16.2.

First of all, I must tell you that when a less volatile solute is dissolved in a more volatile solvent, the boiling point of that solvent goes up. You probably imagine that volatile means flammable or unstable or explosive. If so you imagine incorrectly. No, in chemistry to say that one compound is more volatile than another simply means that it is easier to evaporate, that its boiling point is lower. So, for example, the boiling point of salt-water is higher than that of water. The boiling point of mead is higher than that of alcohol and lower than that of water.

Imagine if you will, a tea-kettle, called by chemists a pot on the stove filled with mead. The heat is on and the mead comes to a boil. If it were pure alcohol it would boil at 78C; if it were pure water it would boil at 100C. But because it is a solution of alcohol in water, it boils somewhere in between, let us say, at 90C. Now if we were to collect a bit of the steam coming out of the spout, what do you suppose we would find? The alcohol "wants" to boil at 78C so at 90C it is really ready to fly the coop, so to speak. By contrast, the water does not really "want" to boil until 100C so at 90C it is in no hurry to bolt. Put another way, because alcohol is the more volatile of the two, the steam will be richer in alcohol than the original mead.

Imagine further that you place a cold dinner plate over the spout of the tea-kettle. The steam will condense into droplets of liquid on the cold surface. This liquid will have the same concentration of alcohol as the steam, that is to say, it is richer in alcohol than the original mead. We could collect this liquid and place it into another tea-kettle, albeit a tiny one. Being richer in alcohol, it would come to a boil at a lower temperature than mead, perhaps 85C. You are probably thinking that we could then collect steam from this new kettle, condense it on a second cold plate, and dribble this even richer liquid into a third kettle. If so, your idea would be gathering steam.

But it is not actually necessary to use such a cumbersome arrangement. If, instead of a dinner plate, we mounted a tube, called by chemists a column on the spout of the tea-kettle, the steam would condense on the inside. As the bottom of the tube gets hot from the rising steam, the drops of condensate will boil again, as it would have in a second kettle. The new steam rises a little further up the tube until it finds a place cool enough to condense and the whole process starts all over again. A simple tube, hot at the bottom and cooler at the top, is all that is needed to take the place of a whole brigade of tea-kettles and the longer the tube is, the richer in alcohol the distillate will be.

Figure 16-1. Distillation as a Process

Imagine further that we attach another tube, a condenser to the top of the column. The only real difference between the column and the condenser is that the column is vertical, so that condensing liquid dribbles back down toward the pot while the condenser has a downward slant so that condensing liquid can exit the still. The condenser might be cooled by air, but it is more efficient if it is cooled by water. The place where column and condenser meet is called the head. Imagine now a thermometer at the head. What would it read during a distillation? At the beginning there is boiling mead in the pot but the head is still at room temperature. The steam begins to rise, heating the bottom of the column, but the head remains at room temperature. Eventually, though, hot steam reaches the head where it simultaneously passes into the condenser. If we were distilling pure alcohol, the thermometer would read 78C and the temperature would remain constant throughout the distillation. If we were distilling pure water, the temperature would remain at 100C throughout. But with a solution, the head temperature can be anywhere in this range, lower when the distillate is more alcoholic, higher when it is less so. Unlike pure substances, which boil at a single, fixed temperature, solutions boil over a range of temperatures and this range can be used to monitor the concentration of the solution.

Actually, there may come a point where the boiling point of the distillate is lower than that of the alcohol. The steam coming off of this distillate will be no richer in alcohol than the liquid and so further distillation is ineffective. For ethanol-water this azeotrope boils at 78.17C, 0.23C lower than pure alcohol, and consists of 95% ethanol and 5% water. "Denatured" alcohol is simply 95% ethanol with little bit of poison added to discourage people from drinking it.

Figure 16-1 shows the distillation process in schematic form. The first reactor looks like a furnace, but one which contains liquid instead of solid. The second reactor is the condenser. Together, they make up a still. No chemical reaction takes place in this process. The process physically separates two liquids that differ in boiling point. The usual conventions are followed; reactants enter from the left of the figure, waste products exit the top and bottom, and the main product exits to the right.

