Birch Reduction Mechanism: Features & Examples

Birch Reduction Mechanism, first discovered by the Australian chemist Arthur Birch in 1944, is an organic chemical reaction observed in aromatic compounds having a benzenoid ring. It is a redox reaction, carried out using sodium or potassium metal dissolved in liquid ammonia in the presence of alcohol. The reaction is initiated by the formation of the radical anion by the addition of solvated electrons to the aromatic ring. On undergoing this reaction, the aromatic compounds that have a benzenoid ring are converted into 1,4-cyclohexadiene. The process is important in synthetic organic chemistry. This article elaborates upon the mechanism of Birch’s Reduction.

Key Terms: Aromatic Compounds, Benzenoid Ring, Reduction, Birch’s Reduction Mechanism, Liquid Ammonia (NH3), Organic Alcohol (ROH)

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Redox Reactions

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Redox reactions are made up of oxidation and reduction reactions:

• Oxidation: It is the addition of an oxygen/electronegative element or removal of a hydrogen/ electropositive element. It involves the loss of electrons.

Examples:

2Mg (s) + O₂ (g) = 2MgO (s)

2H₂S (g) + O₂ (g) = 2S (s) + 2H₂O (l)

• Reduction: It is the removal of oxygen/electronegative element or the addition of hydrogen or electropositive element. It involves the gain of electrons.

Examples:

2FeCl₃ (aq) + H₂ (g) = 2FeCl₂ (aq) + 2HCl

ZnO + C = Zn + CO

• Redox Reactions: As reduction and oxidation reactions do not occur independent of each other, together they are known as redox reactions.

Example:

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Common Redox Reactions

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Some common redox reactions include:

• Combustion Reaction: It involves the reaction of a particular compound with molecular oxygen to form oxygen-containing products.

Example: 2C₈H₁₈ + 25O₂ → 16CO₂(g) + 18H₂O

• Disproportionation Reaction (Auto-oxidation): It involves the oxidation and reduction of a single reactant.

Example: 3ClO⁻(aq) → ClO₃⁻(aq)+2Cl⁻(aq)

• Single Replacement Reaction (Single Displacement Reaction): It is a reaction wherein two elements within a compound switch their places.

Example: Zn(s) + 2HCl(aq) → ZnCl₂(aq)+H₂(g)

Aromatic compounds that contain a benzenoid ring undergo Birch reduction reaction and are converted to 1,4-cyclohexadiene, i.e., compounds that have hydrogen atoms attached at the opposite ends of the molecule. The mechanism of Birch’s reduction has been elaborated in the section below.

Also Read: Difference Between Element And Compounds

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Birch’s Reduction

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Birch’s reduction is a redox reaction.

• The reaction converts aromatic compounds having benzenoid rings into 1,4-cyclohexadiene.
• The reaction is carried out using sodium or potassium metal dissolved in liquid ammonia, in the presence of alcohol.

Related Topics to Magnetism :

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Birch’s Reduction Mechanism

The mechanism for Birch’s reduction reaction is as given below:

• The ammonia (gas) at room temperature is condensed by lowering its temperature to -78^oC. This is done using dry ice or acetone cold finger, which is equipment in which the coolant (dry ice or acetone) flows, and helps create a localized cold area.
• The liquified ammonia is then used as a solvent for the alkali metals to be used for the reaction. This solution of liquid ammonia and alkali metals gives intense blue colour. The metals and ammonia react to form solvated electrons and an electrode salt.
• The reduction mechanism begins with the solvated electrons. The addition of solvated electrons to the aromatic ring leads to the formation of the radical anion.
• The alcohol used in the reaction then supplies a proton to the radical anion, as well as to the next to the last carbanion.
• Thus, cyclohexadiene and an alkoxide ion are formed as products.

Birch’s Reduction Mechanism

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Examples of Birch’s Mechanism

Here is the example of the Reduction of Naphthalene:

Reduction of Naphthalene

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Features of Birch’s Reduction Mechanism

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Some of the features of Birch Reduction Mechanism are:

• Alkali metals dissolve inliquid ammonia produce solvated electrons and electride salts. The aromatic rings take up the electrons one by one.
• The absorption of the first electron produces a radical anion.
• Thereafter, the alcohol molecule gives away its hydroxylic hydrogen and a carbon-hydrogen bond is formed.
• Further, when the second electron is picked up, a cyclohexadienyl-type carbanion is formed. This carbanion is protonated by the alcohol used in the reaction.
• The protonation of the cyclohexadienyl carbanion is at its centre.
• The location of the protonation of the radical anion determines the structure of the product formed in the Birch reduction reaction.
• The use of electron-donating groups in the reaction, causes the protonation to occur at the ortho or meta positions (with respect to the substituent).
• The use of electron-withdrawing groups in the reaction, causes the protonation to generally occur at the para position.

Read Also: 

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Things to Remember

• Birch reduction is a redox reaction.
• Aromatic compounds with a benzenoid ring on undergoing birch reduction from cyclohexadiene and an alkoxide ion as products.
• The reaction takes place by using a solution of alkali metals (Na, K, Li) in ammonia, and in the presence of alcohol.
• The intense blue colour of the solution of sodium in liquid ammonia appears due to the presence of the free electrons in the solution.
• Birch reduction of alkynes results in the formation of alkenes.
• Birch reduction of aniline can produce Conjugated enamines.
• Other than liquid ammonia, tetrahydrofuran can be used as an alternate solvent for the alkali metals to be used in the reduction reaction.

Read Also: Class 11 Chemistry Chapter 13 Hydrocarbons

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Previous Year's Questions

1. The organic compound with two asymmetric carbon atoms is...[KEAM]
2. The reaction of propene with HBrHBr in peroxide proceeds through the intermediate…..[KEAM]
3. Which carbide gives methane on hydrolysis?​
4. Bromination of cyclohexene under conditions given below yields :...[JEE Main 2016]
5. Which of the following compounds is not aromatic ?..[JEE Main 2019]
6. In the presence of peroxide, HClHCl and HIHI do not give anti-Markownikoff' s addition to alkenes because :...[JEE Main 2014]
7. 1, 3-butadiene has:...[JIPMER 2000]
8. The distance between two adjacent carbon atoms is largest in...[NEET 1994]
9. The compound that will react most readily with gaseous bromine has the formula...[NEET 2016]
10. Reaction of HBr with propene in the presence of peroxide gives...[NEET 2004]