4. The apparatus used in these experiments must be open to the atmosphere at the receiving end of the condenser.Never heat a closed system, because the pressure buildup may cause the apparatus to explode!
5. Be certain that the water hoses are securely fastened to your condensers so that they will not pop off and cause a flood. If heating mantles or oil baths are to be used, water hoses that come loose may cause water to spray onto elec- trical connections or into the heating sources, either of which is potentially dangerous to you and to those who work around you.
6. Avoid excessive inhalation of organic vapors at all times.
M I N I S C A L E P R O C E D U R E
Preparation Sign in at www.cengage.com/loginto answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicals used or produced in this procedure. Read or review Sections 2.2, 2.4, 2.9, 2.11, and 2.14.
Apparatus A 50-mL round-bottom flask, apparatus for fractional distillation, mag- netic stirring, and flameless heating.
Setting Up Place 10 mL of cyclohexane and 20 mL of toluene in the round- bottom flask, and add a stirbar to ensure smooth boiling. Equip this flask for frac- tional distillation as shown in Figure 2.39. Pack a Hempel or similar distillation column, using the type of packing specified by your instructor. When packing the column, be careful not to break off the glass indentations at the base of the column. Do not pack the column too tightly, because heating a fractional distil- lation apparatus equipped with a column that is too tightly packed is analogous to heating a closed system. Insulate the fractionating column by wrapping it with glasswool. The position of the thermometer in the stillhead is particularly Chapter 4■ LiquidsDistillation and Boiling Points 141
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important; the top of the mercury thermometer bulb should be level with the bot- tom of the sidearm of the distillation head. Clean and dry three 25-mL containers, which may be bottles or Erlenmeyer flasks, for use as receiving flasks, and label them A, B, and C. Place receiver A so that the tip of the vacuum adapter extends inside the neck of the container to minimize evaporation of the distillate. Have your instructor check your assembled apparatus before heating the stillpot.
Distillation Start the magnetic stirrer and begin heating the stillpot using the heating method specified by your instructor. As the mixture is heated, the head temperature will rise to 81 °C (760 torr), which is the normal boiling point of cyclohexane, and distillation will begin. Regulate the heat so that distillation continues steadily at a rate no faster than one drop of distillate every 1–2 sec; if a drop of liquid cannot be seen suspended from the end of the thermometer, the rate of distillation is too fast.
The head temperature will remain at 81 °C for a period of time, but eventually it will either rise or drop slightly. Receiver A should be left in place until this increase or decrease in temperature is observed. As soon as the temperature deviates from 81 °C by more than ±3 °C, change to receiver B and increase the amount of heat supplied to the stillpot. The temperature will again start to rise, and more liquid will distill. Leave receiver B in place until the temperature reaches 110 °C (760 torr), which is the normal boiling point of toluene, and change to receiver C. Continue the distilla- tion until 1–2 mL of liquid remains in the stillpot, and then discontinue heating.
Analysis Record the volumes of the distillate collected in each receiver by means of a graduated cylinder. Allow the liquid in the column to drain into the stillpot, then record the volume of this pot residue. If instructed to do so, submit or save 0.2-mL samples of each fraction A, B, and C for GLC or GC-MS analy- sis (Sec. 6.4).
Comparative Fractional Distillations
Compare the results of a fractional distillation using a packed column, as described in the above experimental procedure, with those obtained using a simple distillation apparatus, an unpacked column, and one packed with a material different from that used originally.
Fractionation of Alternative Binary Mixtures
Subject other binary mixtures to fractional distillation using a variety of apparatus.
These might include 50:50 mixtures of acetone and 1,4-dioxane, hexane and heptane, hexane and toluene, heptane and ethyl benzene, acetone and toluene, and tetrahydrofuran and 1-butanol. You might also wish to explore the efficacy of fractional distillation for separating a binary mixture having components whose boiling points are separated by less than 30 °C. Examples might be tetrahydrofu- ran and acetone, ethyl acetate and 1,4-dioxane, and ethanol and methanol.
Fractional Distillation of Unknowns
Obtain a 50:50 mixture of two unknown solvents from your instructor. These sol- vents will differ in boiling point by more than 20 °C. Possible solvents include hexane, cyclohexane, heptane, octane, toluene, ethyl benzene, acetone, methanol, 1-butanol, tetrahydrofuran, 1,4-dioxane, ethyl acetate, and others listed by your 142 Experimental Organic Chemistry■ Gilbert and Martin
Discovery Experiment
Discovery Experiment
Discovery Experiment
instructor. Look up the boiling points for each of these solvents. Perform a fractional distillation using a packed column as described in the above experimental proce- dure. Prepare a distillation curve with the boiling point on the vertical axis and the volume on the horizontal axis. Based upon your experimentally determined boiling points for the two liquids, identify the components of your mixture.
