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.

Reduction of Functional groups:

Reduction of Alkenes:

Hydrogenation of the carbon-carbon double bonds occurs simply and in most of the cases can be affected under mild conditions. Just a few highly hindered alkenes are resistant to hydrogenation and even these can usually be reduced under more vigorous situations. Platinum and palladium are the most often employed catalysts. Both are extremely active and the preference is found out by the nature of other functional groups in the molecule and by the degree of selectivity needed; platinum generally brings about a more exhaustive reduction, Raney nickel might as well be in several cases.

Therefore, cinnamyl alcohol is reduced to the dihydro compound by Raney nickel in ethanol at 20^oC and 1,2-dimethyl-cyclohexane by hydrogen and platinum oxide in acetic acid is transformed mostly to cis-1,2-dimethylcyclohexane.

Fig: Reduction of cinnamyl alcohol

Rhodium and ruthenium catalysts haven't been much employed in hydrogenation of alkenes; however they at times exhibit helpful selective properties. Rhodium is specifically helpful for hydrogenation of alkenes whenever concomitant Hydrogenolysis of an oxygen function is to be avoided. Therefore, the plant toxin, toxol, on hydrogenation over rhodium-alumina in ethanol was smoothly transformed to the dihydro compound.

Fig: Toxol on hydrogenation

By the platinum and palladium catalysts, on the other hand, widespread Hydrogenolysis occurs and a mixture of products was made.

The simplicity of reduction of an alkene reduced with the degree of replacement of the double bond, and this at times lets selective reduction of one double bond in a molecule that includes several. The selective reduction of carbon-carbon double bonds in compounds including other unsaturated groups can generally be accomplished, apart from in the presence of triple bonds, aromatic nitro groups and acyl halides.

The detailed reaction method is complex and comprises different kinds of metal-carbon bonds. As two hydrogens are added to the double bond from the surface of metal, they are usually both added to the similar face of the double bond. This kind of addition is termed to as syn, merely as the addition of two 'pieces' of a reagent to opposite faces of a double bond is known as anti, addition. In case of catalytic hydrogenation, the alkene molecule is adsorbed to the catalyst surface by one face of the double bond coordinated to the surface; the two hydrogens are both added to this face.

Fig: Mechanism of reduction of Alkenes

Reduction of Alkynes:

The Catalytic hydrogenation of alkynes occurs in a stepwise way, and similar to alkene and alkane can be isolated. Complete reduction of the alkynes to the saturated compound is simply accomplished over platinum, palladium or Raney nickel. A complication that at times arises, specifically by platinum catalysts, is the Hydrogenolysis of propargylic (that is, saturated C-atom adjacent to the triple bond) hydroxyl groups.

Fig: Reduction of Alkynes

More valuable from a synthetic view-point is the partial hydrogenation of alkynes to Z-alkenes. This reaction can be affected in high yield by a palladium-calcium carbonate catalyst that has been partially deactivated via addition of lead acetate (that is, Lindlar's catalyst) or quinoline. It is aided via the fact that the more electrophilic acetylenic compounds are adsorbed on the electron-rich catalyst surface more strongly as compare to the corresponding alkenes. The significant characteristic of such reductions is their high stereo selectivity. In most of the cases, the product comprises very largely of the thermodynamically less stable Z-alkene and partial catalytic hydrogenation of alkynes gives one of the most suitable routes to Z-1,2-disubstituted alkenes. Therefore stearolic acid on reduction over Lindlar's catalyst in ethyl acetate solution affords a product having 95% of the oleic acid.

Fig: Lindlars catalyst in ethyl acetate solution

Partial reduction of the alkynes having Lindlar's catalyst has been invaluable in the synthesis of carotenoids and most of the other natural products with Z-distributed double bonds.

Reduction of Aromatic Compounds:

The reduction of aromatic rings via catalytic hydrogenation is more complex as compare to those of most of the other functional groups, and selective reduction is not simple. The commonest catalysts are platinum and rhodium that can be employed at ordinary temperatures and Raney nickel or ruthenium which need high temperatures and pressures.

The Benzene itself can be reduced to cyclohexane by platinum oxide in acetic acid solution. Derivatives of benzene like benzoic acid, phenol or aniline are reduced more simply. For large scale work the most convenient process is hydrogenation over Raney nickel at around 150-200^oC and 100 to 200atm. hydrogenation of phenols, followed via oxidation of the resultant cyclohexanol is a well-situated process for the large-scale preparation of the substituted cyclohexanones.

