Boiling Points and Distillation

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The odors of both natural and synthetic organic chemicals surround us in our everyday life. Some, like the smells of fruits and many spices, are pleasant, whereas others, like those of decaying meat and rancid butter, can be rather unpleasant. We detect these odors because many organic chemicals are volatile, and in this chapter we explore the theory and practice of distillation, a technique that depends on our ability to coax these chemicals into the gas phase. 

The purification of solids by recrystallization and the use of melting points as a criterion of their purity are discussed in Chapter 3. The techniques used for the purification of liquids involve simple, fractiortal, steam, and vacuum distillation, and 

Two of the laws, Dalton s and Raoult s, that you have encountered in previous courses in chemistry and/or physics are critical to understanding the phenomenon of distillation. We ll review them in this chapter and show how they are applied to all forms of distillation that you will find in this textbook. As you will see, the apparatus for accomplishing distillations is considerably more complex than that needed for recrystallizations (Chap. 3), but with a little experience you will be able to complete a distillation in as little time as it takes to perform a recrystallization. 

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Bodenheimer [2] has described apparatus with which milligram quantities of material can be distilled and boiling points can be determined with an accuracy of 1—2 deg. C (Fig. 123). The end of tube a, which has been drawn out to an internal... 

Distillation. If the impurities in a liquid are non-volatile, the liquid may be purified by direct distillation, the impurities remaining in the distilling-flask. This process is therefore essentially the same as a simple distillation for boiling-point determination, and has been already described on pp. 7-9. 

Reflux 1 ml. of the ether with 5 ml. of freshly distilled, constant boiling point hydriodic acid (Section 11,49,2), b.p. 126-128°, for 2-3 hours. Add 10 ml. of water, distil and collect about 7 ml. of liquid. Decolourise the distillate by the addition of a httle sodium bisulphite, and separate the two layers by means of a dropper pipette (Fig. 11,27,1). Determine the b.p. of the resulting iodide by the Siwoloboff method (Section 11,12) and prepare a crystalline derivative (Section 111,42). 

It is standard practice in chemical laboratories to distill high-boiling-point substances under reduced pressure. Trichloroacetic acid has a standard enthalpy of vaporization of 57.814 kj-mol 1 and a standard entropy of vaporization of 124 J-K 1-mol. Use this information to determine the pressure that one would need to achieve to distill trichloroacetic-acid at 80.°C. 

Distillation is the process by which two or more liquids, or a solution of liquids and solids, are separated as a result of differences in vapor pressure and boiling points through the evaporation and condensation of the mixture components above the boiling liquid mixture. 

Experiment 8.—To 5 c.c. of freshly distilled acetaldehyde in a moderate-sized conical flask one drop of concentrated sulphuric acid is added with cooling. When the vigorous reaction is over, the paraldehyde produced is shaken in a small separating funnel with water in order to remove the sulphuric acid, and the polymeride, which is insoluble in water, is separated if necessary by extraction with ether. After being dried with a little calcium chloride the substance is distilled from a small distilling flask. Boiling point 124°. 

When the reduction is completed, the flask is connected with a condenser and the mixture subjected to distillation. The boiling point of constant boiling hydrobromic acid is 125-126° at 760 mm., but it must be remembered that in distilling the product from the sulfuric acid mixture, the thermometer reading should not be relied upon as an index to the composition of the distillate. Towards the end of the distillation the thermometer may rise to 130° and above, when water with only traces of acid distils from the sulfuric acid residue. Upon redistillation of the product the thermometer reading may be relied upon. For many uses a product free from traces of... 

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