Adelard's motivation for distilling alcohol was to use it as a solvent and so I should probably tell you a bit about solubility. There are essentially three kinds of substance; ionic substances, or salts consist of positive and negative ions and were discussed in Chapter 7; polar substances consist of molecules in which the negative charge is bunched up at one end and the positive charge at the other; non-polar substances consist of molecules in which the charge is evenly distributed throughout the molecule. In general, substances in which the charges are separated, salts and polar substances, are mutually soluble and non-polar substances are mutually soluble, but salts and polar substances are not soluble in non-polar substances and vice versa. "Oil and water do not mix," or, to put it another way, "like dissolves like."

Figure 16-2. Two Water Molecules

Polar molecules, as I have said, have negative charge bunched up at one end and positive charge at the other. For this to happen, one or more atoms must, like the devil, take more than its due. Of electrons, that is. While carbon and hydrogen are rather mild-mannered in this regard, oxygen and nitrogen are greedy. In the water molecule, for example, the oxygen atom hogs electrons from the hydrogen atoms and so the oxygen end is negative and the hydrogen end is positive. When two water molecules bump into one another, as shown in Figure 16-2, the negative oxygen end of one molecule is attracted to the positive hydrogen end of the other. Opposite charges attract, you see. This mutual attraction, or hydrogen bond, is what holds two water molecules together. A large collection of water molecules resembles a dog convention, with the little beggars sniffing at each other's tails.

Ethane, C2H6, is a typical non-polar molecule. Because carbon is really no greedier than hydrogen, as far as electrons are concerned, there is no positive or negative end to the molecule and hence no strong interaction to bind one ethane molecule to another. In contrast to water, ethane molecules are content to roam about passing one another like head-less tail-less dogs in the night.

What happens when a polar liquid like water and a non-polar one like ethane find themselves in the same container? The situation is much like a room filled half with normal dogs and half with head-less, tail-less dogs. The normal dogs will congregate in one part of the room because of their mutual attraction. The headless, tail-less dogs will be pushed to the other side of the room, not so much because they are attracted to one another, but because the normal dogs are less attracted to them than to those of their own kind. So to come back to chemistry, the polar water molecules will collect in one part of the container and the non-polar ethane molecules will be left in the other. To put it another way, ethane is not very soluble in water and vice versa.

Of course, the question of polarity is not black-and-white. There are some molecules that are very polar, some that are a bit polar, and some that are not very polar at all. For example ethanol, C2H5OH, is mid-way between ethane and water in its polarity. One end of the molecule is non-polar like ethane while the other is polar like water. It is as if a normal dog had been sewn onto a head-less tail-less dog. Such a dog, released onto a pack of dogs, would be equally comfortable with either kind. In Figure 16-3 a water molecule is attracted to the oxygen-hydrogen end of the ethanol molecule and vice versa. This mutual attraction is what is responsible for the ability of ethanol to dissolve in water.

Figure 16-3. Ethanol and Water

You may be wondering why ethanol is such a good solvent for so many dyes and medicines. If you think about it, living things, whether yeasts or bacteria, trees or polliwogs, are made up of tiny bags of water called cells. Part of the living thing is polar, the part dissolved in the water. But much of the organism had better be non-polar, the cell walls, skin and bones, or else the whole creature would dissolve in its own juices and be reduced to a blob of goo. Organisms need to move things around and these things must be polar, but once moved to where they belong they need to stay put. So living things are constantly manipulating their components, adding OH's here and there to get them into solution and then chopping them off or covering them up to bring them out again. The beauty of ethanol, or ethyl alcohol, as a solvent is that it can dissolve many organic compounds whether or not they are soluble in water.

I should mention that ethanol is only one member of a class of compounds, the alcohols. Methanol, CH3OH, is also known as wood alcohol since it came originally from the distillation of wood. Isopropanol, C3H7OH, is familiar as rubbing alcohol. Ethylene glycol, C2H4(OH)2, which looks like ethanol with an OH group on each carbon atom, is used as automobile anti-freeze. Glycerol, C3H5(OH)3, like isopropanol with an OH on each carbon, will be discussed at length in Chapter 19, when we discuss soap. Each of these compounds consists of a carbon-hydrogen chain, the non-polar bit, with one or more polar OH groups hanging off of it.

