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LAH violently reacts with water, including atmospheric moisture, to liberate hydrogen gas. The reaction proceeds according to the following idealized equation: [5] Li[AlH 4] + 4 H 2 O → LiOH + Al(OH) 3 + 4 H 2. This reaction provides a useful method to generate hydrogen in the laboratory.
Cyanide is unstable in water, but the reaction is slow until about 170 °C. It undergoes hydrolysis to give ammonia and formate, which are far less toxic than cyanide: [14] CN − + 2 H 2 O → HCO − 2 + NH 3. Cyanide hydrolase is an enzyme that catalyzes this reaction.
The cyanide source can be potassium cyanide (KCN), sodium cyanide (NaCN) or trimethylsilyl cyanide ((CH 3) 3 SiCN). With aromatic aldehydes such as benzaldehyde, the benzoin condensation is a competing reaction. The reaction is used in carbohydrate chemistry as a chain extension method for example that of D-xylose.
Cyanation of ketones or aldehydes yields the corresponding cyanohydrins, which can be done directly with the cyanide ion (the cyanohydrin reaction) or by using bisulfite, followed by displacement of sulfite: [3] [4] Cyanation of aldehyde with bisulfite. A related reaction is hydrocyanation, which installs the elements of H-CN.
In this reaction, the nucleophilic CN − ion attacks the electrophilic carbonyl carbon in the ketone, followed by protonation by HCN, thereby regenerating the cyanide anion. Cyanohydrins are also prepared by displacement of sulfite by cyanide salts: [2] Cyanohydrins are intermediates in the Strecker amino acid synthesis.
Typical solvents for such reactions are ethers. Water and other protic solvents cannot serve as a medium for ionic hydrides because the hydride ion is a stronger base than hydroxide and most hydroxyl anions. Hydrogen gas is liberated in a typical acid-base reaction. + +
Sodium borohydride and lithium aluminium hydride are commonly used for the reduction of organic compounds. [3] [4] These two reagents are on the extremes of reactivity—whereas lithium aluminium hydride reacts with nearly all reducible functional groups, sodium borohydride reacts with a much more limited range of functional groups.
This reaction is akin to steam reforming, the reaction of methane and water to give carbon monoxide and hydrogen. In the Shawinigan Process, hydrocarbons, e.g. propane, are reacted with ammonia. In the laboratory, small amounts of HCN are produced by the addition of acids to cyanide salts of alkali metals: H + + CN − → HCN