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UNITED STATES PATENT OFFICE 2,56,31 METHOD of REDUCING and by DRO GENATING CHEMICA, COMPOUNDS by REACTING WITE: ALUMNUM-CONAN NG BYOFREDES Hermann E

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UNITED STATES PATENT OFFICE 2,56,31 METHOD of REDUCING and by DRO GENATING CHEMICA, COMPOUNDS by REACTING WITE: ALUMNUM-CONAN NG BYOFREDES Hermann E Patented Nov. 27, 1951 2,576,31 UNITED STATES PATENT OFFICE 2,56,31 METHOD OF REDUCING AND BY DRO GENATING CHEMICA, COMPOUNDS BY REACTING WITE: ALUMNUM-CONAN NG BYOFREDES Hermann E. Schlesirager and Albert E. Finholt, Chicago, Ill.; said Schlesinger assignor of one fourth to. Edaa, M. Schlesinger and said Fin holt assignor of one-fourth to Marion H. Finholt No Drawing. Application June 3, 1947, Serial No. 752,286 2 (Cairns. (C. 260-638) 2 This invention relates to methods of making LiAlH4. Although this new compound will be aluminum-containing hydrides and the reactions called lithium aluminum hydride in the present thereof, and also relates to products prepared by application, it may also be called lithium alumi said methods. nohydride or lithium tetrahydroaluminide. In This application is a continuation-in-part of one method of making lithium aluminum hydride, our copending application Serial No. 717,312, filed lithium hydride is reacted with an aluminum December 19, 1946, now Patent No. 2,567,972, halide such as aluminum chloride in the presence issued September 18, 1951. of a suitable liquid medium such as an ether. If We have discovered that these compounds, es the reagents are mixed in the proportions of the pecially the ether soluble lithium aluminum hy 0 following equation, or if an excess of lithium hy dride, are extremely useful chemical reagents. dride is used, the reaction proceeds as follows: - They may be employed for replacing halogens or Organic radicals by hydrogen in a great variety 4Li H--AlCl3->LiAlH4--3LiCl of compounds. As a result, their discovery has led to new methods, safer, more convenient, and 16 The liquid medium used is one in which one of more efficient than those hitherto known, for pro the reaction products, e. g., the lithium chloride ducing hydrides of other elements or for pro is insoluble, and the other, e.g., lithium aluminum ducing derivatives of such hydrides, as well as for hydride, is soluble or vice versa. This provides reducing certain types of organic halides to the easy separation of the reaction products. The corresponding hydrocarbons. Furthermore, these 20 preferred liquid is an ether as in the ether the aluminum-containing hydrides are strong re lithium aluminum hydride is soluble while the ducing agents and have been effectively used in lithium chloride is not. After removing the in reducing a great variety of organic compounds. soluble materials, the solvent is removed by Examples are the reduction of carbon dioxide to evaporation leaving the Solid lithium aluminum derivatives of formaldehyde, of esters to alcohols 25 hydride. The solvent may be evaporated first or aldehydes, of nitro compounds to azo con at ordinary temperatures and pressures and pounds, of nitriles to amines, and the like. In finally at higher temperatures under reduced many instances these reductions are more effi pressures. In this reaction three gram equiva ciently and conveniently carried out than with lents of aluminum chloride are reacted with four hitherto used reducing agents. A particular ad- 30 gram equivalents of lithium hydride. The term vantage of the aluminum-containing hydrogen "gram equivalent' means the quotient of the compounds is the specificity of their reactions; gram molecular weight of the compound by the thus, in general they reduce functional groups, product of the valences of the ions of the Com Such as the carbonyl, nitro, and other reducible pound. groups, of compounds containing an unsaturated 35 In carrying out the above reaction, the two carbon to carbon linkage without attacking the solids may first be mixed with each other in a . Carbon to carbon link. suitable apparatus and the liquid solvent then The new hydrides include a new aluminum hy added. In an alternate procedure, the Solid dride-ether complex, alkali metal aluminum hy lithium hydride may be introduced into the re drides, and alkaline earth metal aluminum hy- 40 action vessel and then the aluminum chloride drides. In general, these are prepared by react may be added, preferably as a Solution in the ing an aluminum halide with an alkali metal hy liquid to be used. This latter is the preferred dride or an alkaline earth metal hydride. procedure. It is also preferred that the air be One of the most important of the new hydrides displaced from the reaction vessel by an inert gas is lithium aluminum hydride having the formula 45 such as dry nitrogen. This is not essential, howa 2,576,811 V 3 4. ever, since the reaction has been carried out in ever, to remove all of the ether by evaporation air of ordinary humidity. without decomposing the hydride. If the ether When the reacting materials are mixed, the is removed at room