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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 2.8 and 2.9.

Apparatus Thermometer, 10-mm75-mm test tube, two clamps, and apparatus for flameless heating.

Setting Up Obtain a liquid from your instructor and determine its boiling point following the technique described in Section 2.8, using the apparatus shown in Figure 2.20 and flameless heating.

M I C R O S C A L E P R O C E D U R E

Apparatus Thiele melting-point apparatus, micro boiling-point apparatus, and a Bunsen burner or microburner.

Setting Up Determine the boiling point of the liquid(s) assigned by your instructor.

Follow the technique presented in Section 2.8 for using the micro boiling-point apparatus (Fig. 2.21). Use a Bunsen burner or a microburner for heating. In the event you do not know the boiling point of the liquid, first determine an approximate boiling-point range by heating the Thiele tube (Fig. 2.17a) fairly rapidly.

Repeat the measurement by heating the tube until the temperature is 20–30 ºC below the approximate boiling point, and then heat the sample at a rate of 4–5 ºC/min to obtain an accurate value. It may be desirable to repeat this procedure to obtain a more reliable boiling point.

W R A P P I N G I T U P

Unless instructed otherwise, return the organic liquids to the appropriate bottle for either nonhalogenated liquids or halogenated liquids.

E X E R C I S E S

1.Refer to Figure 4.1 and answer the following:

a. What total pressure would be required in the system in order for the liquid to boil at 45 °C?

b. At about what temperature would the liquid boil when the total pressure in the system is 300 torr?

2.Describe the relationship between escaping tendency of liquid molecules and vapor pressure.

3.Define the following terms:

a. boiling point b. normal boiling point

c. Dalton’s law of partial pressures d. equilibrium vapor pressure

4.Using Dalton’s law, explain why a fresh cup of tea made with boiling water is not as hot at higher altitudes as it is at sea level.

5.At a given temperature, liquid A has a higher vapor pressure than liquid B.

Which liquid has the higher boiling point?

6.Explain why the boiling point at 760 torr of a solution of water, bp 100 °C (760 torr), and ethylene glycol (HOCH2CH2OH), bp 196–198 °C (760 torr), exceeds 100 °C. For purposes of your answer, consider ethylene glycol as a nonvolatile liquid.

130 Experimental Organic Chemistry■ Gilbert and Martin

7.Why should there be no droplets of water in the oil of a heating bath?

8.Why is the micro boiling-point technique not applicable for boiling points in excess of 200 °C if mineral oil rather than silicone oil is the heating fluid in the Thiele tube?

9.A rotary evaporator (Sec. 2.29) is a device frequently used in the laboratory to remove solvent quickly under vacuum.

a. Why is it possible to effect the removal of solvent at temperatures below their normal boiling points using this device?

b. Other than being faster than simple distillation for removing a given vol- ume of solvent, why might this type of distillation be preferred for isolat- ing a desired product?

4.3 S I M P L E D I S T I L L A T I O N

Simple distillation allows separation of distillates from less-volatile substances that remain as pot residue at the completion of the distillation. In the ideal case, only a single component of the mixture will be volatile, so the distillate will be a pure com- pound. Real life is rarely ideal, however, and it is more common that several volatile components comprise the mixture. Simple distillation allows isolation of the vari- ous components of the mixture in acceptable purity if the difference between the boiling points of each pure substance is greater than 40–50 °C. For example, a mixture of diethyl ether, bp 35 °C (760 torr), and toluene, bp 111 °C (760 torr), could easily be separated by simple distillation, with the ether distilling first. Organic chemists frequently use this technique to separate a desired reaction product from the solvents used for the reaction or its work-up. The solvents are usually more volatile than the product and are readily removed from it by simple distillation.

To understand the principles of distillation, a review of the effect of impurities on the vapor pressure of a pure liquid is necessary. The discussion starts with consid- eration of the consequences of having nonvolatile impurities present and then turns to the more common case of contamination of the liquid with other volatile substances.

