Metal Hydrid Reduction (NaBH4 and LiAlH4)

Metal Hybrid Reduction

Reduction of aldehydes and ketones

Aldehydes and ketones can be reduced to alcohols, hydrocarbons, and amines. Carbonyl compounds have various structures and react differently depending on the decreasing agent and the reducing agent.

Catalytic hydrogenation

Carbonyl groups also have a pi-bond that can be hydrogenated, like those found in alkenes. The hydrogenation of ketones, such as cyclohexanone, is possible under room temperature and pressure of 4 atmospheres using a platinum catalyst.



It is also possible to hydrogenate a double bond by leaving the carbonyl intact, while the double bond may be hydrogenated by hydrogenating both.


Carbonyl groups, however, cannot be hydrogenated catalytically without the double bond between carbon and carbon. Using a Metal Hydride Reduction is the preferred method to reduce a carbonyl group while preserving the double bond between carbon and carbon.

Hydrogen is a relatively inexpensive gas, but hydrogenation reactions can be problematic because an apparatus is usually made up of tanks of hydrogen and a metal pressure vessel. Due to this, metal hydrides have been recommended as an alternative reduction agent.

Sodium borohydride (NBH) and lithium aluminum hydride (LAH) are two valuable reducing agents. Aldehydes and ketones are both reduced to alcohols with LAH and NBH.





Metal hydrides differ significantly in their reactivity. Aldehydes and ketones can both be reduced by LiAlH4, but so are carboxylic acids, esters, amides, and nitriles.

Anhydrous ether is usually used to reduce LiAlH4 because it undergoes violent reactions with water.

LAH reduces more aggressively than NBH. Water and aqueous alcohol can be used as solvents in its reactions. NaBH4, which has a lower reactivity toward water, is preferred as a reagent for reducing aldehyde or ketone.

Aldehydes and ketones are reduced rapidly by NaBH4, while esters are reduced slowly. Therefore, there is no need to reduce an ester group at the same time as an aldehyde or ketone carbonyl group. LAH is unable to accomplish this selective reduction.

A carbonyl group combined with a C=C double bond is sometimes attacked by NBH or LAH, but not isolated C=C double bonds. So, a carbonyl group that is present in both double bonds and carbonyl groups is usually reduced solely at the carbonyl position. Metal hydrides can act as reducing agents in this way, complementary to hydrogen gas.

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