We have discussed what is involved in the fermentation of sugar to alcohol and by this time many of you have direct experience of this process. The fermentation of alcoholic beverages is a technology that dates back to the stone age. The separation of the alcohol from the water, on the other hand, is a relatively recent development. The Chinese were distilling rice beer as early as 800 BC. But the separation of ethyl alcohol as a substance was not achieved in Europe until the 12th century AD. At that time it was regarded by the alchemists as a "fifth essense" or "quintessence," a higher form of water which was capable of dissolving many medicines which were not soluble in ordinary water. The use of alcohol for medicinal purposes is reflected in another common name for alcohol: aqua vitae (water of life), a name which survives as the name of a modern liqueur.
The principle problem of the preparation of alcohol is that yeasts are unable to ferment a liquid which is more than about 18% alcohol. It is useful to view alcohol as a waste product from the yeast's point of view. Beyond about 18% they simply cannot live in their own "urine." We start, then, with a liquid which is between 5% and 15% alcohol and our primary task is to separate the alcohol from the water and other materials. There are two common methods for doing this: freezing and boiling.
We have already discussed the melting and freezing of solutions when we discussed the difference between quartz and glass. Recall that a pure substance melts and freezes at a single temperature, that is, the melting point equals the freezing point. By contrast, a solution melts over a range of temperatures rather than at a single point. Let us recall the behavior of water as it freezes.
Pure water freezes at 0 C. Pure ice melts at 0 C. If I start with water at room temperature and cool it, the temperature will drop. From 25 C (room temperature) to 20 C, 10 C, 5 C, 0 C. But at 0 C something peculiar happens, ice crystals form. By the time that half the water has turned to ice, the temperature is still 0 C. Only after all the water is frozen will the temperature fall to -1 C, -5 C, ... Similarly if I start with ice at -10 C and warm it, it will warm to -5 C, -1 C, 0 C but then the ice starts to melt. The temperature will stay at 0 C until the last bit of ice is melted and then will start to rise again.
Solutions behave differently. Let us take mead, for example. As we cool mead from room temperature, the temperature will fall below 0 C before the first ice crystals form. And since water freezes at a higher temperature than alcohol, the first ice to form will be almost pure water ice. That means the liquid that remains will have a higher percentage of alcohol than the original liquid. Furthermore, this more concentrated solution will freeze at a lower temperature. So unlike the case of pure water (or pure alcohol for that matter), the temperature will continue to drop as the solution freezes. As the water ice freezes out, the temperature gets lower and lower and the liquid gets more and more concentrated in alcohol. Finally, the temperature will get so low that the remaining solution freezes. But if we stop prior to that point, we will have a liquid that is much higher in alcohol than the original liquid.
There are two ways to look at this. On the one hand, we could consider that we are purifying the alcohol by removing the water "impurity." On the other hand, we could consider that we are purifying the water by removing the alcohol "impurity." Which one is the impurity depends on which one you are most interested in. In the final analysis it is better to consider this a separation process rather than a purification process. The separation process that relies on freezing out one component is called recrystalization.
Lets look now at the phenomenon of boiling. Recall that pure water boils at 100 C and water vapor condenses at 100 C. If we heat water from room temperature, the temperature climbs from 25 C to 50 C to 75 C to 100 C. At 100 C the temperature stops climbing and the water begins to boil, producing water vapor at 100 C. As we continue to heat, the temperature stays pegged at 100 C until the last drop of water has boiled away and only then does the temperature begin to climb again.
By contrast, if we heat a solution like mead, the temperature will climb to the boiling point of the substance which boils at the lowest temperature. Since alcohol boils at 78 C and water boils at 100 C, the temperature of the mead will climb to 78 C and begin to boil. Most of the vapor will be alcohol at this temperature since water does not boil until 100 C. This leaves less alcohol and more water in the mead so the temperature continues to climb as the solution boils.
