Introduction:

The features or characteristics of reduction reactions are opposite to such of oxidation reactions. As an outcome, organic molecules lose oxygen and/or gain hydrogen in the reduction reactions.

Definitions:

Reduction is any chemical reaction comprising gaining of electrons. This can still be stated in the sense of addition of hydrogen to an unsaturated group like carbon-carbon double bond, a carbonyl group or an aromatic nucleus, or the addition of hydrogen with concomitant fission of a bond between the two atoms, as in the reduction of a disulphide to a thiol or of an alkyl halide to a hydrocarbon.

Reductions are usually affected either chemically or via catalytic hydrogenation, which is by the addition of molecular hydrogen to compounds beneath the influence of catalyst. Each and every technique has its benefits. In numerous reductions either method might be utilized equally well. Complete reduction of the unsaturated compound can usually be accomplished devoid of undue difficulty; however the main aim is often selective reduction of one group in a molecule in the presence of other unsaturated groups. Both the catalytic and chemical techniques of reduction offer considerable scope in this direction, and the process of choice in a specific case will often based on the selectivity needed and on the stereochemistry of the desired product.

Types of Reduction Reactions:

Catalytic hydrogenation:

Of numerous methods available for the reduction of organic compounds catalytic hydrogenation is one of the suitable. Reaction is simply effected simply via stirring or shaking the substrate by the catalyst in an appropriate solvent, or without a solvent if the substance being reduced is a liquid, in an atmosphere of hydrogen in an apparatus that is arranged in such a way that the uptake of hydrogen can be evaluated. At the end of the reaction, the catalyst is filtered off and the product is recovered from the filtrate, often in a high state of purity. The technique is simply adapted for work on a micro scale, or on a large, even industrial, scale. In most of the cases reaction carries on smoothly at or near room temperature and at atmospheric or slightly elevated pressure. In other cases high temperature (100 to 200^oC) and pressure (100 to 300 atmospheres) are essential, needing special high pressure equipment.

Catalytic hydrogenation might yield simply in the addition of hydrogen to one or more unsaturated groups in the molecule or it might be accompanied via fission of a bond between the atoms. The latter method is termed as Hydrogenolysis.

Most of the common unsaturated groups in organic chemistry, like 

Fig: Common unsaturated groups

And aromatic and heterocyclic nuclei can be decreased catalytically under suitable conditions, however they are not all reduced by equivalent simplicity

Most of the different catalysts have been employed for catalytic hydrogenations; they are mostly finely divided metals, metallic oxides or sulphides. The most generally employed in the laboratory are the platinum metals (that is, platinum, palladium and to a lesser degree rhodium and ruthenium), nickel and copper chromate. The catalysts are not particular and by the exception of copper chromite might be employed for a diversity of different reductions.

For hydrogenation, at high pressure the most general catalysts are Raney nickel and copper chromite. Raney nickel is a porous, finely divided nickel acquired via treating a powdered nickel-aluminium alloy by sodium hydroxide. This is usually utilized at high temperatures and pressures, however by the more active catalysts lots of reactions can be affected at atmospheric pressure and normal temperature. Almost all the unsaturated groups can be reduced by Raney nickel however it is most often employed for reduction of aromatic rings and Hydrogenolysis of the sulphur compounds.  

Most of the hydrogenations carry on satisfactorily under a broad range of conditions, however where a selective reduction is wanted, conditions might be more vital. The choice for catalyst for hydrogenation is regulated by the activity and selectivity needed. Selectivity is a property of the metal; however it as well depends to some degree on the activity of the catalyst and on the reaction conditions. In common, the more active the catalyst the less discriminating it is in its action, and for maximum selectivity reactions must be run by the least active catalyst and under the mildest possible conditions consistent by a reasonable rate of reaction.

Reduction by Hydride-Transfer Reagents:

A number of metal hydrides have been used as reducing agents in the organic chemistry, however the most generally employed are lithium aluminium hydride and sodium borohydride, both of which are commercially available. The other helpful reagent is borane. 

The anions of the two complex hydrides can be considered as derived from the lithium or sodium hydride and either aluminium hydride or borane.

LiH + AlH₃ → Li ⁺ AlH₄^-

NaH + BH₃ → Na⁺ BH₄^-

The anions are nucleophilic reagents and as such they generally attack polarized multiple bonds like C=O or C≡N via transfer of hydride ion to the more positive atom. They don't generally reduce isolated carbon-carbon double or triple bonds.

With both the reagents all four hydrogen  atoms might be employed for reduction being transferred in a stepwise way as described below for the reduction of a ketone by lithium aluminium hydride.