temperature without exten mixture usually becomes warm. Ordinarily, how sive evacuation, the Solid product has a compo ever, this incipient reaction soon stops or ben sition approximating the formula. comes too slow to be readily appreciable. After an induction period, which may be only a few minutes in length or may last for Several hours, the reaction again sets in, usually with Such vigor Its composition, however, depends on the time that cooling of the mixture is necessary. This O and temperature during ether removal. Thus by induction period is undesirable for several rea heating the Solid at 90° C. in vacuo for several sons. It is difficult or impossible to foretell when hours, the composition may reach proportions the vigorous reaction will begin. It is therefore corresponding to 6Al H3 O(Cahs) 2. Irrespective necessary that the reaction mixture be closely of its composition, the Solid is at best only watched in order that the cooling may be begun slightly soluble in ether. Nevertheless, either the before the reaction has become too vigorous for original solution of aluminum hydride or the safety. It has been found that the induction ether insoluble Solids may be used in place of period may be eliminated by adding to the lithium lithium aluminum hydride. hydride a relatively small amount of previously As can be seen from the first and second equa prepared lithium aluminum hydride, preferably 20 tions set out herein, lithium hydride and alumi dissolved in the liquid that is used in the reac num chloride may be reacted together to form tions. When this is done, the reaction proceeds either lithium aluminum hydride or aluminum smoothly on addition of the aluminum halide. hydride. The final product is determined by the Under these conditions, the rate of reaction may proportions of the reacting materials. Thus, to be conveniently controlled by controlling the rate 25 prepare lithium aluminum hydride four moles or of addition of the aluminum halide, which is pref more of lithium hydride are reacted per mole of erably added in the form of a Solution. Among aluminum chloride. If... the final product is to be the preferred solvents are the ethers such as di aluminum hydride, only three moles of lithium ethyl ether, dibutyl ether, dioxane, and any other hydride are reacted per mole of aluminum chlo liquid ether non-reactive toward lithium alumi 30 ride. AS has been pointed out above, it is possible num hydride. For best results, the liduid used to use aluminum halides other than the chloride. should be anhydrous and the aluminum halide It is also possible to use hydrides of alkali metals should be reasonably free from hydrogen halide. other than lithium, or hydrides of the alkaline In all of the operations, moisture should be ex earth metals. cluded although it is not necessary to work under 35 The new compound lithium aluminum hydride absolutely anhvdro is conditions. The alkali is a white solid that is stable in dry air at room metal and alkaline earth metal hydrides that are temperature. It may be heated without appre used should be of good quality. It is preferred clable decomposition to temperatures below 100' that each of these materials be used in a finely . C. in a vacuum. Above 100° C., it decomposes powdered condition, preferably between 100 and 40 slowly, but the rate of decomposition increases 200 mesh. with rise in temperature. At 150° C., the decom Although it is preferred that a solvent such as position can be observed by color changes after an ether be used, this is not absolutely neces a relatively short time. The products of decom sary. The aluminum halide and the hydride may position are lithium hydride, aluminum, and hy be reacted in the absence of a liquid, but the re 45 drogen. The new compound is soluble in diethyl action is sometimes difficult to control. ether to the extent of about 25 grams per 100 Aluminum hydride is closely related to the grams solvent. It is also soluble in other ethers. lithium aluminum hydride in its chemical prop The new compound reacts with water to give erties. Aluminum hydride may be prepared by hydrogen, and either lithium hydroxide and alu reacting aluminum halides with either lithium 50 minum hydroxide, or lithium aluminate. This hydride or lithium aluminum hydride. Any reaction is quite rapid. In spite of this, the com similar hydride of an alkali metal or an alkaline pound does not decompose very rapidly on ex earth metal may be used in place of the lithium posure to air of even fairly high humidity. It compound in producing the aluminum hydride. is believed that this is true because the solid When the reagents are mixed in the proper pro 55 becomes coated with a protective layer of reac portions the reaction of aluminum chloride and tion product. lithium hydride proceeds as follows: In a typical embodiment of the method of making lithium aluminum hydride, 0.02 mole of anhydrous aluminum chloride was mixed with 60 0.50 mole of lithium hydride under dry nitrogen The reaction between aluminum chloride and in a flask.
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