Reduction Reactions
Reactions involving addition of Hydrogen or removal of Oxygen.
By chemca Team • Updated Jan 2026
Reduction in organic chemistry generally involves the addition of hydrogen (hydrogenation) or the removal of oxygen. Common reducing agents range from catalytic metals ($Ni, Pt, Pd$) to complex metal hydrides ($LiAlH_4, NaBH_4$).
1. Catalytic Hydrogenation
Alkenes & Alkynes to Alkanes
Unsaturated hydrocarbons react with hydrogen gas in the presence of finely divided metal catalysts ($Ni, Pt, \text{or } Pd$) to form alkanes.
$$CH_2=CH_2 + H_2 \xrightarrow{Ni, \Delta} CH_3-CH_3$$
Partial Reduction (Alkynes):
Using Lindlar's Catalyst ($Pd/CaCO_3$, poisoned) yields Cis-alkenes.
$$R-C \equiv C-R + H_2 \xrightarrow{Lindlar's Cat.} R-CH=CH-R \text{ (Cis)}$$
2. Reduction of Carbonyls to Alkanes
Deoxygenation Methods
The carbonyl group ($>C=O$) of aldehydes and ketones can be completely reduced to a methylene group ($-CH_2-$).
A. Clemmensen Reduction:
Uses Zinc Amalgam ($Zn-Hg$) and Conc. $HCl$. Best for compounds stable in acid.
$$R-CO-R' \xrightarrow{Zn-Hg/HCl} R-CH_2-R' + H_2O$$
B. Wolff-Kishner Reduction:
Uses Hydrazine ($NH_2NH_2$) and strong base ($KOH$) in ethylene glycol. Best for acid-sensitive compounds.
$$R-CO-R' \xrightarrow{NH_2NH_2/KOH, \Delta} R-CH_2-R' + N_2$$
3. Reduction by Metal Hydrides
To Alcohols
A. Lithium Aluminium Hydride ($LiAlH_4$):
Strong reducing agent. Reduces Acids, Esters, Aldehydes, and Ketones to Alcohols. Reduces Nitriles/Amides to Amines.
$$R-COOH \xrightarrow{1. LiAlH_4, 2. H_3O^+} R-CH_2OH$$
B. Sodium Borohydride ($NaBH_4$):
Mild reducing agent. Selectively reduces Aldehydes and Ketones to Alcohols. Does not reduce Acids or Esters.
$$R-CHO \xrightarrow{NaBH_4} R-CH_2OH$$
Selectivity: Neither $LiAlH_4$ nor $NaBH_4$ typically reduces isolated Carbon-Carbon double bonds ($C=C$).
4. Specific Named Reductions
Partial Reductions to Aldehydes
A. Rosenmund Reduction:
Acid chlorides to Aldehydes using $H_2/Pd-BaSO_4$.
$$R-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO + HCl$$
B. Stephen Reaction:
Nitriles to Aldehydes using $SnCl_2/HCl$ followed by hydrolysis.
$$R-CN \xrightarrow{SnCl_2/HCl} R-CH=NH \xrightarrow{H_3O^+} R-CHO$$
C. Reduction of Nitro Compounds:
Nitrobenzene to Aniline using $Sn/HCl$ or $Fe/HCl$.
$$C_6H_5NO_2 + 6[H] \xrightarrow{Sn/HCl} C_6H_5NH_2 + 2H_2O$$
Knowledge Check
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