Lithium Aluminum Hydride Powder CAS 16853-85-3

Lithium aluminum hydride powder is an inorganic compound with the chemical formula of LiAlH4. It is a white crystalline powder, insoluble in hydrocarbons, soluble in ether, tetrahydrofuran, soluble in tetrahydrofuran, ether and dimethyl solvent, slightly soluble in dibutyl ether, and almost insoluble in dioxane and hydrocarbon compounds. Lithium aluminum hydride is stable in dry air, and violently hydrolyzes in humid air, releasing a large amount of H2 and burning. It can reduce ketones, aldehydes, acids, esters, anhydrides, quinones, etc. to alcohols, halogenated hydrocarbons to hydrocarbons, and nitriles to primary amines. Lithium aluminum hydride is widely used in medicine, pesticides, spices, dyes and other industries due to its excellent reducibility, and is used as a reducing agent in other organic synthesis. At the same time, lithium aluminum hydride will release a lot of heat energy when burning, and it is also used as an additive for missiles and launch vehicles.

Use of Lithium Aluminum Hydride Powder:

1. One step reduction with lithium aluminum hydride β- The ketamine is used to prepare 1,3-amino alcohol. The one-step reduction of N, N-dimethyl-3-hydroxy-3-arylpropylamine can be achieved by selecting a suitable reaction substrate (3-dimethylamino-1-aryl-2-propenone) and refluxing excessive lithium aluminum hydride in tetrahydrofuran solvent. The process is to reduce the alkene bond first and then the carbonyl group. This also provides an example, that is, under certain conditions, lithium aluminum hydride can reduce carbon carbon double bonds.

2. In the exploration of the antibacterial drug moxifloxacin, 8-benzyl-7,9-dioxo-2,8-diazobicyclo [4.3.0] nonane can be reduced to 8-benzyl-2,8-diazobicyclo [4.3.0] nonane with lithium hydride, and the optimal yield of 8-benzyl-2,8-diazobicyclo [4.3.0] nonane can reach 93.7%.

3. In the process of synthesizing balofloxacin, 3-aminopyridine was selected as the reaction material, and 3-formamidopyridine was prepared by oxidation with formic acid. Then, 3-formamidopyridine was reduced with lithium aluminum hydride as the reducing agent to obtain 3-methylaminopyridine, with a yield of 85%.

The chemical properties of lithium aluminum hydride powder are shown in the following aspects:

1. Thermal decomposition reaction:

Lithium aluminum hydride is metastable at room temperature. During long-term storage, lithium aluminum hydride will be decomposed into Li3AlH6 and LiH. This process can be accelerated by such cocatalytic elements as titanium, iron and vanadium. When heating lithium aluminum hydride, the reaction mechanism is divided into three steps:

3LiAlH4 → Li3AlH6 + 2Al + 3H2↑（R1）

2Li3AlH6 → 6LiH + 2Al + 3H2↑（R2）

2LiH + 2Al → 2LiAl +H2↑（R3）

R1 usually starts with the melting of lithium aluminum hydride, with a temperature range of 150~170 ℃, and then decomposes into Li3AlH6 immediately, but R1 is carried out below the melting point of LiAlH4. At about 200 ℃, Li3AlH6 decomposes into LiH and Al (R2), and then decomposes into LiAl (R3) above 400 ℃. In practice, reaction R1 is irreversible, while R3 is reversible, and the equilibrium pressure at 500 ℃ is 25 kPa. The reaction of R1 and R2 can occur at room temperature with appropriate catalyst.

2. Hydrolysis reaction:

LiAlH4 reacts violently and explosively with water and releases hydrogen:

LiAlH4 + 2H2O → LiAlO2 + 4H2↑

LiAlH4 + 4H2O → LiOH +Al(OH)3+ 4H2↑

Since the hydrogen released is quantitative, this reaction can be used to determine the content of lithium aluminum hydride in the sample. In order to prevent the reaction from being too violent, some dioxane, ethylene glycol dimethyl ether or tetrahydrofuran are often added as the diluent. This reaction provides a useful laboratory method for hydrogen production. The samples exposed to air for a long time will usually turn white because the samples have absorbed enough water to form a white mixture consisting of lithium hydroxide and aluminum hydroxide.

