Question

How do you convert Benzoic acid to Benzene ?

Hint:

Remember: \[ RCOONa \xrightarrow[\Delta]{NaOH/CaO} RH \] This reaction is called decarboxylation. For aromatic compounds: \[ C_6H_5COOH \rightarrow C_6H_5COONa \rightarrow C_6H_6 \] Benzoic acid loses one carbon atom and gives benzene.

Answer 1

Concept: Carboxylic acids can be converted into hydrocarbons containing one carbon atom less by the process of decarboxylation. In decarboxylation, the carboxyl group \((-COOH)\) is removed as carbon dioxide. The reaction is generally carried out by heating the sodium salt of the carboxylic acid with soda lime, which is a mixture of sodium hydroxide \((NaOH)\) and calcium oxide \((CaO)\). For aromatic carboxylic acids, decarboxylation produces the corresponding aromatic hydrocarbon.

Step 1: Conversion of benzoic acid into sodium benzoate. Benzoic acid first reacts with sodium hydroxide to form sodium benzoate. \[ C_6H_5COOH + NaOH \rightarrow C_6H_5COONa + H_2O \] Thus, sodium benzoate is obtained. \[ \boxed{C_6H_5COONa} \]

Step 2: Decarboxylation using soda lime. Sodium benzoate is heated with soda lime. Soda lime consists of: \[ NaOH + CaO \] During heating, the carboxyl group is removed in the form of carbon dioxide. The reaction is \[ C_6H_5COONa + NaOH \xrightarrow[\Delta]{CaO} C_6H_6 + Na_2CO_3 \] The product obtained is benzene. \[ \boxed{C_6H_6} \]

Step 3: Understanding the carbon count. Benzoic acid contains seven carbon atoms. \[ C_6H_5COOH \] During decarboxylation, one carbon atom is removed as carbon dioxide. Therefore, the product contains one carbon atom less. \[ 7 \; \text{carbons} \longrightarrow 6 \; \text{carbons} \] Hence benzene is formed.

Step 4: Writing the complete conversion sequence. \[ C_6H_5COOH \xrightarrow{NaOH} C_6H_5COONa \xrightarrow[\Delta]{NaOH/CaO} C_6H_6 \] This is the standard laboratory method for converting benzoic acid into benzene.

Final Answer: \[ \boxed{ C_6H_5COOH \xrightarrow{NaOH} C_6H_5COONa \xrightarrow[\Delta]{NaOH/CaO} C_6H_6 } \] or \[ \boxed{ C_6H_5COONa + NaOH \xrightarrow[\Delta]{CaO} C_6H_6 + Na_2CO_3 } \]