Hydride, aluminum

Lithium aluminum hydride has some solubility in ether solvents but tends to form a slurry when relatively large amounts of the reagent are used. The solubility of LiAlH4 in ether is 35-40 g/100 g of ether but obtaining this solubility requires concentration of a more dilute solution, The solubility is 13g/100 g in THF 

Reacts with caustic soda solution forming gelatinous precipitate of aluminum hydroxide (hydrous aluminum oxide) yields aluminum monobasic stearate, A1(OH)2[OOC(CH2)i6CH3] when its solution is mixed with a solution of sodium stearate. 

Formula AIH3 MW 30.005 Structure polymeric, containing residual ether  

It is used as a reducing agent, and also as a catalyst for polymerization reaction. 

Colorless cubic crystal very unstable decomposes in water AH°/ -11.0 kcal/mol (-46.0kJ/mol) 

Aluminum hydride is prepared by the reaction of lithium hydride with aluminum chloride in diethyl ether 

LABORATORY CHEMICAL SAFETY SUMMARY LITHIUM ALUMINUM HYDRIDE 

Autoignition Temperature Toxicity Data Major Hazards 

White to gray crystalline solid Decomposes above 125° C Reacts vigorously with water Odorless solid Ignites in moist or heated air 

Reacts violently with water, acids, and many oxygenated compounds may ignite in moist air corrosive to skin, eyes, and mucous membranes. 

Lithium aluminum hydride is highly corrosive to the skin, eyes, and mucous membranes. Contact with moisture forms lithium hydroxide, which can cause severe bums. Powdered LAH forms dusts that can pose an inhalation hazard. Ingestion of this substance may cause aching muscles, nausea, vomiting, dizziness, and unconsciousness and may be fatal. Ingestion can result in gas embohsm due to the formation of hydrogen. 

Carbon Dioxide, Methyl Ethers. Decomposes violently in dimethyl or di-(2-methoxyethyl) ethers if carbon dioxide is present in the solution. If aluminum chloride is also present, the decomposition is explosive.4 

Tetrazoles. Complexes of aluminum hydride and a variety of tetrazoles are explosive.5 

Wear nitrile rubber gloves, laboratory coat, and eye protection, and work from behind body shield where possible. Keep available pulverized dolomite or dry graphite for firefighting. Eliminate all sources of ignition. Follow waste disposal procedure. 

Wear nitrile rubber gloves, laboratory coat, and eye protection. Work in the fume hood. Cover the hydride with a 1 1 1 mixture by weight of sodium or calcium carbonate, clay cat litter (bentonite), and sand. Mix carefully. Place material in a large container behind a safety shield in the hood. Slowly add dry butyl alcohol (31 mL per gram of aluminum hydride). After reaction ceases, slowly and cautiously add water (three times the volume of alcohol added). Neutralize with 6 M hydrochloric acid (prepared by adding concentrated acid to an equal volume of cold water), and let stand until solids settle. Decant the liquid into drain and discard the solid residue as normal refuse.7,8 

Reactions for Spillage and Waste Disposal AlHj + 3CH CH,CH,CH1OH - AI(OCHjCH,CH2CH,)j + 31 Ij 

Solubility soluble in ether (35 g/100 mL cone of more dil soln necessary) soluble in THF (13 g/100 mL) modestly soluble in other ethers reacts violently with H2O and protic solvents. 

Form Supplied in colorless or gray solid 0.5-1 M solution in diglyme, 1,2-dimethoxyethane, ether, or tetrahydrofuran the LiAlH4-2THF complex is available as a 1M solution in toluene. 

Analysis of Reagent Purity Metal Hydrides Technical Bulletin No. 401 describes an apparatus and methodology for assay by means of hydrogen evolution. See also Rickborn and Quartucci.  

Handling, Storage, and Precaution the dry solid and solutions are highly flammable and must be stored in the absence of moisture. Cans or bottles of L1A1H4 should be flushed with N2 and kept tightly sealed to preclude contact with oxygen and moisture. Lumps should be crushed only in a glove bag or dry box. 

Essential Reactions for Organic Synthesis, First Edition. Edited by Philip L. Fuchs. 2016 John Wiley Sons, Ltd. Published 2016 by John ley Sons, Ltd. 

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The conversion of carboxylic acid derivatives (halides, esters and lactones, tertiary amides and lactams, nitriles) into aldehydes can be achieved with bulky aluminum hydrides (e.g. DIBAL = diisobutylaluminum hydride, lithium trialkoxyalanates). Simple addition of three equivalents of an alcohol to LiAlH, in THF solution produces those deactivated and selective reagents, e.g. lithium triisopropoxyalanate, LiAlH(OPr )j (J. Malek, 1972). 

Synthesis by high-dilution techniques requires slow admixture of reagents ( 8-24 hrs) or very large volumes of solvents 100 1/mmol). Fast reactions can also be carried out in suitable flow cells (J.L. Dye, 1973). High dilution conditions have been used in the dilactam formation from l,8-diamino-3,6-dioxaoctane and 3,6-dioxaoctanedioyl dichloride in benzene. The amide groups were reduced with lithium aluminum hydride, and a second cyclization with the same dichloride was then carried out. The new bicyclic compound was reduced with diborane. This ligand envelops metal ions completely and is therefore called a cryptand (B. Dietrich, 1969). 

For most laboratory scale reductions of aldehydes and ketones catalytic hydro genation has been replaced by methods based on metal hydride reducing agents The two most common reagents are sodium borohydride and lithium aluminum hydride... 

Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do except that they function as hydride donors rather than as carbanion sources Figure 15 2 outlines the general mechanism for the sodium borohydride reduction of an aldehyde or ketone (R2C=0) Two points are especially important about this process... 

The mechanism of lithium aluminum hydride reduction of aldehydes and ketones IS analogous to that of sodium borohydride except that the reduction and hydrolysis... 

Neither sodium borohydride nor lithium aluminum hydride reduces isolated carbon-carbon double bonds This makes possible the selective reduction of a carbonyl group m a molecule that contains both carbon-carbon and carbon-oxygen double bonds... 

Carboxylic acids are exceedingly difficult to reduce Acetic acid for example is often used as a solvent in catalytic hydrogenations because it is inert under the reaction con ditions A very powerful reducing agent is required to convert a carboxylic acid to a pri mary alcohol Lithium aluminum hydride is that reducing agent... 

Sodium borohydride is not nearly as potent a hydride donor as lithium aluminum hydride and does not reduce carboxylic acids... 

Lithium aluminum hydride is the reagent of choice for reducing esters to alcohols O... 

Give the structure of an ester that will yield a mixture contain mg equimolar amounts of 1 propanol and 2 propanol on reduction with lithium aluminum hydride... 

Which of the isomeric C5H12O alcohols can be prepared by lithium aluminum hydride reduction of... 

In contrast to alcohols with their nch chemical reactivity ethers (compounds contain mg a C—O—C unit) undergo relatively few chemical reactions As you saw when we discussed Grignard reagents m Chapter 14 and lithium aluminum hydride reduc tions m Chapter 15 this lack of reactivity of ethers makes them valuable as solvents m a number of synthetically important transformations In the present chapter you will learn of the conditions m which an ether linkage acts as a functional group as well as the methods by which ethers are prepared... 

Epoxides are reduced to alcohols on treatment with lithium aluminum hydride Hydride is transferred to the less substituted carbon... 

Epoxidation of an alkene followed by lithium aluminum hydride reduction of the result mg epoxide gives the same alcohol that would be obtained by acid catalyzed hydration (Section 610) of the alkene... 

Reduction to alcohols (Section 15 2) Aide hydes are reduced to primary alcohols and ketones are reduced to secondary alcohols by a variety of reducing agents Catalytic hydrogenation over a metal catalyst and reduction with sodium borohydride or lithium aluminum hydride are general methods... 

Lithium aluminum hydride reduction (Sec tion 15 3) Carboxylic acids are reduced to primary alcohols by the powerful reducing agent lithium aluminum hydride... 

The reaction of esters with Gngnard reagents and with lithium aluminum hydride both useful m the synthesis of alcohols were described earlier They are reviewed m Table 20 4 on page 848... 

Reduction with lithium aluminum hydride (Sec tion15 3) Lithium alumi num hydride cleaves es ters to yield two alcohols... 

Section 20 9 Esters react with Gngnard reagents and are reduced by lithium aluminum hydride (Table 20 4)... 

Alkyl azides prepared by nucleophilic substitution of alkyl halides by sodium azide as shown m the first entry of Table 22 3 are reduced to alkylammes by a variety of reagents including lithium aluminum hydride... 

Reduction of an azide a nitrile or a nitro compound furnishes a primary amine A method that provides access to primary secondary or tertiary amines is reduction of the carbonyl group of an amide by lithium aluminum hydride... 

Alkyl azides prepared by nucleophilic substitution by azide ion in primary or secondary alkyl halides are reduced to primary alkylamines by lithium aluminum hydride or by catalytic hydrogenation... 

Reduction of amides (Section 22 9) Lithi um aluminum hydride reduces the car bonyl group of an amide to a methylene group Primary secondary or tertiary amines may be prepared by proper choice of the starting amide R and R may be ei ther alkyl or aryl... 

The carbonyl group of carbohydrates can be reduced to an alcohol function Typi cal procedures include catalytic hydrogenation and sodium borohydnde reduction Lithium aluminum hydride is not suitable because it is not compatible with the solvents (water alcohols) that are required to dissolve carbohydrates The products of carbohydrate reduc tion are called alditols Because these alditols lack a carbonyl group they are of course incapable of forming cyclic hemiacetals and exist exclusively m noncyclic forms... 

Lithium aluminum hydride, see Lithium tetrahydri-doaluminate... 

Ethyl ether Eiquid air, chlorine, chromium(VI) oxide, lithium aluminum hydride, ozone, perchloric acid, peroxides... 

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

Trifluoroethanol was first prepared by the catalytic reduction of trifluoroacetic anhydride [407-25-0] (58). Other methods iaclude the catalytic hydrogeaatioa of trifluoroacetamide [354-38-1] (59), the lithium aluminum hydride reductioa of trifluoroacetyl chloride [354-32-5] (60) or of trifluoroacetic acid or its esters (61,62), and the acetolysis of 2-chloro-l,l,l-trifluoroethane [75-88-7] followed by hydrolysis (60). More recently, the hydrogenation of... 

Amines of the formula n 2n+ be prepared by the lithium aluminum hydride reduction of the corresponding amide, hydrogenolysis... 

Tetrafluoropyridine can be prepared ia 75% yield from the hydrogenation of pentafluoropyridine under free-radical (catalytic) or nucleophilic (lithium aluminum hydride) conditions (416,419). No practical uses for 2,3,5,6-tetra uoropyridine are known. 

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