Now suppose we start with mead that is 5% alcohol. At 78 C most of the vapor will be alcohol but there will be some water as well. Say this vapor is 60% alcohol. If we were to condense this vapor into a liquid and then bring it to a boil again, the new liquid would be 60% alcohol, but the new vapor would be even higher in alcohol because there was less water to begin with. So the new vapor is, say, 80% alcohol. We can repeat this process over and over again and the alcohol content will increase with each new vaporization step.
Actually it is not really necessary to break it up into steps. If we pass the vapor through a tube, some of it will condense on the walls of the tube. As more hot vapor rises, some of the condenses vapor will re-evaporate and move farther up the tube. This process is repeated over and over again. As the line of condensation, or reflux line moves up the tube, or column, the refluxing liquid gets more and more alcoholic. The liquid in the original flask or pot gets less and less alcoholic. So again we are really performing a separation rather than a purification. This kind of separation is called distillation.
So which is better, recrystalization or distillation? Well, it depends on what you want. Both will separate two components of a solution, in our case alcohol from water. In the case of ethanol and water, higher ethanol concentrations can be achieved from distillation than from recrystalization. For our project, then, we will choose to distill our mead to get a product that is as high in alcohol as possible. The test we will apply will be whether the product will burn (about 50% or 100 proof).
It is illegal to distill alcohol without a permit from the Bureau of Alcohol, Tobacco, and Firearms (BATF). Recall that adults 21 years of age and older are allowed to brew up to 100 gallons of wine or beer for their own consumption. Thus there are homebrewing magazines, clubs, classes, and suppliers in great numbers. There is no legal equivalent for distilled alcoholic beverages. You are not allowed to make your own whiskey or vodka, even for your own personal\ use.
There is a provision in the law for the experimental distillation of alcohol as a fuel. This is not a loophole. It requires a permit from the BATF and they are serious about the fuel part. The permit is not hard to get but its provisions are strictly enforced. I applied for and received a permit from the BATF for the distillation of alcohol fuel in this class. It allows for the distillation of ethyl alcohol in stills owned by the Chemistry Department and the alcohol must be used as fuel on the premises. It is under this permit that you may attempt this project.
It may interest you to know that the Chemistry Department purchases 190 proof (95%) ethyl alcohol in 55 gallon drums for $3.72 per gallon. This alcohol cannot be used for beverage purposes. If it were, it would be subject to a Federal tax of $12.83 per gallon, i.e. the tax is more than three times the purchase price.
When we speak of alcohol in everday language, we are usually talking about ethanol, or ethyl alcohol. We already know the formula for this compound, C2H5OH, but we will find that in organic chemistry, there may be several different compounds with the same name. Increasingly, we will need to provide more information on the structure of a compound in order to discuss its properties. Here are several representations of the ethanol molecule:
|Ball and Stick|
H H | | H-C-C-OH | | H H
As we go from left to right, we get less and less information about the molecule, but the formulas become easier to draw or type. A chemist learns to see that all five of these representations denote the same molecule. We will use any of these formats depending on the circumstances.
As you well know, ethanol is soluble in water (e.g. beer, wine, mixed drinks). But looking at the structures above and recalling the principles learned in the soap project, we can undestand why this is so. The OH group in ethanol looks like half a water molecule. In the charge distribution, this shows up as a deep blue (positive) charge right next to a deep red (negative) charge. The polarity of the OH group causes a strong attractive force with other OH groups and so alcohols tend to be soluble in water.
You may not realize it, but there is a whole class of organic compounds called alcohols. You are probably familiar with some of them:
H | H-C-OH | H
|gasoline antifreeze, Sterno, "wood alcohol"|
H H | | H-C-C-OH | | H H
H H H | | | H-C-C-C-H | | | H O H H
What makes these compounds alcohols is the OH group. What distinguishes different alcohols from each other is the number of carbon atoms. The name of the alcohol tells you the number of carbons:
|Prefix||Number of Carbons|
Some alcohols have more than one OH group:
H H | | H-C-C-H | | O O H H
H H H | | | H-C-C-C-H | | | O O O H H H
|component of soap, drugstore "glycerine"|
Once we can make and purify ethanol, we can make a variety of organic compounds from it. We will use this as a model for an industrial complex. Each of these substances can be purified by distillation.