Figure 16-4. Acetic Acid and Water

When ethanol is oxidized by bacteria (Equation 16-1(a)) it becomes acetic acid, which makes up about 5% of vinegar. Acetic acid, CH3COOH, is one member of the class of compounds, the organic acids. You will recall that oxygen is an electron hog and with two of them pulling electron density away from the poor hydrogen, it is left with a greater positive charge than it had in either ethanol or water. Since electrons are what hold the molecule together, this acidic hydrogen or proton has a tendency to abandon ship if it happens to bump into an alkali. This kind of metathesis reaction, illustrated in Equation 16-1(b), is characteristic of an acid-base reaction; the acid donates a proton to the base.

Equation 16-1. Reactions of Ethanol and Acetic Acid

Now, like sodium hydroxide, ethanol has an OH group and you are possibly wondering whether a metathesis reaction is possible between ethanol and acetic acid. If so, you are wondering in the right direction. Equation 16-1(a) shows the metathesis reaction producing ethyl acetate, which is a member of the class of compounds, the esters. Esters are the organic counterparts of inorganic salts, being produced as a condensation of an acid and an alcohol. If your mead contained bacteria and oxygen you no doubt produced some ethyl acetate by accident. Unlike alcohols and acids, however, esters have no OH groups and the negative oxygen atom is buried deep inside the molecule. Consequently ethyl acetate is not very polar and makes an excellent solvent for non-polar compounds. It is a popular choice for fingernail polish remover.

Figure 16-5. Ethyl Acetate

I have still to tell you what salt, or, as I like to call it, tbmu, has to do with anything. In those days our condensers were short, air-cooled jobbies, good enough for separating substances like gold from mercury, whose boiling points differ by hundreds of degrees. I did not suspect, and was not prepared to efficiently separate substances like alcohol from water, whose boiling points differ by only 22 degrees. As I have said, when a less volatile solute is dissolved in a more volatile solvent, the boiling point of that solvent goes up. Since salt dissolves in water but not in ethanol, it raises the boiling point of the water but not the ethanol, making their boiling points farther apart than they would normally be and consequently making it possible to separate them even with an inefficient air-cooled condenser. When you do not suspect that such a thing is possible, there is, of course, no reason to worry about condenser efficiency, but after my discovery it became clear that most of the alcoholic genie was going out the window rather than into the bottle. And while the painter himself tried to keep me bottled up, I was too volatile for him to keep a lid on me; I spread my bejeezical wings and flew the coop shortly after he gave up the ghost.

WarningMaterial Safety
 

Locate an MSDS for ethanol (CAS 64-17-5). Summarize the hazardous properties of this material in your notebook, including the identity of the company which produced the MSDS and the potential health effects for eye contact, skin contact, inhalation, and ingestion. Also include the LD50 (oral, rat) and the NFPA diamond for ethanol.[1]

While it is possible to drink enough beer, wine or mead to make yourself seriously ill, it is even easier with distilled spirits. An eight-ounce glass of ethanol is the equivalent of more than a half-gallon of wine or a gallon of beer. Remember, it is the dose that makes the poison. You should also be aware that ethanol dissolves things that are not soluble in more dilute alcoholic solutions. Consequently, poisonous compounds from bottles or stills may dissolve in ethanol that would not have dissolved in mead. Finally, you should be aware that ethanol is not the only volatile compound present in fermented beverages so when you distill alcohol you may concentrate other potentially toxic compounds, ethyl acetate, for example.

You should wear safety glasses while working on this project. Your distilled spirit may be carefully burned or saved for use in a spirit lamp, as shown in Figure 16-8. The contents of the pot may be flushed down the drain.

NoteResearch and Development
 

You are probably wondering what will be on the quiz.

Notes

[1]

The NFPA diamond was introduced in Section 15.2. You may substitute HMIS or Saf-T-Data ratings at your convenience.