Clearly, the same favorable water-alcohol hydrogen bonds are still possible with these larger alcohols. The difference, of course, is that the larger alcohols have larger nonpolar, hydrophobic regions in addition to their hydrophilic hydroxyl group. 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. See special feature on distilled water in Unit 2. Compare how well polar, slightly polar, and nonpolar liquids dissolve substances. In our experiment in Activity 4, we found that water dissolves ionic salts and polar covalent compounds such as alcohol.
We also saw that water is far less effective as a solvent for nonpolar covalent compounds such as oil. However, a list of substances in seawater suggests that water can dissolve small quantities of almost any substance. B The properties of water e.
To understand how water dissolves substances, let us concentrate first on compounds that water dissolves easily — the ionic and polar covalent compounds.
With these compounds it is the exceptionally strong polarity of water that gives it its dissolving power. The ionic salt sodium chloride NaCl is a good model of how this dissolving takes place.
See Fig. The bonding between the ions and water is strong, and shortly the ions are as strongly attracted to the water as to each other. As other water molecules collide with the ion-containing clusters, they knock them off, casting them into the solution. An ion surrounded by water is called a hydrated ion. A similar process occurs in the dissolving of polar covalent compounds except that the water is attracted to the poles of the dissolving polar compound.
For example, sugar is a large polar molecule with negatively charged OH groups that help sugar easily dissolve in water. Water is not attracted to everything. Because water molecules are polar, they are more attracted to molecules that are also polar or that have a charge like an ion.
Some kinds of molecules, like oils and fats, are nonpolar. These nonpolar molecules have no charge, and so water is not very attracted to them. Molecules of nonpolar compounds, such as oil and gasoline, even when mixed well into water, tend to separate from the water when the mixing stops. The key point is that you should have been able to pile many more drops of water on the penny than drops of alcohol.
The explanation here is pretty much identical to the one above. The water molecules have much more cohesion that alcohol molecules, because they grab onto one another through hydrogen bonds. Because a sphere allows for the maximum number of connections between water molecules, the expanding drop of water continues to maintain that shape. At a certain point, the sphere expands to where it overflows the penny, and water spills out over the side. However long you observed for, you should have noted that it takes much longer for water to evaporate than alcohol.
In other words, the amount of water that evaporated was much less than the amount of alcohol that evaporated. Because of hydrogen bonding. The water molecules, like the alcohol molecules, are vibrating and moving. But hydrogen bonds keep the water molecules from jumping away from the surface of the drop, so that the drop of water evaporates very slowly.
Consequently, when alcohol molecules accelerate to a high enough speed to jump away from the surface of the liquid, they take off. There are no hydrogen bonds holding them back. As molecule after molecule of alcohol jumps away from the liquid, the volume of the liquid decreases…until all of it has evaporated. As they absorb heat from your skin, they start to move faster and faster.
As they start to evaporate, they carry their heat energy away from your skin. You perceive that loss of heat energy as coolness. Salt will NOT dissolve in alcohol. Science is all about repeatable, verifiable results. Solutions are homogeneous mixtures of two or more pure substances. For our purposes, we will generally be discussing solutions containing a single solute and water as the solvent. What is a solvent? In crudest terms it is the molecule in the mixture with the highest concentration.
That is to say if you had a liter of salt and 2 grams of water. In that case, the salt would be the solvent and the water the solute. But this type of mixture would be useless so why bother to make it??? When we do place solutes and solvents together, there is what we call the solution process. You can think of it as being similar to what you would experience if you tried to squeeze into an already packed elevator.
Everyone has to adjust to "find their space" again. Now just like in the elevator, molecules will adjust differently dependent on the type of molecule making an entrance. And also like in an elevator there will come a point when no more people can be added. For a solution, this point is called the saturation point and the solution itself is called a saturated solution.
At the point of saturation, no more solute will dissolve in the solvent. Rather the process of dissolving and precipitation are both occurring simultaneously and at the same rate. Generally speaking only certain molecules will dissolve in water to begin with. The old phrase "like dissolves like" or "birds of a feather flock together" is very true with respect to what degree solutes are soluble or miscible in different solvents.
At very low concentrations, almost all molecules are somewhat soluble in all solvents. But by trend, ionic and polar solutes are more soluble in polar solvents and non-polar molecules are soluble in non-polar mostly organic solvents.
The units of concentration we just discussed are used to describe the degree to which a solute is soluble in a solvent. 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.
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