W R A P P I N G I T U P
Unless directed to do otherwise, pour the pot residue into the container for nonhalo- genated organic liquids and return the distillation fractions to a bottle marked
“Recovered Cyclohexane and Toluene.”
E X E R C I S E S
1.Define the following terms:
a. fractional distillation f. mole fraction
b. head temperature g. height equivalent to a theoretical plate (HETP) c. pot temperature h. temperature gradient
d. Raoult’s law i. Dalton’s law e. ideal solution j. reflux ratio
2.Specify whether a simple distillation or a fractional distillation would be more suitable for each of the following purifications, and briefly justify your choice.
a. Preparing drinking water from sea water.
b. Separating benzene, bp 80 °C (760 torr), from toluene, bp 111 °C (760 torr).
c. Obtaining gasoline from crude oil.
d. Removing diethyl ether, bp 35 °C (760 torr), from p-dichlorobenzene (s), mp 174–175 °C.
3.Sketch and completely label the apparatus required for fractional distillation.
4.Explain why a packed fractional distillation column is more efficient than an unpacked column for separating two closely boiling liquids.
5. If heat is supplied to the stillpot too rapidly, the ability to separate two liquids by fractional distillation may be drastically reduced. In terms of the theory of distil- lation presented in the discussion, explain why this is so.
6.Explain why the column of a fractional distillation apparatus should be aligned as near to the vertical as possible.
7.Explain the role of the stirbar normally added to a liquid that is to be heated to boiling.
8.The top of the mercury bulb of the thermometer placed at the head of a distil- lation apparatus should be adjacent to the exit opening to the condenser.
Explain the effect on the observed temperature reading if the bulb is placed (a) below the opening to the condenser or (b) above the opening.
9.Calculate the mole fraction of each compound in a mixture containing 15.0 g of cyclohexane and 5.0 g of toluene.
Chapter 4■ LiquidsDistillation and Boiling Points 143
10.In the fractional distillation of your mixture of cyclohexane and toluene, what can be learned about the efficiency of the separation on the basis of the relative volumes of fractions A, B, and C?
11. a. A mixture of 60 mol % n-propylcyclohexane and 40 mol % n-propylben- zene is distilled through a simple distillation apparatus; assume that no fractionation occurs during the distillation. The boiling temperature is found to be 157 °C (760 torr) as the first small amount of distillate is col- lected. The standard vapor pressures of n-propylcyclohexane and n-propyl- benzene are known to be 769 torr and 725 torr, respectively, at 157.3 °C.
Calculate the percentage of each of the two components in the first few drops of distillate.
b. A mixture of 50 mol % benzene and 50 mol % toluene is distilled under exactly the same conditions as in Part a. Using Figure 4.3, determine the distillation temperature and the percentage composition of the first few drops of distillate.
c. The normal boiling points of n-propylcyclohexane and n-propylbenzene are 156 °C and 159 °C, respectively. Compare the distillation results in Parts a and b. Which of the two mixtures would require the more efficient frac- tional distillation column for separation of the components? Why?
12.Examine the boiling-point–composition diagram for mixtures of toluene and benzene given in Figure 4.3.
a. Assume you are given a mixture of these two liquids of composition 70 mol % toluene and 30 mol % benzene and that it is necessary to effect a fractional distillation that will afford at least some benzene of greater than 99% purity. What would be the minimum number of theoretical plates required in the fractional distillation column chosen to accomplish this separation?
b. Assume that you are provided with a 20-cm Vigreux column having an HETP of 10 cm in order to distill a mixture of 48 mol % benzene and 52 mol % toluene. What would be the composition of the first small amount of distillate that you obtained?
13.At 50 °C, the vapor pressures for methanol and ethanol are 406 and 222 torr, respectively. Given a mixture at 50 °C that contains 0.2 mol of methanol and 0.1 mol of ethanol, compute the partial pressures of each liquid and the total pressure.
14.Figure 4.6 shows a temperature (°C) vs. composition diagram for a mixture of acetone and ethyl acetate at 760 torr. You can calculate the mole fraction of ethyl acetate at any given point of the plot by subtracting the mole fraction of acetone from one because of the relationship mole fraction of acetone + mole fraction of ethyl acetate1. Answer the following questions using Figure 4.6.
a. Specify the normal boiling point of acetone and of ethyl acetate.
b. A mixture of acetone and ethyl acetate of unknown ratio begins to boil at 65 °C (760 torr). What is the composition of this binary mixture in terms of the mole fraction of acetone and that of ethyl acetate?
c. For a mixture comprised of 50 mol % acetone and 50 mol % ethyl acetate:
i. At what temperature will the mixture begin to boil?
ii. When the mixture begins to boil, what is the mol % acetone in the vapor?
