Boiling Points and Distillation

The action of boiling is a process familiar to anybody who has cooked pasta or brewed tea. As heat is applied to a pan of water, the temperature of the water increases until it reaches 100°C (212°F). At this temperature, additional heat causes the water to bubble vigorously and transforms liquid water into gaseous water, or steam. Most organic liquids will behave in a similar fashion. On heating, the temperature of the liquid increases until a certain temperature called the boiling point is reached. Additional heating causes the liquid to vaporize accompanied by vigorous bubbling of the liquid. The boiling point of a substance is a physical property of a substance and can be useful for characterizing that substance. The process of heating a substance until it vaporizes, cooling the vapors, and collecting the condensed liquid is the basis of a commonly used purification technique called distillation. Vapor Pressure and Boiling Point

If a liquid is placed in an empty, closed container, some molecules at the surface of the liquid evaporate into the empty space above the liquid. Once vaporized, some of the molecules in the vapor condense into the liquid in a competing process. As the space above the liquid becomes occupied with molecules of vaporized liquid, the pressure of the vapor above the liquid rises until it reaches a certain value. When the pressure stabilizes, the rates of evaporation and condensation are equal. The pressure of the vapor under these conditions is called the equilibrium vapor pressure. The equilibrium vapor pressure of a liquid is temperature dependent. As the temperature of the liquid is raised, more molecules vaporize and the equilibrium vapor pressure increases. The graph in Figure 11 shows the relationship between vapor pressure and temperature for three substances, dichloromethane, water, and d-limonene.

Figure 11. Vapor pressure-temperature diagram for dichloromethane, water and d-limonene.

The normal boiling point of a substance is defined as the temperature at which the vapor pressure of that substance equals atmospheric pressure, 760 mmHg. The normal boiling points of dichloromethane, water, and d-limonene are, respectively, 40.2°C, 100°C, and 175°C. If the barometric pressure is less than 760 mmHg, the temperature at which a substance boils will be less than the normal boiling point. When measuring boiling points, it is important to record the barometric pressure at the time of the measurement. In the chemical literature, there are several different methods of reporting boiling points with the barometric pressure. For example, the boiling point of 2,5-hexanedione at 754 mmHg is 194°C and is

reported in the following formats: bp754 194°C, bp 194°C/754 mmHg, bp 194°C (754 mmHg), or 194°C754. If a boiling point is reported without a pressure it is assumed to be the normal boiling point and the pressure is 760 mmHg.

To compare an observed boiling point to one reported in the chemical literature, it is often necessary to compensate for differences in pressure. The following formula provides a good estimate to correct an observed boiling point to a normal boiling point:

∆t = 0.00012 (760 - p) (t + 273) where ∆t is the correction in degrees Centigrade to be added to the observed boiling point t at barometric pressure p. Another rule of thumb for estimating boiling points near 760 mmHg states that the observed boiling point will differ from the normal boiling point by 0.5° for every 10 mmHg difference in pressure. Distillation of a Single Volatile Liquid

The vaporization of a liquid and condensation of the resulting vapor is the basis for a method of purification called distillation. Organic liquids containing very small amounts of impurities, non-volatile substances, or substances with very high boiling points are easily purified by simple distillation. A typical simple distillation setup is shown in Figure 12. It consists of a flask containing the material to be distilled, an adapter to hold a thermometer and to connect the flask to a water-cooled condenser, and a flask to hold the condensed material.

Figure 12. Simple distillation apparatus

Distillation Apparatus

The distillation flask. The distillation flask is a round-bottom flask and is often referred to as the pot. The liquid to be distilled should fill the distillation flask to 1/2 to 2/3 of its capacity. If the flask is too small, the liquid is likely to bump or foam over into the receiving flask without vaporizing. If the flask is too large, a substantial amount of the liquid may be lost as vapor filling the flask. This residual volume is called the take-up volume.

To promote even boiling of the liquid, pieces of porous clay plate or silicon carbide chips are added to the liquid before starting to heat the liquid. The irregular chips provide sites for bubbles of vapor to form. An alternative method for promoting even boiling is to agitate the liquid with a magnetic stirrer as it is being heated. CAUTION: Never add boiling chips or a stir bar to a hot liquid. This can cause a seemingly calm liquid to boil suddenly and violently.

The distilling adapter. The adapter connects the distillation flask, the condenser, and the thermometer. This type of adapter is often referred to as a distillation head. The ground glass joints must be lined up and mated tightly to avoid leakage of the vaporized liquid. Leakage will result in loss of some of the liquid and will pollute the laboratory environment. The position of the thermometer is adjusted so the bulb is below the adapter sidearm connected to the condenser. In order to get reliable readings on the thermometer during the distillation, the vapors of the heated liquid must totally surround and contact the thermometer bulb.

