The Positive Test for Nitrogen with Lassaigne Extract

The Lassaigne extract is a traditional method used to detect the presence of nitrogen in organic compounds. It involves converting the nitrogen-containing compound into a sodium fusion extract, which is then subjected to a series of tests to identify the presence of nitrogen. One of the tests is the formation of a blue-colored precipitate with iron(III) chloride solution. Among the given options, the compound that forms a blue precipitate with iron(III) chloride is Fe4[Fe(CN)6]3, which is option A.

Explanation:

To understand why Fe4[Fe(CN)6]3 forms a blue precipitate with iron(III) chloride, we need to examine the structure and properties of the compound.

- Fe4[Fe(CN)6]3: This compound consists of iron atoms coordinated to cyanide ligands. The central iron atom is surrounded by six cyanide ligands, forming a hexacyanoferrate(III) complex. The overall charge of the complex is 3-, which is balanced by four iron(III) ions. The presence of the cyanide ligands is crucial for the formation of the blue precipitate.

- Iron(III) chloride: Iron(III) chloride (FeCl3) is a compound that dissociates in water to form Fe3+ ions and chloride ions (Cl-). The Fe3+ ions have an empty d orbital, which allows them to form coordination complexes with ligands.

When Fe4[Fe(CN)6]3 is treated with iron(III) chloride, the Fe3+ ions from iron(III) chloride coordinate with the cyanide ligands in Fe4[Fe(CN)6]3, forming a complex known as Prussian blue. Prussian blue is a deep blue-colored precipitate, indicating the presence of nitrogen in the organic compound.

Conclusion:

Among the given options, Fe4[Fe(CN)6]3 is the compound that forms a blue precipitate with iron(III) chloride in the Lassaigne test for nitrogen. This is because Fe4[Fe(CN)6]3 contains cyanide ligands that can coordinate with Fe3+ ions from iron(III) chloride to form Prussian blue. Therefore, option A is the correct answer.