Recall that when mead "goes sour," bacteria digest the ethanol in the presence of
oxygen. The reaction is a typical oxidation:
CH3CH2OH + O2 -----> CH3COOH + H2O
The product is acetic acid (old name) or ethanoic acid, the acid present in vinegar. Vinegar is typically 5% acetic acid. We can purify it by distillation to produce concentrated acetic acid, called glacial acetic acid. Like alcohols, acetic acid contains an OH group and so it is, of course, soluble in water.
|Ball and Stick|
HO H | | O=C-C-H | H
The other alcohols can also be oxidized to their corresponding acids (though not typically by bacteria).
Recall that concentrated sulfuric acid can literally suck the water out of compounds containing hydrogen and oxygen. We saw this in class when we added sulfuric acid to sucrose (sugar). The acid sucked out the water leaving charcoal behind. If we do the same thing to ethanol, sulfuric acid will suck out as much water as it can, but there will be leftover hydrogens. Two reactions are possible, depending on temperature:
The sulfuric acid is written over the arrow to show that it promotes the reaction but is not changed by the reaction. In this case, it serves simply to remove the water produced by the reaction.
The product H2C=CH2, ethylene, is a gas and it is a feedstock for a great deal of industrial chemistry. U.S. production of ethyene was 15.9 billion kg in 1989, the fourth largest volume. It is used mainly to produce polyethylene, the plastic from which milk cartons are made. While it could be produced from ethanol as described, it is more economical to produce it from petroleum. With no polar OH groups, it is, of course, not soluble in water.
|Ball and Stick|
H H | | C=C | | H H
The product CH3CH2OCH2CH3 is ethyl ether, or just ether for short. It is a very popular laboratory solvent. Because of its combination of flammability and low boiling point, it poses an explosion hazard. Cocaine users use this solvent when "free basing" and there have been several celebrity explosions from careless use of ether. It is also used as an anesthetic in its own right. Since it has no OH group, it is non-polar and insoluble in water.
|Ball and Stick|
H H H H | | | | H-C-C-O-C-C-H | | | | H H H H
Just as an acid and an alkali react to form a salt, an organic acid and an alcohol react to form an ester. Fats, for example, were esters of glycerol and three fatty acids. When acetic acid and ethanol react, they form ethyl acetate. Notice that this naming convention follows the same "first name-last name" form that we used for inorganic salts.
H H H H | | | | H-C-C-OH + HO H -----> H-C-C---O H + HOH | | | | | | | | H H O=C-C-H H H O=C-C-H | | H H
With no exposed OH groups, ethyl acetate is non-polar and hence not soluble in water. It is, in fact, one of the most popular non-polar solvents. It is used, for example, as fingernail polish remover.
|Ball and Stick|
H H | | H-C-C---O H | | | | H H O=C-C-H | H
If we use an acid to put ethanol and acetic acid together, we can use an alkali to take them apart:
If this looks like the equation for the saponification of soap, it should. It is the same reaction except that we have ethanol instead of glycerol, and acetic acid instead of a fatty acid.
All of these compounds synthesized from ethanol can be separated by distillation because their boiling points differ from each other, from water, and from sulfuric acid:
The alcohol quiz will consist of three questions on the following topics:
You will distill 500 mL of your own mead. There is a still set up in Gilmer 213 which you may use only during the designated period. You may not remove any alcohol from the lab! Dr. Dunn must personally monitor all distillations.
Mead will not burn, but ethanol will. I will attempt to light your "beeshine." If it burns, you pass. If not, you fail. But of course you can try again (once per day).