methyl group on acetic acid, but not the larger hydrophobic effect of the 6-carbon benzene group on benzoic acid.
Now, try slowly adding some aqueous sodium hydroxide to the flask containing undissolved benzoic acid. As the solvent becomes more and more basic, the benzoic acid begins to dissolve, until it is completely in solution.
What is happening here is that the benzoic acid is being converted to its conjugate base, benzoate. The neutral carboxylic acid group was not hydrophilic enough to make up for the hydrophobic benzene ring, but the carboxylate group, with its full negative charge , is much more hydrophilic.
Now, the balance is tipped in favor of water solubility, as the powerfully hydrophilic anion part of the molecule drags the hydrophobic part, kicking and screaming, if a benzene ring can kick and scream into solution. If you want to precipitate the benzoic acid back out of solution, you can simply add enough hydrochloric acid to neutralize the solution and reprotonate the carboxylate.
If you are taking a lab component of your organic chemistry course, you will probably do at least one experiment in which you will use this phenomenon to separate an organic acid like benzoic acid from a hydrocarbon compound like biphenyl. Similar arguments can be made to rationalize the solubility of different organic compounds in nonpolar or slightly polar solvents.
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