Patent GB-792572-A (GB)
Title: Process for the preparation of aliphatic ª‡-unsubstituted ª‡:ª‰-unsaturated carboxylic acids and their derivatives
IPC: C07C51/377
Filed: 1954-08-06
Abstract: Aliphatic a -unsubstituted a  : b -unsaturated carboxylic acids or their nitriles or amides are prepared by dehydration of the corresponding a -hydroxy-acids or their nitriles, amides or esters by treatment, at 520 DEG  to 700 DEG  C., preferably at 580 DEG  to 650 DEG  C., with phosphoric acid, pyrophosphoric acid, meta- or poly-phosphoric acids containing not more than 6 phosphorus atoms, hydrogen chloride, hydrogen bromide or mixtures of these acids, present in amount of 1 to 90 per cent, preferably 20 to 40 per cent, by weight of the total weight of inorganic acids and organic starting material.  Water and/or sulphuric acid may also be present in the reaction mixture.  Carboxylic acids and their esters are converted to the corresponding unsaturated acids while nitriles and amides are initially coverted to the corresponding unsaturated nitriles and amides; in the presence of aqueous hydrochloric or hydrobromic acid or of sulphuric acid however, these are at least partially hydrolysed to the corresponding acids.  In one method of performing the dehydration, which is advantageously conducted under reduced pressure, the reactants are first vaporized and the vapours, which may be diluted with oxygen-free inert gases, e.g. nitrogen or oxygen-free steam, are passed simultaneously through the heated reaction tower. Alternatively, the reactants may be introduced simultaneously into the tower in a finely-divided state, e.g. by forcing them through nozzles, the resultant mixture being diluted with oxygen-free inert gases.  The reaction tower is preferably brick-lined and the dehydrating agents, when they are not injected, may be supported on carriers.  The reaction tower may be heated externally or, alternatively, direct contact of the spray cone of the reaction mixture with hot inert oxygen-free gases, such as burnt gas, may be used to attain the required temperature.  Two forms of apparatus suitable for use in the process are described with the aid of drawings.  In one of these the reactants are forced by means of pressure nitrogen into the top of an electrically-heated vertical reaction furnace and the product passes out through the conical shaped lower end of the furnace.  In the other apparatus burnt gases are produced in a horizontal reaction chamber and are passed through a distributer ring to the top of the main vertical reaction chamber where they meet and heat the reactants, the remainder of the apparatus being as described above.  In examples: (1) lactic acid nitrile is dehydrated in a quartz tube in presence of phosphoric acid and water at 600 DEG  C. and 20 mm. for a reaction time of 0.28 seconds, the chief product, separated by distillation, being acrylonitrile; at 760 mm. and for a reaction time of 6.9 seconds the yield of acrylonitrile is decreased; (2) a -hydroxybutyric acid nitrile is dehydrated as in (1), the reaction time being 0.34 seconds, and the chief product is crotonic acid nitrile; (3) the process of (1) is repeated except that the dehydrating agent is hydrochloric acid and the reaction time is 0.26 seconds, the products being, chiefly, acrylic acid and some acrylonitrile; the same products are obtained if hydrobromic acid replaces the hydrochloric acid; (4) ethyl lactate dehydrated with phosphoric acid and water at 620 DEG  C. and 20 mm. for 0.41 second yields chiefly acrylic acid; (5) the lactic acid nitrile of (1) is replaced by lactic acid, the reaction time being 0.47 seconds, and the product is chiefly acrylic acid; (6) the first apparatus described above is used for the dehydration of lactic acid nitrile in presence of phosphoric acid and steam at 650 DEG  C. and 100 mm. for 4.6 seconds, the gases produced being cooled, separated from phosphoric acid and steam, washed with dilute phosphoric acid and then with a weakly alkaline solution of lactic acid nitrile and finally condensed under pressure to give chiefly acrylonitrile, separated phosphoric acid being recycled; (7) the second apparatus described above is used for the dehydration of lactic acid nitrile with phosphoric acid at 600 DEG  to 620 DEG  C. and 760 mm. for 3.1 seconds, the product giving, on distillation, chiefly acrylonitrile.  A list of further starting materials, including unsubstituted, N-lower alkyl substituted and N : N-di-(lower alkyl) substituted amides, and the products obtained therefrom by the process of the invention is given.