Fig: Reduction of Aromatic Compounds

Reduction of the benzene derivatives carrying oxygen or nitrogen functions in benzylic positions is complicated by the simple Hydrogenolysis of such groups, specifically over palladium catalysts. Preferential reduction of the benzene ring in such compounds is best accomplished by ruthenium or preferably with rhodium, catalysts that can be employed under mild conditions. Therefore mandelic acid is readily transformed to hexahydromandelic acid over rhodium-alumina in methanol solution, while, with palladium, Hydrogenolysis to phenylacetic acid is the major reaction.

Fig: Reduction of benzene derivatives

By polycyclic aromatic compounds it is frequently possible, by varying the conditions, to get either partially or fully reduced products. Therefore, naphthalene can be transformed to tetrahydro or decahydro compound over Raney nickel based on the temperature. With anthracene and phenanthrene the 9,10-dihydro compounds are acquired by hydrogenation over copper chromite, however in general, aromatic rings are not reduced by this catalyst. To get more completely hydrogenated compounds more active catalysts should be employed.

Reduction of Aldehydes and ketones:

The reduction of ketones or aldehydes converts their C=O groups to alcohol groups (ROH), or to CH₂ groups.

a) Conversion of C=O group to OH:

Aldehydes and ketones are simply reduced to the corresponding primary and secondary alcohols, correspondingly.

Fig: Conversion of C=O group to OH

Most of the different reducing agents might be employed. For laboratory applications, the complex metal hydrides are specifically efficient. Lithium aluminium hydride (LiAlH₄) is a powerful reducing agent which has been employed for this aim.

Reactions by LiAlH4 are generally carried out by adding an ether solution of the aldehyde or ketone to an ether solution of LiAlH₄. Reduction is fast even at -78^oC (that is, dry ice temperature). At the end of reaction, the alcohol is present as a mixture of lithium and aluminium salts and should be released by hydrolysis.

Fig: Reactions with LiAlH₄

The reagent as well reduces most of the other oxygen- and nitrogen- containing functional groups. The main demerit of the reagent is its cost, which renders it is helpful merely for fairly small-scale laboratory applications, and the hazards comprised in handling it.

Sodium borohydride, NaBH₄, offers some benefits. This hydride is much less reactive as compare to LiAlH₄ and is as a result more selective. Of the functional groups which are reduced via LiAlH₄, only aldehydes and ketones are reduced at a reasonable rate via NaBH₄. The reagent is fairly stable in aqueous and in alcoholic solution, particularly at basic pH. The illustration below describes the selectivity that might be accomplished with the reagent.

Fig: Selectivity achieved with the reagent

The carbonyl group is reduced quickly and quantitatively via diborane in ether or THF. The initial product is the ester of boric acid and an alcohol, a trialkyl borate. This material is quickly hydrolyzed on treatment with water.

(CH₃)₂CHCHO + B₂H₆ → [(CH₃)₂CHCH₂O)₃B + (H₂O) → (CH₃)₂CHCH₂OH

Aldehydes and ketones might as well be reduced to alcohols via hydrogen gas in the presence of a metal catalyst (that is, catalytic hydrogenation). The major benefits of this process are that it is comparatively simple to accomplish and generally affords quantitative yield of product as no complicated work-up procedure is needed. Though, it suffers from the demerits that most of the catalysts employed (Pd, Pt, Ru, Rh) are comparatively expensive and that other functional groups (C=C, -C≡C-, NO2, -C≡N) as well react.

Fig: Aldehydes and ketones reduction to alcohols by hydrogen

b) Conversion of C=O group to CH₂:

The complete reduction to alkane (CH₂) is not possible by employing either metal-hydride reduction or catalytic hydrogenation of aldehydes and ketones. Such methods will stop at the alcohol oxidation state. We can transform the C=O group of ketones and aldehydes to a CH₂ group by the Clemmensen reduction or the Wolff-Kishner reaction.

=> Clemmensen Reduction:

We perform this reaction via treating an aldehyde or ketone by zinc amalgam (that is, Zn treated by mercury metal (Hg) in the aqueous HCl). As this reaction employs aqueous HCl, it is not helpful for compounds which are sensitive to acid. A plausible reaction procedure for the reduction is as illustrated below.

Fig: Clemmensen reduction of carbonyls

=> Wolff-Kishner Reaction:

The procedure of Wolff-Kishner reaction comprises nucleophilic addition of hydrazine to the C=O group to manufacture the intermediate hydrazone. This intermediate reacts further with H₂O in the reaction mixture to form N₂ and the final organic product. Since the reaction medium is basic, we can make use of the Wolff-Kishner reaction with compounds sensitive to the acidic conditions of the Clemmensen reduction.

Fig: Wolff-Kishner Reaction

One disadvantage of the Wolff-Kishner reaction is that it comprises the use of strong base (KOH) at elevated temperatures, conditions that might influence other base sensitive functional groups in the molecule.

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