Fractional distillation theory

An account of the elementary theory of fractional distillation is given in Section 1,4. Fractionating columns suitable for both elementary and advanced work will be described in the following pages. 

The student should first study the elementary theory of fractional distillation given in Sections 1,4-1,5. The experimental technique for simple fractional distillation is described in Section 11,15. 

The mixture was heated to reflux for 24 hours. The chloroformic reaction mixture was washed with water, and then dried over anhydrous sodium sulfate. The chloroform was evaporated off and the oil obtained was fractionally distilled under a pressure of 0.2 mm Hg. The fraction distilling at 140°C to 160°C, being the desired product indicated above, was collected and crystallized. Yield 94 g (32% of theory) MP 7B°C (after recrystallization in petroleum ether). 

To show you the theory of the next topic, Class 3, Fractional Distillation. 

Detailed information about the theory of fractional distillation will be found in J. Eggert, Lehrbuch der physikalischen Chemie, 3rd Edition, 1931, p. 248. 

On the Theory of Steam Distillation.—The ideal case occurs when the substance to be distilled is insoluble, or, more accurately, sparingly soluble in water (examples toluene, bromobenzene, nitrobenzene) so that the vapour pressures of water and the substance do not affect each other, or hardly so. The case of substances which are miscible with water (alcohol, acetic acid) is quite different and involves the more complicated theory of fractional distillation. Let us consider the first case only and take as our example bromobenzene, which boils at 155°. If we warm this liquid with water, its vapour pressure will rise in the manner shown by its own vapour pressure curve and independently of that of water. Ebullition will begin when the sum of the vapour pressures of the two substances has become equal to the prevailing atmospheric pressure. This is the case, as we can find from the vapour pressure curves, at 95-25° under a pressure of 760 mm. 

Martin and Synge first proposed the plate theory in 1941, whereby they merely compared the GC separation to fractional distillation. Thus, the theoretical plate is the portion of the column wherein the solute is in complete equilibrium with the mobile and the stationary phase. 

Physical Methods of Organic Chemistry , Interscience, NY, Vol 4(1951), pp 1 to 174 (Theory of distillation) 175 to 316 (Ordinary fractional distillation) 317 to 387 (Extractive and azeotropic distillation) 389 to 461 (Distillation of liquefied gases and low-boiling liquids) 463 to 494 (Distillation under moder-... 

To describe the formal theory of fractional distillation, let us consider the boiling-point diagram of a near-ideal A/B binary solution, as shown in Fig. 7.10. The solution is initially at high concentration x of the high-boiling component B. Consider the following four steps, as illustrated in the figure ... 

The product is colored red by the presence of vanadium tetrachloride and contains considerable dissolved chlorine. These impurities may be removed by fractional distillation followed by a distillation from metallic sodium. The last few milliliters of vanadium oxytrichloride should not be removed from the sodium by a direct flame, as superheating often causes an explosion. A fractionating udder is convenient for collecting the product without exposing it to the moisture in the air. The side tube should be connected to a drying tube. Yield 70 g. (87 per cent of theory) boiling point 124.5 to 125.5° at 744 mm. 

A mixture of 560 g of potassium carbonate, 700 ml of ethanol (96%), 404 g of 1,3-dibromopropane and 260 g of ethyl acetoacetate was heated with stirring to go 60°C. After the reaction had subsided, the reaction mixture was refluxed for 5 hours. Then the bulk of the alcohol was distilled off under ordinary pressure and the residue was mixed with 1.5 L of water. The resulting oily layer was separated, and the aqueous phase was extracted with benzene and the benzene layer was combined with the oil. After drying with sodium sulfate the benzene was distilled off and the residue was fractionally distilled 250 g (73% of theory) of 2-methyl-3-carbethoxy-5,6-dihydropyrane of boiling point 105°-108°C were obtained. 

A mixture of 61 grams l-phenyl-l-oxo-2-(N-methyl-N-ethanolamino)-propane hydrochloride and 100 cc 98-100% formic acid was refluxed at the boiling point at atmospheric pressure for 45 minutes on an oil bath. Thereafter, the oil bath temperature was increased to 180°C and as much of the excess unreacted formic acid as possible was distilled off. A vigorous evolution of carbon dioxide developed during the distillation, which ceased after approximately 45 additional minutes. The honey-yellow syrup which remained as the distillation residue was worked up by admixing it with about six volumes of water and adjusting the aqueous mixture to alkaline reaction with concentrated sodium hydroxide. An oily phase separated out which was extracted with ether. The ether extract was washed with water and dried over potassium carbonate. The solvent was distilled off and the distillation residue was fractionally distilled in vacuo. The base boils at 132°-133°C at 12 mm. The yield was 93% of theory. Reaction with tartaric acid gave the final product. 

Problem 15 Discuss the theory of fractional distillation of binary solutions. 

Martin and Synge (3) introduced the important concept of theoretical plates into chromatography. Their concept was derived from partition theory and random statistics, and was related to similar ideas developed for extraction and fractional distillation. They supposed that the column could be divided into a number of sections called theoretical plates, and that solutes (dissolved compounds) could be expected to achieve equilibrium between the two phases (mobile and stationary) that exist within each plate. The chromatographic process, like an extraction process, can be visualized to occur when mobile phase (solvent) is transferred to the next plate, where a new equilibrium is established. Theoretical plate numbers of 1000 or more are common for HPLC columns, which means that 1000 separate equilibria must be established to obtain the same degree of separation by solvent... 

As soon as the reaction is complete, the product is transferred to a separatory funnel containing cold water. The oily, colourless, heavy layer which separates from the aqueous layer is washed twice with cold water, dried over calcium chloride and fractionally distilled from a water-bath. The fraction passing over between 69° and 72° C. is collected. Yield about 70% of theory. ... 

Theory of fractional distillation. We shall now discuss some consequences of Duhem s equation which are of importance in... 

To showyou the theory ofthe next topic. Class 3 Fractional Distillation. 

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