Reverse azeotropic distillation

With reversible reactions, recycling is warranted when improvement in conversion can be realized by removing some of the product in a separator and returning only unconverted material. In some CSTR operations, the product is removed continuously by extraction or azeotropic distillation. The gasoline addi-... 

The formation of imines mkes place by a mechanism that is the reverse of the hydrolysis. Preparative proc ures often ensure completion of the reaction by removing water as it is formed by azeotropic distillation or by the use of an irreversible dehydrating agent. 

Although the previous two sections of this chapter emphasized hydrolytic processes, two mechanisms that led to O- or N-acylation were considered. In the discussion of acid-catalyzed ester hydrolysis, it was pointed out that this reaction is reversible (p. 475). Thus, it is possible to acylate alcohols by reaction with a carboxyhc acid. To drive the reaction forward, the alcohol is usually used in large excess, and it may also be necessary to remove water as it is formed. This can be done by azeotropic distillation in some cases. 

With reversible reactions, sufficient improvement in conversion sometimes can be realized from removing the product to warrant a recycle operation. This can be done by sending the product to a separator and returning only unconverted material. Some systems, moreover, lend themselves to continuous removal of product in equipment integrated with the reactor. Extraction is thus employed in problem P4.06.13 and azeotropic distillation in problems P4.06.14 and P4.06.15. The gasoline additive, methyl-tert-butyl ether, is made in a distillation column where reaction and simultaneous separation take place. 

The water formed in the reaction is continuously removed from the reaction mixture by azeotropic distillation, in order to avoid reversible reaction between water and ester. The progress of reaction can be followed either by measuring the amount of water or by determining the amount of unreacted acid in aliquots withdrawn at regular intervals of time. The reaction can be carried out either in presence of a catalyst, i.e. a weak acid like p-toluene sulphonic acid (a strong acid can hydrolyse the polymer, formed) or in absence of the catalyst. 

Acetal formation is reversible (K for MeCHO/EtOH is 0-0125) but the position of equilibrium will be influenced by the relative proportions of R OH and H2O present. Preparative acetal formation is thus normally carried out in excess R OH with an anhydrous acid catalyst. The equilibrium may be shifted over to the right either by azeotropic distillation to remove H2O as it is formed, or by using excess acid catalyst (e.g. passing HCl gas continuously) to convert H2O into the non-nucleophilic H3O . Hydrolysis of an acetal back to the parent carbonyl compound may be effected readily with dilute acid. Acetals are, however, resistant to hydrolysis induced by bases—there is no proton that can be removed from an oxygen atom, cf. the base-induced hydrolysis of hydrates this results in acetals being very useful protecting groups for the C=0 function, which is itself very susceptible to the attack of bases (cf. p. 224). Such protection thus allows base-catalysed elimination of HBr from the acetal (27), followed by ready hydrolysis of the resultant unsatu-... 

Obviously, the condensation of a carbonyl group with a diol produces 1 mol of water and because of the reversibility of the reaction (hydrolysis of the acetal), yields are lowered if this by-product is not removed. For such a purpose, there are essentially two possibilities (1) the continuous removal of water by an azeotropic distillation with a solvent mainly chosen for its boiling point (petroleum ether, benzene, toluene, xylene, for instance) (2) the presence of a desiccant (the most commonly taken is copper(II)sulfate, but sodium sulfate or molecular sieves have been also used) molecules known to be water scavengers, such as ortho-esters or dialkylsulfites, have also been suggested, even if they are seldom used in carbohydrate chemistry. 

Enol ether 13 is prepared from butanal 12 by acetalization with alcohol PMBOH 35. The resulting acetal 40 is subjected to elimination with phosphinic acid 36. Acetalization proceeds via nucleophilic attack of the alcohol on the protonated aldehyde 37, dehydratization of the hemiacetal 38 and further nucleophilic attack on the carbenium ion 39. Since all steps are reversible, the created water has to be removed to achieve quantitative turnover. This is carried out by the use of water binding agents or solvents (dry Na2S04, CaCl2, orthoesters) or azeotropic distillation. 

New phenomena compared to nonreactive Langmuir systems are the same as in the binary case - that is, the existence of combined waves due to the occurrence of inflection points of the equilibrium functions y(x) or Y(X) and limitations on feasible product composition due to adsorptivity reversal similar to azeotropic distillation. Nonreactive examples for the latter were treated in Refs. [6 - 8], reactive examples will be discussed in the next section. 

Sodium di(ethylhexyl)sulfosuccinate (Aerosol-OT, sodium docusate) [577-11-7] M 444.6. Dissolve it in MeOH and the inorganic salts which precipitate are filtered off. Water is added and the solution is extracted several times with hexane. The residue is evaporated to one-fifth its original volume, benzene is added and azeotropic distillation is continued until no water remains. The solvent is evaporated. The white residual soHd is crushed and dried in vacuo over P2O5 for 48hours [El Seoud Fendler J Chem Soc, Fcrcukiy Trans 1 71 452 1975]. [Beilstein 4 IV 114.] It solubilises major myelin trans membrane proteolipids, and forms reverse micelles in hydrocarbon solvents. 

Ketimines. The formation of ketimines from aldehydes or ketones and amines is reversible and, in general, it is necessary to remove the water formed. Azeotropic distillation has been usually used. Kyba1 recommends use of molecular... 

Sulphonation of aromatic compounds with sulphuric acid can take place under a wide variety of conditions depending upon the aromatic compound being used. The reaction is a reversible one, as shown in equation 4, and so either a large excess of mineral acid is required or the water must be removed, either by azeotropic distillation or by another process. One such procedure involves the addition of thionyl chloride to the sulphonating mixture. In this case, any water that is produced reacts with the thionyl chloride to form HC1 and sulphur dioxide6. 

In this section, you will prepare N-cinnamyl-m-nitroaniline (9) by a sequence beginning with the condensation of cinnamaldehyde (5) with nx-nitroaniline (6), followed by reduction of the intermediate imine 7 with sodium borohydride, as shown in Equations 17.12-17.14. The formation of the imine is reversible, but the reaction is driven to completion by azeotropic distillation. Because cyclohexane and water form a minimum-boiling azeotrope (Sec. 4.4), the water generated by the condensation of 5 and 6 is continuously removed by distilling the cyclohexane-water azeotrope throughout the course of the reaction. 

The dehydration of 34 to 29 is a reversible process that is driven to completion by removing the water from the reaction mixture. This is conveniently done by using toluene as the solvent for the reaction. Water and toluene form an azeotrope (Sec. 4.4), so azeotropic distillation allows the continuous separation of water as dehydration occurs. To minimize the amount of solvent that is required for distillations of this type, a Dean-Stark trap (Fig. 18.15) is commonly used. Because such traps are often not available in the undergraduate laboratory, an operational equivalent may be devised by assembling the apparatus in a way such that water, but not toluene, can be prevented from returning to the reaction flask (Fig. 18.16). 

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