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To aldehydes
Nitriles can also be reduced to aldehydes. The Stephen aldehyde synthesis uses Tin(II) chloride and hydrochloric acid to yield an aldehyde via the hydrolysis of a resulting iminum salt. Aldehydes can also form using a hydrogen donor followed by in-situ hydrolysis of an imine. Useful reagents for this reaction include formic acid with a hydrogenation catalysis or metal hydrides which are used to add one mol of hydrogen to the nitrile. For example, sodium borohydride reduces nitriles in alcoholic solvents with a CoCl2 catalyst or Raney nickel. Reducing agent Diisobutylaluminium hydride, or DIBAL-H, is another commonly used metal hydride. DIBAL-H acts as a proton source, adding a hydride ion to the carbon of the nitrile. The resulting imine is a relatively stable intermediate that can be hydrolzed to the aldehyde.

Mechanism
DIBAL-H is added in controlled amounts at low temperatures to achieve partial reduction of the nitrile. The aluminum atom in DIBAL acts as a Lewis acid, accepting an electron pair from the nitrile. The nitrile is then reduced by the transfer of a hydride ion to the carbon of the carbon-nitrile triple bond, producing an imine. After a workup with water, the aluminum complex is hydrolyzed to produce the desired aldehyde. Because the hydrolytic workup generates the aldehyde at the end, the nitrile does not undergo over-reduction.