| 摘要 |
<p>Esters or acetals are prepared from a dilute aqueous solution of a water-soluble organic compound, which is a glycol or a polyhydric alcohol or contains an aldehyde or carboxylic acid group by adding as appropriate a water-soluble ester- or acetal-forming compound, which is carboxylic, alcoholic or aldehydic, reacting the added compound with the already dissolved compound under ester- or acetal-forming conditions and during said reaction preferentially solvent extracting the resulting ester or acetal from the aqueous mixture with a water-immiscible or partially immiscible solvent by carrying out the reaction in the presence of the solvent or by intermittently extracting the reaction mixture with successive portions of the solvent, so that the equilibrium in the aqueous phase is displaced in the direction of the acetal or ester formation and recovering the organic extract. In a modification, excess of the ester- or acetal-forming compound is used to serve as the solvent, provided it is not completely miscible with the aqueous solution. The dilute aqueous solutions used are those containing up to 20% w/v of the water soluble organic compound. The reactions of a di- or polyhydric alcohol with an aldehyde to form an acetal, a carboxylic acid with an alcohol to form an ester and a hydroxy carboxylic acid with an aldehyde to form a cyclic acetal ester, are mentioned; if two ester- or acetal-forming functional groups are present, a di-acetal or di-ester may be prepared; use of a suitably substituted compound, e.g. glyoxylic acid enables simultaneous acetalisation and esterification to occur. An acid catalyst may be present and the reaction extraction can be effected continuously in an extraction column. The ester or acetal may be separated from at least part of the solvent in the extract by distillation, crystallisation or solvent extraction. The recovered ester or acetal may be decomposed by hydrolysis to yield the original water soluble organic compound in separated concentrated form, and the ester- or acetal-forming compound originally added. Alternatively, the original water-soluble organic compound may be recovered from the ester or acetal by transesterification or transacetalisation in a solvent for the ester or acetal in which the original water soluble organic compound is only slightly soluble. Examples describe the reaction as above of dilute aqueous solutions of (1) 1:3-or 2:3-butane diol with acetaldehyde yielding the cyclic acetal; (2) glycerol with acetaldehyde to yield the cyclic acetal; (3) erythritol with acetaldehyde yielding the di-acetal; (4) acetic acid with methanol yielding methyl acetate; (5) lactic acid with acetaldehyde yielding 2:5-dimethyl-1:3-dioxolan; (6) glyoxylic acid with methanol and butylene glycol to form the cyclic diol acetal of glyoxylic acid methyl ester; (7) pyrrolidone carboxylic acid and methanol forming the methyl ester, typical solvents used being xylene, methylene chloride, dichloro benzene, benzophenone, chloroform and diphenylene oxide. Examples (12) and (13) describe the hydrolysis of the butane diol acetal obtained in (1), by heating the acetal with a limited amount of water and formic or sulphuric acid as catalyst and distilling off the liberated acetaldehyde and excess water. The treatment of dilute aqueous glycollic acid and glyoxal solutions is also mentioned. Reference is also made to the recovery of glyoxylic acid in the form of its butane diol acetal which is heated with acetaldehyde to liberate the glyoxylic acid.</p> |
