Question

Carry out the following conversion
Propanoic acid to Ethane

Hint:

Reduction of carboxylic acids can yield alcohols, and further reduction of alcohols can yield alkanes.

Answer 1

To solve the problem, we need to convert propanoic acid to ethane.

1. Understanding the Structures:
Propanoic acid has the formula $ \text{CH}_3\text{CH}_2\text{COOH} $, a three-carbon carboxylic acid. Ethane has the formula $ \text{C}_2\text{H}_6 $, a two-carbon alkane. The conversion involves reducing the carbon chain from three to two carbons and removing the carboxyl group, replacing it with hydrogens to form an alkane.

2. Decarboxylation to Form Ethane:
A common method to convert a carboxylic acid to an alkane with one less carbon is decarboxylation. For propanoic acid, we can use the soda lime decarboxylation reaction, where the acid is heated with soda lime (a mixture of $ \text{NaOH} $ and $ \text{CaO} $). This removes the carboxyl group as $ \text{CO}_2 $, yielding an alkane.

$ \text{CH}_3\text{CH}_2\text{COOH} + \text{soda lime} \xrightarrow{\text{heat}} \text{C}_2\text{H}_6 + \text{CO}_2 $

3. Reaction Mechanism:
In the presence of soda lime, propanoic acid first forms its sodium salt, sodium propanoate ($ \text{CH}_3\text{CH}_2\text{COONa} $). Upon heating, the carboxylate group is removed as $ \text{CO}_2 $, and a hydrogen from the soda lime replaces it, forming ethane ($ \text{C}_2\text{H}_6 $).

$ \text{CH}_3\text{CH}_2\text{COONa} \xrightarrow{\text{heat, soda lime}} \text{C}_2\text{H}_6 + \text{Na}_2\text{CO}_3 $

4. Alternative Route Consideration:
Another possible route is converting propanoic acid to ethane via reduction, but this is less direct. For example, reducing propanoic acid to 1-propanol ($ \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} $) using $ \text{LiAlH}_4 $, followed by dehydration to propene ($ \text{CH}_3\text{CH}=\text{CH}_2 $), and then hydrogenation to propane ($ \text{C}_3\text{H}_8 $), followed by further chain shortening, is complex and not practical for direct conversion to ethane. Decarboxylation is the most efficient method.

5. Finalizing the Process:
The soda lime decarboxylation directly yields ethane from propanoic acid by removing the carboxyl carbon, reducing the chain length from three to two carbons, and saturating the molecule with hydrogens.

Final Answer:
Propanoic acid can be converted to ethane by heating with soda lime, resulting in decarboxylation to form $ \text{C}_2\text{H}_6 $.