The condenser. The condenser cools the vapor causing it to reliquify and directs this condensate to the receiving flask. The most common type of condenser is the water-jacketed type shown in Figure 12. The water supply is connected to the condenser with rubber hoses. The water flows in the lower hose connection (most remote from the distillation flask) and out the upper hose connection. Before turning the water flow on, check the hose connections carefully to ensure that they are secure and will not pop off. An extra margin of security can be gained by twisting wire around the hose connections. The water flow is adjusted so there is a slow, constant flow of cold water to the condenser. During the distillation of very high boiling liquids, it is common practice to cool the condenser with air instead of water. The thermal shock to glassware from a large temperature difference between cold water and hot vapor can crack the glassware.

The receiving flask. The container to collect the liquified vapor is called the receiver. It may be a round-bottom flask, an Erlenmeyer flask, a bottle, or a graduated cylinder. In Figure 12, the receiver is connected to the condenser with an adapter. The sole purpose of the adapter in this case is to direct the condensate from the condenser to the receiver. One important feature of the setup should be noted at this point—the system is open to the atmosphere. If the liquid being collected has a low boiling point, it is good practice to cool the receiving flask with a cold water bath.

Heat is slowly applied to the distillation flask. The amount of heat to apply is determined by the rate of distillation. The liquid should gently bubble and vaporize. As vapor rises from the liquid, it moves up the apparatus raising the temperature of the apparatus. The vapor will fill the distillation flask and most of the distillation head. The thermometer bulb should be completely surrounded by the vapor. If vapor creeps past the thermometer bulb without contacting it, the measured boiling point will be low. The vapor condenses in the condenser and drips into the receiving flask. Typically, the liquid should drip into the receiving flask at a rate of about 10 drops per minute. If the rate of distillation is too rapid, the heat applied to the distillation flask must be decreased. With too rapid a rate, the measured boiling point is likely to be inaccurate and the purity of the distilled liquid will be compromised.

The behavior of the measured boiling point during the course of a simple distillation is graphed in Figure 13. As the liquid vaporizes and the vapor comes into contact with the thermometer bulb, the temperature rises. The temperature stabilizes at the boiling point and most of the liquid distills. The temperature drops when there is no liquid left in the distillation flask.

Figure 13. Boiling point of a pure substance as a function of amount of liquid distilled

The first material that distills before the temperature stabilizes is called the forerun. The forerun will contain any low boiling impurities and is usually discarded after checking its purity. The material collected after the temperature stabilizes is the purified substance. Usually the temperature stabilizes slightly below the literature value for the boiling point and then slowly creeps up. When the temperature begins to drop, the distillation is halted by removing the heat from the distillation flask. As a safety precaution, distillations are never carried out to dryness. The residual liquid plus the liquid from the take-up volume is the price one pays for purity. The boiling point of the collected material is actually a range rather than a point. Distillation of Mixtures

There are two general types of mixtures to consider, mixtures of miscible liquids and mixtures of immiscible liquids. Their behavior on distillation is very different from one another and each type has very well defined uses in organic chemistry. Miscible liquids are soluble in each other in all ratios. Immiscible liquids do not dissolve in one another to any extent. Water is immiscible with most organic substances and, for our purposes, will always be one of the components in a mixture of immiscible liquids. Mixtures obey Dalton's law of partial pressures which states that vapor pressure above a mixture is equal to the sum of the vapor pressures of the individual components. For example, for a two component mixture:

where PA and PB are the partial pressures of components A and B respectively. The difference in the behavior of the two types of mixtures on distillation arises from the differences in partial pressures. Mixtures of miscible liquids. In a mixture of miscible substances, the partial pressure of a component depends on the vapor pressure of the pure component and the relative amount of the component in the mixture. This relationship is stated as Raoult's law—the partial pressure of a component in an ideal solution is equal to the vapor pressure of the pure component multiplied by its mole fraction:

BAtotal PPP +=

The mole fraction of component A, XA is defined as:

For a mixture containing only two components, A and B, the mole fraction is:

Combining Dalton's law of partial pressures and Raoult's law to a mixture of A and B, the vapor pressure of the mixture is:

Since the vapor pressures of the pure components increase with temperature, the vapor pressure above

the mixture will increase. When the vapor pressure of the mixture reaches 760 mmHg, the mixture will boil. For ideal solutions, i.e., solutions that obey Raoult's law, the boiling point of the mixture will be between the boiling points of the pure components.