144 Experimental Organic Chemistry■ Gilbert and Martin
iii. What is the boiling point of the liquid formed by condensation of the vapor from (ii) above?
iv. How many theoretical plates (distillation stages) will be necessary to isolate acetone of at least 90% purity?
d. A mixture of acetone and ethyl acetate of unknown molar ratio is heated to boiling. Immediately after boiling commences, a sample of the vapor is found to contain 0.8 mol fraction of ethyl acetate. What is the composition in mol % of the original binary mixture and what is its boiling point?
e. Would you expect a solution comprised of 30 mol % acetone and 70 mol % ethyl acetate to boil above or below ca. 67 °C in Denver, CO, which is at an elevation of one mile above sea level? Explain your answer.
4.5 S T E A M D I S T I L L A T I O N
The separation and purification of volatile organic compounds that are immiscible or nearly immiscible with water can often be accomplished by steam distillation.
The technique normally involves the codistillation of a mixture of organic liquids and water, although some organic solids can also be separated and purified by this means. Of the various distillation methods, steam distillation is utilized least fre- quently, owing to the rather stringent limitations on the types of substances for which it can be used. These limitations as well as the virtues of this technique are revealed by considering the principles underlying steam distillation.
Theory and Discussion The partial pressure Piof each component i of a mixture of immiscible, volatile sub- stances at a given temperature is equal to the vapor pressure P˚iof the pure com- pound at the same temperature (Eq. 4.6) and does not depend on the mole fraction of the compound in the mixture. In other words, each component of the mixture vapor- izes independently of the others. This behavior contrasts sharply with that exhibited by solutions of miscible liquids, for which the partial pressure of each constituent of the mixture depends on its mole fraction in the solution (Raoult’s law, Eq. 4.2).
PiP˚i (4.6)
Chapter 4■ LiquidsDistillation and Boiling Points 145
Vapor line
Liquid line 60
65 70 75 80
55
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Temperature (°C)
Mole Fraction Acetone Figure 4.6
Temperature–composition diagram for acetone and ethyl acetate (Exercise 14).
You may recall that the total pressure, PT, of a mixture of gases is equal to the sum of the partial pressures of the constituent gases, according to Dalton’s law (Eq. 4.4). This means that the total vapor pressure of a mixture of immiscible, volatile compounds is given by Equation 4.7. This expression shows that the total vapor pressure of the mixture at any temperature is always higher than the vapor pressure of even the most volatile component at that temperature, owing to the contributions of the vapor pressures of the other constituents in the mixture. The boiling temperature of a mixture of immiscible compounds must then be lower than that of the lowest-boiling component.
PTP˚aP˚bã ã ãP˚i (4.7) Application of the principles just outlined is seen in an analysis of the steam dis- tillation of an immiscible mixture of water, bp 100 °C (760 torr), and bromobenzene, bp 156 °C (760 torr). Figure 4.7 is a plot of the vapor pressure versus temperature for each pure substance and for a mixture of these compounds. Analysis of this graph shows that the mixture should boil at about 95 °C (760 torr), the temperature at which the total vapor pressure equals standard atmospheric pressure. As theory predicts, this temperature is below the boiling point of water, which is the lowest-boiling com- ponent in this example.
The ability to distill a compound at the relatively low temperature of 100 °C or less by means of a steam distillation is often of tremendous value, particularly in the purification of substances that are heat-sensitive and would therefore decom- pose at higher temperatures. It is useful also in the separation of compounds from reaction mixtures that contain large amounts of nonvolatile residues such as inor- ganic salts.
The composition of the condensate from a steam distillation depends upon the molar masses of the compounds being distilled and upon their respective vapor pres- sures at the temperature at which the mixture steam-distils. To illustrate this, consider a mixture of two immiscible components, A and B. If the vapors of A and B behave as ideal gases, the ideal gas law can be applied and Equations 4.8a and 4.8b are obtained.
In these two expressions, P˚ is the vapor pressure of the pure liquid, V is the volume 146 Experimental Organic Chemistry■ Gilbert and Martin
Mixture of bromobenzene
and water
Water
Bromobenzene
Pressure (torr)
Temperature (°C) 700
600 500 400 300 200 100 0 800
0 20 40 60 80 100 120 140 160
Figure 4.7
Vapor pressure–temperature graph for pure bromobenzene, pure water, and a mixture of bro- mobenzene and water.
in which the gas is contained, g is the weight in grams of the component in the gas phase, M is its molar mass, R is the gas constant, and T is the absolute temperature in kelvins (K). Dividing Equation 4.8a by Equation 4.8b gives Equation 4.9.
