Explain why sodium bicarbonate test only provide information for Carboxylic acid group and doesn't provide for unsaturation and aldehyde

Answer 1

The sodium bicarbonate test is a specific qualitative test that is used primarily to detect the presence of carboxylic acids. Here’s how it works and why it doesn't provide information about unsaturation (double or triple bonds) or aldehydes:

Mechanism of the Sodium Bicarbonate Test:

1. Reactivity with Carboxylic Acids:
   • Carboxylic acids have a functional group represented as –COOH. These acids are proton donors and can donate a hydrogen ion (H+).
   • When a carboxylic acid is mixed with sodium bicarbonate (NaHCO₃), a reaction occurs where carbon dioxide (CO₂) gas is produced. This reaction is represented as: \[ \text{RCOOH} + \text{NaHCO}_3 \rightarrow \text{RCOONa} + \text{H}_2\text{O} + \text{CO}_2 \uparrow \]
   • The evolution of CO₂ gas, which is usually observed as bubbling or fizzing, indicates the presence of a carboxylic acid.

Reasons for Limited Scope:

1. Unsaturation:

   • Unsaturation generally refers to carbon-carbon multiple bonds (like double or triple bonds) in alkenes and alkynes.
   • These compounds do not have a functional group that reacts with sodium bicarbonate to produce CO₂, so the test does not yield relevant information about unsaturation.
   • To identify unsaturation, other tests like the bromine test (decolorization of bromine water) or the Baeyer's test (potassium permanganate test) are more appropriate.

2. Aldehydes:

   • Aldehydes have the functional group –CHO. They do not react with sodium bicarbonate to produce CO₂ either.
   • While some aldehydes can undergo certain reactions (e.g., oxidation), they generally do not produce the characteristic reaction with bicarbonate. Instead, they typically require tests like the Tollens' test or the Schiff's test for identification.
   • Aldehydes lack the acidic hydrogen that characterizes carboxylic acids, which is essential for the bicarbonate test's mechanism.

Conclusion:

Thus, the sodium bicarbonate test is selective for carboxylic acids due to their ability to release protons and react with bicarbonate to form carbon dioxide. It does not provide information regarding the presence of unsaturation or aldehydes, as these compounds lack the necessary reactive groups to undergo the same or similar reaction. Different tests are required to specifically identify those functional groups.