Reactions in Which Sodium–Mercury Amalgam (Na–Hg) is Used
Sodium–mercury amalgam, written as Na–Hg, is an important reagent used in both organic and inorganic chemistry. It mainly acts as a mild and controlled reducing agent. Because of its gentle action, it is preferred over metallic sodium in many reactions.
What is Sodium–Mercury Amalgam?
Sodium amalgam is a solution of sodium metal in mercury. Mercury controls the high reactivity of sodium and allows the gradual release of electrons or nascent hydrogen.
Due to this property, sodium amalgam is widely used in laboratory reductions.
1. Reduction of Aldehydes
Sodium amalgam reduces aldehydes to primary alcohols in aqueous or alcoholic medium.
General Reaction:
R–CHO + [H] → R–CH2OH
Here, sodium amalgam acts as a source of nascent hydrogen.
2. Reduction of Ketones
Ketones are reduced to secondary alcohols using sodium amalgam.
General Reaction:
R–CO–R′ + [H] → R–CHOH–R′
This reaction is useful when mild reduction conditions are required.
3. Reduction of Nitro Compounds
Nitro compounds are reduced to amines using sodium amalgam.
General Reaction:
R–NO2 + Na–Hg + H2O → R–NH2
This reaction is preferred when strong reducing agents like tin and hydrochloric acid are not suitable.
4. Reduction of Unsaturated Compounds
Alkenes and alkynes can be reduced by sodium amalgam under controlled conditions.
Example:
R–CH=CH–R → R–CH2–CH2–R
The hydrogen produced from sodium amalgam adds across the multiple bond.
5. Reduction of Oximes and Imines
Oximes and imines are reduced to corresponding amines.
Example:
R–CH=NOH + Na–Hg → R–CH2–NH2
This reaction is important in organic synthesis.
6. Reduction of Carbonyl Compounds in Aqueous Medium
Sodium amalgam reduces carbonyl compounds such as aldehydes and ketones in aqueous medium without affecting sensitive functional groups.
Hence, it is useful for selective reduction reactions.
7. Reduction of Metal Ions (Inorganic Chemistry)
Sodium amalgam is also used to reduce metal ions in solution.
Example:
Fe3+ + Na–Hg → Fe2+
This reaction is commonly used in redox chemistry experiments.
8. Preparation of Alcohols from Acyl Chlorides
Acyl chlorides can be reduced to primary alcohols using sodium amalgam.
General Reaction:
R–COCl + Na–Hg + H2O → R–CH2OH
This method avoids harsh reducing conditions.
Why Sodium–Mercury Amalgam is Preferred
- It is safer than metallic sodium
- It provides controlled reduction
- It is a mild reducing agent
- It is suitable for selective reactions
One-Line Examination Answer
Sodium–mercury amalgam is used as a mild reducing agent in reactions such as reduction of aldehydes, ketones, nitro compounds, oximes, imines, and metal ions.
Educational content prepared for chemistry students.
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