P˚AVA(gA/MA)(RT) (4.8a)
P˚BVB( gB/MB)(RT) (4.8b)
(4.9) Because the RT factors in the numerator and denominator are identical and because the volume in which the gases are contained is the same for both (VAVB), these terms in Equation 4.9 cancel to yield Equation 4.10.
(4.10) Now let the immiscible mixture of A and B consist of bromobenzene and water, whose molar masses are 157 g/mol and 18 g/mol, respectively, and whose vapor pressures at 95 °C, as determined from Figure 4.6, are 120 torr and 640 torr, respec- tively. The composition of the distillate at this temperature can be calculated from Equation 4.10 as shown in Equation 4.11. This calculation indicates that on the basis of weight, more bromobenzene than water is contained in the steam distillate, even though the vapor pressure of the bromobenzene is much lower at the temperature of the distillation.
(4.11) Organic compounds generally have molar masses much higher than that of water, so it is possible to steam-distill compounds having vapor pressures of only about 5 torr at 100 °C with a fair efficiency on a weight-to-weight basis. Thus, solids that have vapor pressures of at least this magnitude can be purified by steam distil- lation. Examples are camphor, used in perfumes, and naphthalene, present in some brands of mothballs. The rather high vapor pressures of these solids is evidenced by the fact that their odors are detectable at room temperature.
In summary, steam distillation provides a method for separating and purifying moderately volatile liquid and solid organic compounds that are insoluble or nearly insoluble in water from nonvolatile compounds. Although relatively mild conditions are used in steam distillation, it cannot be used for substances that decompose on pro- longed contact with steam or hot water, that react with water, or that have vapor pres- sures of 5 torr or less at 100 °C, all of which are significant limitations to the method.
4.6 S T E A M D I S T I L L A T I O N : I S O L A T I O N O F C I T R A L F R O M L E M O N G R A S S O I L
Citral is a naturally occurring oil mainly comprised of two isomeric unsaturated alde- hydes, geranial (1) and neral (2), that are extremely difficult to separate. These isomers differ only in the spatial orientation of the substituents about the carbon-carbon dou- ble bond that bears the aldehyde moiety (–CHO). This qualifies 1 and 2 as stereoiso- mers and more specifically as diastereomers, the definitions of which are found in
gbromobenzene
gwater (120)(157) (640)(18) 1.64
1 Grams of A
Grams of B (P°A)(molar mass of A) (P°B)(molar mass of B) PA°VA
P°BVB gA/MA(RT) gB/MB(RT)
Chapter 4■ LiquidsDistillation and Boiling Points 147
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Section 7.1. In addition, they are sometimes also called geometric isomers, an older term that is used to describe stereoisomers that differ because the three-dimensional distribution of their substituents is the result of a double bond.
Citral possesses a lemonlike odor and taste. Although this odor and taste is pleasant to humans, it is less attractive to other species. For example, ants secrete citral to ward off potential predators, so citral is functioning as a defense pheromone.
There are several commercial applications of citral. For example, it
may be added to perfumes whenever a lemonlike essence is desired, and it is used as an intermediate for the synthesis of vitamin A (3).
The commercial importance of citral has stimulated an extensive search for its presence in nature. One source is the oil from the skins of lemons and oranges, although it is only a minor component of this oil. However, citral is the major con- stituent of the oil obtained from lemon grass, and in fact 75–85% of the crude oil derived from pressing lemon grass is this natural product.
Citral is a chemically labile substance, and its isolation therefore presents a challenge. This task is simplified by the fact that citral, bp 229 °C (760 torr), is rela- tively volatile and has low solubility in water. These properties make it a suitable candidate for steam distillation, a technique that allows distillation of citral from crude lemon grass oil at a temperature below 100 °C, which is far below its normal boiling point. Neutral conditions are maintained in steam distillation, as is the par- tial exclusion of atmospheric oxygen, so the possibility of oxidation and/or poly- merization of citral is minimized.
Steam distillation can be performed by two different methods, as described in Section 2.16. The first involves using an external steam source, which may be either a laboratory steam line or a steam generator, and passing the steam through a mix- ture of lemon grass oil and water. This method has some advantages, but it is experimentally more difficult than an alternative method, which involves heating a mixture of lemon grass oil and water and collecting the distillate. Although this latter procedure is simpler from the experimental standpoint, there are some limi- tations to its application. For example, in the steam distillation of only slightly volatile substances, a large initial volume of water will be required, or water must be added as the distillation proceeds, perhaps by means of an addition funnel.
Nonetheless, the technique for internal generation of steam is well suited for this experiment.
CH3
OH H3C CH3 CH3 CH3
3 Vitamin A CH3
CH3
CHO CH3
CH3
CH3 CH3
CHO 2
1
Geranial Neral
148 Experimental Organic Chemistry■ Gilbert and Martin
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