Question

During Lassaigne's test, the elements present in an organic compound are converted from:

• A. Covalent form to ionic form
• B. Covalent form to covalent form
• C. Ionic form to ionic form
• D. Ionic form to covalent form

Hint:

Lassaigne's test converts covalently bonded elements into ionic sodium salts for their detection.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question focuses on the underlying chemical principle of Lassaigne's test, which is a qualitative analysis method used to detect the presence of Nitrogen, Sulfur, and Halogens in organic compounds. Because these elements are bonded within a covalent framework in organic molecules, they do not exist as free ions and cannot be detected by standard inorganic reagents. The test is designed to convert these non-ionizable elements into a form that can be tested in an aqueous solution.
Step 2: Key Formula or Approach:
The "Sodium Fusion" method is the core approach here. By heating the organic compound with highly reactive sodium metal, the covalent bonds are broken, and the atoms are forced to react with sodium to form stable, water-soluble ionic salts. The general reactions are:
For Nitrogen: $Na + C + N \rightarrow NaCN$ (Sodium Cyanide)
For Sulfur: $2Na + S \rightarrow Na_2S$ (Sodium Sulfide)
For Halogens: $Na + X \rightarrow NaX$ (Sodium Halide)
Step 3: Detailed Explanation:

Initial State: In organic molecules (like urea, thiourea, or chlorobenzene), the Nitrogen, Sulfur, and Halogens are part of the carbon chain or rings. They are held together by covalent bonds, where electrons are shared. These molecules do not dissociate into ions when dissolved in water, meaning we cannot use simple tests like adding silver nitrate to detect a halogen.
Transformation Process: During the fusion process, the organic compound is heated with a pellet of sodium metal until the tube becomes red hot. The intense heat and the strong reducing nature of sodium cause the covalent bonds to rupture.
Ionic Product Formation: The free atoms then bond with sodium. Carbon and Nitrogen from the organic matter combine with sodium to form the cyanide ion ($CN^-$) in the salt $NaCN$. Sulfur forms the sulfide ion ($S^{2-}$) in $Na_2S$, and Halogens (Cl, Br, I) form halide ions ($X^-$) in $NaX$.
Detection: These resulting salts are ionic. When the red-hot tube is plunged into distilled water, these ionic salts dissolve completely. This "sodium fusion extract" can then be tested using standard inorganic qualitative procedures (like the Prussian blue test for Nitrogen or the Lead Acetate test for Sulfur).
Step 4: Final Answer:
The fundamental shift in the Lassaigne's test is the conversion of elements from their original covalent form into a detectable ionic form. Thus, option (B) is the correct answer.