Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

Select the correct statements regarding Lassaigne's test? (a) Li and \(\mathrm{K}\) are not used in this test (b) \(\mathrm{NH}_{4} \mathrm{Cl}, \mathrm{NH}_{2}-\mathrm{NH}_{2}\) Can't be detected by this test. (c) Urea, Acetamide can not give this test. (d) Benzamide, Azobenzene can give this test.

Step-by-Step Solution

Answer

Statements (a), (b), and (c) are correct.

Step 1: Understand Lassaigne's Test

Lassaigne's test is used to detect the presence of halogens, nitrogen, and sulfur in organic compounds. It involves fusing the organic compound with sodium metal to convert these elements into detectable ions.

Step 2: Analyze Statement (a)

Evaluate if lithium (Li) and potassium (K) are used in Lassaigne's test. Lassaigne's test specifically uses sodium (Na) and does not require Li or K, making this statement correct.

Step 3: Analyze Statement (b)

Determine if ammonium chloride (04Cl) and hydrazine (02−82) can be detected. Lassaigne's test does not detect nitrogen in ammonium salts and hydrazines, as these do not form appropriate cyanides or thiocyanates, making this statement correct.

Step 4: Analyze Statement (c)

Evaluate if urea and acetamide can be tested. These compounds decompose and produce ammonia gas instead of forming sodium cyanide, so they do not give a positive Lassaigne's test, making this statement correct.

Step 5: Analyze Statement (d)

Determine if benzamide and azobenzene pass the test. Both compounds contain nitrogen, but benzamide requires special conditions to form sodium cyanide, and azobenzene usually doesn't convert to sodium cyanide, so this statement is incorrect.

Key Concepts

Detection of Halogens

Lassaigne's test is a classic chemical process for identifying halogens such as chlorine, bromine, and iodine in organic compounds. The test starts with the reaction of an organic sample with sodium metal to form corresponding sodium halides. Once the sodium extract, also known as Lassaigne's extract, is prepared, it can be tested for halogens.

• In the presence of halogens, these compounds convert into sodium halides.
• On further reaction with silver nitrate (\( \text{AgNO}_3 \)), a white precipitate forms for chlorine, a cream color for bromine, and a yellow precipitate for iodine.
• The color of the precipitate helps in identifying which halogen is present.

This simple conversion methodology is what makes Lassaigne's test so effective in detecting halogens in an organic compound.

Detection of Nitrogen

The detection of nitrogen involves its conversion into cyanide ions. When an organic compound is fused with sodium, nitrogen is converted into sodium cyanide. This cyanide can then be detected using the Prussian blue test.

• Sodium cyanide from the reaction is reacted with ferrous sulfate (\( \text{FeSO}_4 \)).
• On addition of hydrochloric acid (\( \text{HCl} \)), sodium ferrocyanide is formed.
• This sodium ferrocyanide reacts with ferric chloride [(\( \text{FeCl}_3 \)] to form Prussian blue (\( \text{Fe}_4[\text{Fe(CN)}_6]_3 \)), indicating the presence of nitrogen.

Some compounds, like ammonium salts, even though they contain nitrogen, do not typically form sodium cyanide during Lassaigne's test and thus cannot be detected using this method. This is an important limitation to acknowledge.

Detection of Sulfur

Lassaigne's test also facilitates the detection of sulfur, converting it into soluble sodium sulfide. When sodium fuses with the organic compound, any sulfur present forms sodium sulfide.

• To check for sulfur, the sodium extract is treated with sodium nitroprusside.
• A purple color indicates the presence of sulfur attributable to the formation of a sulfur-nitroprusside complex.
• This distinct color change is an easy marker for identifying sulfur in an unknown sample.

Compounds that contain sulfur do convert efficiently to sodium sulfide during the test, making it straightforward to detect sulfur-containing organic compounds.