Lassaigne's Test for Detection of Nitrogen

Lassaigne's test is widely used for the detection of nitrogen in organic compounds. The test is based on the conversion of nitrogen-containing compounds into sodium cyanide, followed by the formation of a Prussian blue precipitate upon reaction with iron(III) ions. However, the test fails in certain cases.

Explanation of Failure

The Lassaigne's test fails in compounds that contain nitrogen in the form of a carbonyl group (C=O). This is because the carbonyl group reacts with sodium metal to form sodium cyanide, which can then react with iron(III) ions to form a Prussian blue precipitate. As a result, the test gives a false positive for the presence of nitrogen.

Examples of Compounds where Lassaigne's Test Fails

The following are some examples of compounds where the Lassaigne's test fails:

- NH2 - CO - NH - NH2 (urea): Urea contains nitrogen in the form of a carbonyl group, which reacts with sodium metal to form sodium cyanide. This can then react with iron(III) ions to form a Prussian blue precipitate, giving a false positive for the presence of nitrogen.

- NH2 - CO - NH2 (biuret): Biuret also contains nitrogen in the form of a carbonyl group, which can react with sodium metal to form sodium cyanide. This can then react with iron(III) ions to form a Prussian blue precipitate, giving a false positive for the presence of nitrogen.

- Ph - NH - NH2 (aniline): Aniline contains a nitrogen atom that is attached to a benzene ring. Although this nitrogen atom is not directly attached to a carbonyl group, it can still give a false positive in the Lassaigne's test. This is because aniline can undergo oxidation to form a compound called nitrosobenzene, which reacts with sodium metal to form sodium cyanide. This can then react with iron(III) ions to form a Prussian blue precipitate, giving a false positive for the presence of nitrogen.

Conclusion

In conclusion, Lassaigne's test is a useful method for the detection of nitrogen in organic compounds. However, the test fails in compounds that contain nitrogen in the form of a carbonyl group. Therefore, it is important to use other methods such as the Kjeldahl method or the Dumas method for the determination of nitrogen in such compounds.