Fig: Reduction of ketone with lithium aluminium hydride

There is proof that in borohydride reductions a more complex path might be followed. For reduction with lithium aluminium hydride (however not by sodium borohydride) each consecutive transfer of hydride ion occurs more slowly as compare to the one before, and this has been exploited for the preparation of modified reagents that are less reactive and more selective than lithium aluminium hydride itself by substitution of two or three of the hydrogen atoms of the anion via alkoxy groups.

Lithium aluminium hydride is a more influential reducing agent as compare to sodium boriohydride and reduces most of the generally encountered organic functional groups. This readily reacts with water and other compounds that have active hydrogen atoms and should be employed under anhydrous conditions in a non-hydroxylic solvent; ether and tetrahydrofuran are generally used. Sodium borohydride reacts slowly by water and most alcohols at room temperature and reductions with this reagent are frequently effected in ethanol solution. Being less reactive as compare to lithium aluminium hydride, it is more discriminating in its action. At room temperature in ethanol it readily reduces aldehydes and ketones however it doesn't usually attack esters or amides and it is usually possible to reduce aldehydes and ketones selectively by sodium borohydride in the presence of a diversity of other functional groups. A few typical illustrations are described below: 

Fig: Ethanol readily reduces to aldehydes and ketones

Lithium borohydride is as well at times used. This is a more powerful reducing agent as compare to sodium borohydride and selective in its action however it consists of the benefit that it is soluble in ether and tetrahdrofuran. The exemption to the general rule that carbon-carbon double bonds are not attacked via hydride reducing agents is found in the reduction of β-aryl-αβ-unsaturated carbonyl compounds having lithium aluminium hydride, where the carbon-carbon double bond is frequently reduced and also the carbonyl group. Even in such cases, though, selective reduction of the carbonyl group can usually be accomplished by working at low temperatures or by employing sodium borohydride or aluminium hydride as the reducing agent.

Fig: Reduction of the carbonyl group

Lithium aluminium hydride and sodium borohydride have perhaps found their most extensive use in the reduction of carbonyl compounds. Aldehydes, carboxylic acids, ketones, esters and lactones can all be reduced smoothly to the corresponding alcohols under mild conditions. Reaction by lithium aluminium hydride is the process of choice for the reduction of carboxylic acids to primary alcohols. Substituted amides are transformed to amines or aldehydes, based on the experimental conditions. To effect selective reduction of the ester, the keto group of ethylacetoacetate for illustration must be protected as its acetal, and the ester reduced by lithium aluminium hydride. Mild acid hydrolysis then re-produces the ketone to provide the β-keto-alcohol.

Fig: Keto group of ethylacetoacetate

It will be noted that on reduction of ethyl acetoacetate with lithium aluminium hydride both the ester and ketone functional groups are reduced to afford 1,3-butandiol, though, only the keto group is reduced by the milder sodium borohydride to provide ethyl-3-hydroxybutanoate.

Fig: Reduction of ethyl acetoacetate with lithium aluminium hydride

Reduction by dissolving metals:

The chemical processes of reduction are of two major types: Those which occur by addition of electrons to the unsaturated compound followed or accompanied via transfer of protons; and those which occur by addition of hydride ion followed in the separate step by protonation.

Reductions that follow the first path are usually affected by a metal, the source of electrons and a proton donor which might be water, an alcohol or the acid. They can yield either in the addition of hydrogen atoms to a multiple bond or in fission of a single bond between the atoms, generally, in practice, a single bond between carbon and a heteroatom. In such reactions an electron is transferred from the metal surface (or from the metal in solution) to the organic molecule being reduced, providing, in the case of addition to a multiple bond, an anion radical that in most of the cases is instantly protonated. The resultant radical afterward takes up the other electron from the metal to form an anion that might be protonated instantly or remain as the anion until work-up. In the absence of a proton source dimerization or polymerization of the anion-radical might occur. In several cases a second electron might be added to the anion-radical to form a di-anion, or two anions in case of fission reactions. 

The metals generally used in such reductions comprise the alkali metals, calcium, zinc, magnesium, tin and iron. The alkali metals are frequently employed in solution in liquid ammonia or as suspensions in inert solvents like ether or toluene, often with addition of an alcohol or water to act as the proton source. Most of the reductions are as well affected via direct addition of sodium or, principally, zinc, tin, or iron, to a solution of the compound being reduced in the hydroxylic solvent like ethanol, acetic acid or an aqueous mineral acid.

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