Problem 41 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

A mixture of sodium benzoate and sodalime on heating produces: (a) \(\mathrm{CH}_{4}\) |AFMC 1999] (b) \(\mathrm{C}_{6} \mathrm{H}_{6}\) (c) both (a) and (b) (d) none of these

Step-by-Step Solution

Answer

The mixture of sodium benzoate and sodalime on heating produces benzene ( \( \text{C}_6\text{H}_6 \ )). Option (b) is correct.

Step 1: Understand the reaction components

Sodium benzoate is \( ext{C}_6 ext{H}_5 ext{COONa} \), which is the sodium salt of benzoic acid. Sodalime is a mixture commonly composed of calcium oxide (CaO) and sodium hydroxide (NaOH). You need to determine the product of heating these substances together.

Step 2: Identify the reaction type

The reaction involving a sodium benzoate and sodalime is a decarboxylation reaction, where a carboxyl group (\( -COONa \)) is removed as carbon dioxide, leading to the formation of an alkane.

Step 3: Write the chemical equation

The chemical reaction for the decarboxylation of sodium benzoate with sodalime can be represented as: \(\text{C}_6\text{H}_5\text{COONa} + \text{NaOH} \xrightarrow{\Delta} \text{C}_6\text{H}_6 + \text{Na}_2\text{CO}_3}\)This results in the formation of benzene (\( \text{C}_6\text{H}_6 \)).

Step 4: Evaluate the options

The reaction primarily produces benzene (\( \text{C}_6\text{H}_6 \)), so only option (b) \( \text{C}_6\text{H}_6 \) is produced.

Key Concepts

Sodium Benzoate

Sodium benzoate is a common food preservative that also plays a crucial role in organic chemistry reactions. It is the sodium salt of benzoic acid, often symbolized as \( \text{C}_6\text{H}_5\text{COONa} \). In chemical terms, sodium benzoate can be seen as a benzoic acid where the hydrogen atom in the carboxyl group is replaced by a sodium ion. This gives it unique properties which make it useful in various reactions.

One of the key reactions involving sodium benzoate is the decarboxylation reaction. In this reaction, sodium benzoate acts as a source of the phenyl (\( \text{C}_6\text{H}_5 \)) group when it loses a carboxyl group. This is often facilitated by heating in the presence of sodium hydroxide (a component of sodalime). Understanding the role of sodium benzoate in these reactions is essential for predicting the formation of familiar aromatic compounds, such as benzene.

Sodalime Reaction

The sodalime reaction is a well-known laboratory technique used for decarboxylation processes. Sodalime itself is a mixture, primarily consisting of calcium oxide (CaO) and sodium hydroxide (NaOH). This combination is key for facilitating the reaction where a carboxyl group is removed from a carboxylic acid salt.

In the context of sodium benzoate, the sodalime aids in the removal of the \(-COONa\) group. This is a necessary step for generating benzene from sodium benzoate. The alkaline nature of sodium hydroxide in sodalime also makes it effective in driving these reactions to completion.

**Important Points About Sodalime Reaction:**

• It generally involves heating the mixture of a carboxylate salt and sodalime.
• The reaction is a decarboxylation process, which means it results in the removal of carbon dioxide.
• Often results in the formation of aromatic hydrocarbons like benzene when aromatic carboxylate salts are used.

Benzene Formation

Benzene is a fundamental aromatic hydrocarbon, represented by the chemical formula \( \text{C}_6\text{H}_6 \). In decarboxylation reactions involving sodium benzoate and sodalime, benzene is the primary product. The process removes the carboxyl group and leaves behind the phenyl group, which forms benzene.

The reaction that yields benzene from sodium benzoate through decarboxylation is not just important for theoretical chemistry. Benzene is widely utilized in industrial chemical synthesis and serves as a building block for many other compounds, including polyesters, resins, and nylon.

**Characteristics of Benzene Formation:**

• It's a clear example of a decarboxylation reaction in organic chemistry.
• The reaction is driven by the high-temperature conditions that facilitate the breakdown of sodium benzoate.
• Benzene's distinct ring structure is formed from its six carbon atoms, exhibiting stability due to resonance.

Understanding benzene formation from sodium benzoate offers insights into aromatic compound synthesis and illustrates the effective use of decarboxylation reactions.