Water will dissolve most molecules held together by ionic bonds because, as a polar molecule, water can stabilize positive ions by surrounding them with negative dipoles and stabilize negative ions by surrounding them with positive dipoles. And water will also dissolve alcohols like ethanol, isopropyl alcohol, and n-butanol because polar water molecules are strongly attracted to polar alcohol molecules. Alcohol molecules are polar because they contain a hydroxyl (-OH) group. The oxygen atom in the hydroxyl group is more electronegative than either the carbon or hydrogen atom it is chemically bonded to, so the oxygen atom will pull the electron density towards itself, creating a negative dipole.

 

(methanol)

The solubility of alcohol molecules in water follows an interesting pattern. Smaller alcohol molecules tend to be more soluble than larger alcohol molecules. While small alcohol molecules like methanol and ethanol are completely miscible in water, alcohol molecules larger than n-octanol (C8H17OH) tend not to be soluble in water at all.

ethanol

n-propanol

n-butanol

n-pentanol

n-hexanol

n-heptanol

n-octanol

The reason why n-octanol is essentially insoluble in water has to do with its long carbon chain. Carbon and hydrogen atoms have similar electronegativities, so they share their electrons equally in their covalent bonds. This means that the carbon and hydrogen atoms not chemically bonded to the oxygen atom are nonpolar and not attracted (or attractive) to polar water molecules at all. A few water molecules will be attracted to the hydroxyl (-OH) group at one end of the n-octanol molecule, but the rest of the water molecules, the ones that cannot get close to the hydroxyl group, will end up clustering together instead of surrounding the nonpolar carbon chain. Just like positive and negative ions are not stable unless surrounded by oppositely charged particles, polar water molecules are not stable unless their positive and negative dipoles are surrounded by oppositely charged dipoles.

stable configurations

unstable configuration

Another way to think about the insolubility of n-octanol in water is in terms of energy. Water molecules are like small magnets that are strongly attracted to each other, so it takes energy to pull them apart and separate them. When isopropyl alcohol is dissolved in water, the isopropyl alcohol molecules and water molecules mix on the molecular level. This means that water molecules are separated from each other and then clustered around isopropyl alcohol molecules. Energy is absorbed when the water molecules are pulled apart, and then energy is released when water molecules are regrouped with isopropyl alcohol molecules. The amount of energy absorbed and released depends on the strength of the intermolecular attraction between those molecules. In the case of isopropyl alcohol and water, the attraction between isopropyl alcohol and water molecules is so strong that it makes up for the loss of water-to-water molecular attraction… and, overall, heat is released.

For n-octanol and water, the attraction between n-octanol molecules and water molecules is too weak (because of the long, nonpolar carbon chain) to make up for the loss of water-to-water intermolecular attraction. So the energy to pull the water molecules apart does not exist, and the water molecules remain in a pure water liquid state.

He

water
(polar)

oxygen
(nonpolar)

carbon dioxide
(nonpolar)

helium
(nonpolar)

This relationship between polarity and solubility in water is reflected in the relative solubilities of carbon dioxide (CO2), oxygen (O2), and helium (He) gas. While all three of these substances are considered to be nonpolar, carbon dioxide is slightly more polar than oxygen, which is slightly more polar than helium. At 0 °C, 3.36 g of CO2 will dissolve in one liter of water, 0.070 g of O2 will dissolve in one liter of water, and 0.0017 g of He will dissolve in one liter of water.

Different Structures of Butanol

So what the heck is n-butanol anyway? Butanol is the name we give to any molecule with a hydroxyl (-OH) group chemically bonded to a four-carbon backbone. But there are actually four different ways to chemically bond a hydroxyl group to a four-carbon backbone.

n-butanol

isobutanol

sec-butanol

tert-butanol

The prefix “n” is given to the alcohol molecule that has its hydroxyl group chemically bonded to the end of a linear (non-branching) chain of carbon atoms. While all four of these molecules are alcohols and behave like alcohols, their different geometries do affect their physical and chemical properties. For example, tert-butanol is less chemically reactive than other forms of butanol because its hydroxyl group is surrounded on three sides by carbon atoms. This limits physical access to the hydroxyl groups, and reduces the probability that another molecule will react with the hydroxyl group by colliding with it and breaking the O-H chemical bond.

alcohol	density (g/cm3	melting point (°C)	boiling point (°C)	solubility in water (g/L) at 20 °C
n-butanol	0.810	-90	118	77
isobutanol	0.802	-102	108	87
sec-butanol	0.806	-115	99	350
tert-butanol	0.781	-25	82	∞

When butanol is manufactured, n-butanol and isobutanol are typically both produced at the same time in the same overall chemical reaction. When chemical reactions are driven by heat and molecular collisions (and not on the surface of controlled catalysts), it is very difficult to restrict products to a single type of molecule. The mixture of n-butanol and isobutanol can then be separated into pure substances.

You will see many different names for the same molecule depending on the naming system being used. n-Butanol is also known as 1-butanol, butan-1-ol, and n-butyl alcohol.

The situation for triglycerides (the primary molecule in vegetable oils and animal fats) is even worse. A triglyceride consists of a glycerol chemically bonded to three fatty acids. A fatty acid molecule is basically a long chain of carbon atoms attached to a carboxyl (-OOH) group. This carboxyl group enables a fatty acid to chemically bond with the glycerol. Most fatty acids in naturally occurring triglycerides contain between 16 and 20 carbon atoms.

typical triglyceride molecule

Because of their long, nonpolar carbon chains, triglycerides are insoluble in water. This is why oil and water do not mix. However, it is imprecise to say that oil and water molecules repel or do not “like” each other. Water and oil molecules are actually almost completely neutral towards each other (water molecules are slightly attracted to the glycerol end of a triglyceride molecule). Oil and water do not mix only because water molecules are so strongly attracted to other water molecules. If you placed 1000 small wooden blocks in a box with 1000 small magnets, and shook the box for five minutes, what would happen? All of the magnets would clump together. Now, magnets do not have anything against wooden blocks, but they would end up separating anyways.

So far, the only solvent we have looked at has been water. But there are many other liquid solvents in use today. Toluene is used as paint-thinner, hexane is used in spot removers, tetradecane is used in laboratory settings, and mineral oil is used in industrial cleaners. Notice how the pattern of alcohol solubility in these four solvents is completely opposite to the pattern in water: larger (more nonpolar) alcohol molecules tend to be more soluble than smaller (more polar) alcohol molecules. The key difference between these four solvents and water is that these solvents are all based on nonpolar molecules (mineral oil is a mixture of nonpolar hydrocarbons). And the benefit of nonpolar solvents, unlike water and other polar solvents, is that nonpolar solvents dissolve nonpolar molecules.

toluene

hexane

tetradecane

As a rule of thumb, “like dissolves like”… polar molecules dissolve in polar solvents and nonpolar molecules dissolve in nonpolar solvents. Substances that dissolve in polar solvents are often described as “water-soluble” and substances that dissolve in nonpolar solvents are often described as “fat-soluble.” While some substances do dissolve in both water and fat, in general, it is one or the other. So if you are trying to dissolve fat-soluble substances such as greases, oils, or petroleum products, you would not use water as your solvent. None of those substances are water-soluble. Instead, you would use a nonpolar solvent like hexane.