3. Ammonolysis reaction:

The ether or tetrahydrofuran solution of LiAlH4 can react violently with ammonia to release hydrogen:

LiAlH4+4NH3 → LiAl (NH2) 4+2H2 ↑

When the amount of ammonia is insufficient, the following reaction occurs:

2LiAlH4+5NH3 → [LiAlH (NH2) 2] 2NH+6H2 ↑

When the NH3/LiAlH4 ratio is smaller, all three hydrogen in ammonia can be replaced:

3LiAlH4+NH3 → (LiAlH3) 3N+3H2 ↑

4. Coordination reaction:

Lithium aluminum hydride can react with almost all halides to generate corresponding coordination aluminum hydride. When the coordination aluminum hydride is unstable, it will be decomposed into corresponding hydride. The general formula is:

nLiAlH4+MXn → M (AlH4) n+nLiX, M (AlH4) n → MHn+nAlH3

Therefore, many metal or non-metallic hydrides can be prepared by this method, such as

LiAlH4+4NaCl → 4NaH+LiCl+AlCl3

5. Double decomposition reaction:

Lithium aluminum hydride can conduct double decomposition reaction with NaH in tetrahydrofuran to efficiently produce sodium aluminum hydride (NaAlH4):

LiAlH4+NaH → NaAlH4+LiH

Potassium aluminum hydride (KAlH4) can be prepared in a similar way with diethylene glycol dimethyl ether as solvent:

LiAlH4+KH → KAlH4+LiH

6. Reduction reaction:

Lithium aluminum hydride can reduce many organic compounds, and its ether or tetrahydrofuran solution is commonly used in practice. The reduction ability of lithium aluminum hydride is stronger than that of the related sodium borohydride, because the Al-H bond is weaker than the B-H bond. Due to inconvenient storage and use, bis (2-methoxyethoxy) sodium aluminum hydride (red aluminum), a derivative of lithium aluminum hydride, is commonly used as a reducing agent in industry, but lithium aluminum hydride is still used in small-scale industrial production.

The functional groups that can be reduced by lithium aluminum hydride mainly include:

1. Haloalkanes are reduced to alkanes. The reaction of iodoalkanes is the fastest, followed by bromoalkanes and chloroalkanes. In this reaction, the primary haloalkanes (primary haloalkanes) have good performance, and the obtained products undergo configuration transformation, so this reaction is considered to be SN2 mechanism. The secondary halogenated alkanes (secondary halogenated alkanes) can also be reduced by this method. The tertiary halogenated hydrocarbons (tertiary halogenated alkanes) are prone to elimination reaction, so this method is not applicable. Lithium aluminum hydride can only be used to reduce alkynes with alcohol groups nearby, and cannot be used to reduce simple olefins and aromatic hydrocarbons.

2. Silicon halides are reduced to silane, such as

LiAlH4+SiCl4 → SiH4+LiCl+AlCl3

3. Carbonyl compounds (except amides) are reduced to alcohols, such as esters and carboxylic acids can be reduced to primary alcohols by lithium aluminum hydride. Before the discovery of the method of reducing esters by lithium aluminum hydride, the ester was generally reduced by the Bouver Brown reduction reaction, that is, boiling metal sodium anhydrous alcohol as the reducing agent, but this reaction is difficult to carry out. Aldehydes and ketones can also be reduced to alcohols by lithium aluminum hydride, but more mild reagents such as NaBH4 are generally used for reduction. α,β- Unsaturated ketones will be reduced to allyl alcohol.

4. Epoxy compound. When the epoxy compound is reduced, lithium aluminum hydride reagent will attack the low steric hindrance end of the epoxy compound, usually generating secondary or tertiary alcohols. Cyclohexane oxide will be preferentially reduced to α Alcohols of bonds (upright bonds).

5. Amides and imides are reduced to amines. The yield of such reactions is generally high, and the reaction of N, N-substituted raw materials is much faster than others.

6. Nitrile is reduced to primary amine. In addition, oximes, nitro compounds and alkyl azides can be reduced to amines. Quaternary ammonium cations can be reduced to corresponding tertiary amines.

7. Reaction with alcohol to produce alkoxy lithium aluminum hydride:

LiAlH4 + ROH → LiAl(OR)H3 + H2↑

LiAlH4 + 2ROH → LiAl(OR)2H2 + 2H2↑

LiAlH4 + 3ROH → LiAl(OR)3H + 3H2↑

LiAl (OR)₂H₂ is a suitable reagent for reducing amides to aldehydes, LiAl(OC(CH₃)₃)₃H is a suitable reagent for reducing acyl chlorides to aldehydes, and lithium aluminum hydride cannot partially reduce acyl chlorides to corresponding aldehydes, because lithium aluminum hydride can completely reduce the latter to primary alcohol, so it is necessary to use a milder tri tert butyloxy lithium aluminum hydride (LiAl(OC(CH₃)₃)₃H) to reduce acyl chlorides. The reaction of lithium tri tert butoxy aluminum hydride with acyl chloride is much faster than that with aldehyde. For example, the addition of sulfoxide chloride in isovaleric acid will generate isovaleryl chloride. At this time, lithium tri tert butoxy aluminum hydride can be used to reduce isovaleryl chloride to isovaleraldehyde, and the yield can reach 65%.

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