LiAlH4, reaction with water

Flammable sohd ignites in moist or heated air. It nndergoes vigorous exothermic reaction with water evolving hydrogen [2.36 L(H2)/g of LiAlH4] ... 

All reactions are carried out under an atmosphere of dry nitrogen gas unless otherwise stated. Lithium bis(trimethylsilyl)amide (Aldrich), 1,4-dicyanoben-zene (Aldrich), chlorotrimethylsilane (Aldrich), and 1,3,5-triazine (Aldrich) are used as received. Benzonitrile (Fisher) is vacuum distilled from P2Os prior to use. Solvents are purified by distillation as follows hexane, over calcium hydride diethyl ether, over LiAlH4 ( Caution. Reacts violently with water) toluene, over sodium acetonitrile, over P2Os. 

Treating an aldehyde or a ketone with NaBH4 or LiAlH4, followed by water or some other proton source, affords an alcohol. This is an addition reaction because the elements of H2 are added across the n bond, but it is also a reduction because the product alcohol has fewer C-0 bonds than the starting carbonyl compound. 

Sodium borohydride is one of the weakest hydride donors available. The fact that it can be used in water is evidence of this as more powerful hydride donors such as lithium aluminium hydride, LiAlH4, react violently with water. Sodium borohydride reacts with both aldehydes and ketones, though the reaction with ketones is slower for example, benzaldehyde is reduced about 400 times faster than acetophenone in isopropanol. 

With 2,4-substituted, bispidones of lower symmetry and analoguous 1,5-dicarboxylic acid esters (e.g., compounds 10-48 in Table 1) the reduction under Wolff-Kishner conditions fails, but reaction with complex hydrides leads to the corresponding bispidoles. The treatment with LiAUTj yields a 1 1 mixture of the epimeric tris alcohols, whereas reduction with NaBELj at ambient temperature in various solvents leads to the epimeric mono-alcohols in different ratios up to epimerically pure compounds (see Scheme 9) (116, 117), which can be further reduced to the tris alcohols with LiAlH4. For example, the reduction of 14 with NaBH4 in dry methanol yields 65% syn- (84b) and 35% anri-product (84a), while the reaction in a mixture of dioxane-water leads exclusively to the anti-configuration of alcohol 84a (117). 

The azide ion is a better nucleophile than amines, but it has to be reduced to the amine after nucleophilic substitution. Lithium aluminum hydride (LiAlH4) in ether followed by treatment with water reduces the azide ion to the amine. Figures 13-11 and 13-12 illustrate two examples of this reaction. 

The excess of LiAlH4 can be decomposed with moist ether or, carefully, with sodium carbonate solution or water. For larger batches the decomposition is effected by a reagent that does not evolve hydrogen on reaction with LiAlH4 ethyl acetate is very suitable, since its reduction product (ethanol) rarely interferes in subsequent working up. 

The chemistry of acid anhydrides is similar to that of acid chlorides. Although anhydrides react more slowly than acid chlorides, the kinds of reactions the two groups undergo are the same. Thus, acid anhydrides react with water to form acids, with alcohols to form esters, with amines to form amides, and with LiAlH4 to form primary alcohols (Figure 21.7, p. 864). 

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