Without going into the mathematical explanation, the important consequence of Raoult's law is that the vapor above a boiling mixture is enriched in the lower boiling component. By carrying out a distillation carefully, it is possible to collect portions of the distillate, which are considerably enhanced in the amount of lower boiling component and higher boiling component. If the difference in the boiling points of the two components is large, a careful distillation can separate the mixture into its two components.

In the figure below, are shown two curves representing the temperature of the distilling vapor as a function of the volume of distillate. The curve on the left represents an idealized distillation in which the lower boiling component distills completely and then the higher boiling component distills. The curve on the right represents a more empirical curve in which the distillate at the beginning of the distillation is enhanced in the amount of the lower boiling component and the distillate at the later stages of the distillation is enriched in the higher boiling component.

Ao

AA XPP =

mixturein molesofnumber totalA of moles ofnumber

=AX

B of molesofnumber A ofmolesofnumber A of moles ofnumber

+=AX

Bo

BAo

Atotal XPXPP +=

By careful control of temperature and by using columns designed to increase the surface area that the distilling vapors come in contact with, it is possible to make the empirical curve more closely resemble the curve on the left than the one on the right. This technique is referred to as fractional distillation.

The apparatus for a fractional distillation (Figure 14) is similar to the apparatus for a simple distillation with an extension added to increase the path the vapor has to travel. This extension is called a fractionating column. There are many types of fractionating columns that are used in fractional distillation. They are all similar in that the surface area, which contacts the distilling vapor, is increased. The larger the surface area contacted by the vapor, the more efficient the column is in separating the components. There are columns which are open, columns with glass indentations called Vigruex columns, and columns which are loosely packed with glass, metal or ceramic material (Figure 15).

The fractionating column is often insulated to keep the temperature of the column nearly constant. If the temperature of the column fluctuates widely, it is difficult to maintain a slow, constant distillation rate. Sophisticated fractionating columns have regulated heating coils built into them.

Figure 14. Fractional Distillation Setup

Figure 15. Types of fractionating columns: a) Vigruex b) open c) packed column

Mixtures of immiscible liquids. In an immiscible mixture, the partial pressure of a component is equal to the vapor pressure of the pure component. For an immiscible mixture Dalton's law becomes:

where PoA and PoB are the vapor pressures of pure A and pure B. When a mixture of immiscible liquids is heated, it will boil at a temperature which is less than the boiling point of either of the components. Both components will be present in the vapor. For a specific example consider a mixture of limonene and water. At a little over 97°C the vapor pressure of water is 695 mmHg and the vapor pressure of limonene is 65 mmHg. Since the sum of the vapor pressures equals 760 mmHg, the mixture boils. The mixture will continue to boil at this temperature as long as any limonene is present in the mixture. When all of the limonene is gone, the boiling point rises to 100°C, the boiling point of pure water. On condensing the vapor, the limonene and water, being immiscible in one another, separate into two phases.

This technique is referred to as steam distillation and can be an effective method for isolating organic materials from complex mixtures. The earliest isolations of organic substances from natural materials were done using steam distillation. One distinct advantage of steam distillation is the lower temperature required to isolate the organic substance. In the example with limonene, limonene is volatilized at 97°C whereas its normal boiling point is 175°C.

The universal gas law, PV = nRT, also holds for partial pressures:

and

oB

oAtotal PPP +=

RTnVP AA =

RTnVP BB =

where PA and PB are the partial pressures of A and B in the vapor and nA and nB are the number of moles of A and B in the vapor. Since the volume V, temperature T, and gas constant R are the same for both A and B and the partial pressures of A and B are equal to the vapor pressures of pure A and pure B, the ratio of the vapor pressures of A and B is equal to the ratio of the number of moles of A and B in the vapor:

B

Ao

B

oA

nn

PP

=

Since the numbers of moles of each component are equal to the weight of the component divided by its molecular weight, the ratio becomes:

BB

AAo

B

oA

MWmMWm

PP

//

=

where mA and mB are the weights of A and B and MWA and MWB are the molecular weights of A and B. Applying this to the limonene example, the amount of water required to distill 0.5 g of d-limonene can be calculated.

mwater = 0.7 g

Figure 16. Steam Distillation Setup

oneneonene

waterwatero

B

oA

MWmMWm

PP

limlim //

=

136/5.018/

65695 waterm

=

The setup shown (Figure 16) is a modified simple distillation apparatus wherein a claisen head is

added in between the boiling flask and the distilling adapter. The purpose of this is to provide a second opening into the system to accomodate a source of steam or the addition of water.

In actual practice, considerably more water is usually distilled to ensure that all of the organic material

has distilled over. One way of determining when all of the organic material has distilled is to check the condensate. If it is clear and is only one phase, the steam distillation is complete.