Consider a homogeneous solution composed of a nonvolatile impurity and a pure liquid; for the present purpose, these are taken as sugar and water, respec- tively. As a nonvolatile impurity, the sugar reduces the vapor pressure of the water because it lowers the concentration of the volatile constituent in the liquid phase. The consequence of this is shown graphically in Figure 4.2. In this figure, Curve 1 corresponds to the dependence of the temperature upon the Chapter 4■ LiquidsDistillation and Boiling Points 131

Temperature (°C)

Vapor pressure (torr)

40 50 60 70 80 90 100 110

500 760

1 2 Figure 4.2

Diagram of dependence of vapor pressure on temperature. Curve 1 is for pure water, and Curve 2 is for a solution of water and sugar.

vapor pressure of pure water and intersects the 760-torr line at 100 °C. Curve 2 is for a solution having a particular concentration of sugar in water. Note that the presence of the nonvolatile impurity reduces the vapor pressure at any temperature by a constant amount, in accord with Raoult’s law as discussed later. The temperature at which this curve intersects the 760-torr line is higher because of the lower vapor pressure, and consequently the temperature of the boiling solution, 105 °C, is higher.

Despite the presence of sugar in the solution, the head temperature (Sec. 2.13) in the distillation will be the same as for pure water, namely, 100 °C (760 torr), since the water condensing on the thermometer bulb is now uncontaminated by the nonvolatile impurity. The pot temperature will be elevated, however, owing to the decreased vapor pressure of the solution (Fig. 4.2). As water distills, the pot temperature will progressively rise because the concentration of the sugar in the stillpot increases, further lowering the vapor pressure of the water. Nevertheless, the head temperature will remain constant, just as though pure water were being distilled.

The quantitative relationship between vapor pressure and composition of homogeneous liquid mixtures is known as Raoult’s law (Eq. 4.2). The factor PX represents the partial pressure of component X, and it is equal to the vapor pressure, P°X, of pure X at a given temperature times the mole fraction NXof X in the mixture. The mole fraction of X is defined as the fraction of all molecules present in the liquid mixture that are molecules of X. It is obtained by dividing the number of moles of X in a mixture by the sum of the number of moles of all components (Eq. 4.3). Raoult’s law is strictly applicable only to ideal solutions, which are defined as those in which the interactions between like molecules are the same as those between unlike molecules. Fortunately, many organic solutions approximate the behavior of ideal solutions, so the following mathematical treatment applies to them as well.

PXP˚XNX (4.2)

(4.3) Note that the partial vapor pressure of X above an ideal solution depends only on its mole fraction in solution and it is completely independent of the vapor pressures of the other volatile components of the solution. If all components other than X are nonvolatile, the total vapor pressure of the mixture will be equal to the partial pressure of X, since the vapor pressure of nonvolatile compounds may be taken as zero. Accordingly, the distillate from such a mixture will always be pure X. This is the case for the distillation of a solution of sugar and water, as discussed earlier.

When a mixture contains two or more volatile components, the total vapor pressure is equal to the sum of the partial vapor pressures of each such component.

This is known as Dalton’s law (Eq. 4.4), where PX, PY, and PZrefer to the vapor pressures of the volatile components. The process of distilling such a liquid mixture may be significantly different from that of simple distillation, because the vapors above the liquid phase will now contain some of each of the volatile components. Separation of the liquids in this case may require the use of fractional distillation, which is discussed in Section 4.4.

PtotalPXPYPZã ã ã (4.4)

NX nX

nX nY nZ Á

132 Experimental Organic Chemistry■ Gilbert and Martin

See Who was Raoult?

See Who was Dalton?

E X P E R I M E N T A L P R O C E D U R E S

Simple Distillation

Purpose To demonstrate the technique for purification of a volatile organic liquid containing a nonvolatile impurity.

S A F E T Y A L E R T

Use paper towels to clean up spilled liquids; discard the towels as directed