Question

The compound formed in the positive test for nitrogen with the Lassaigne solution of an organic compound is: (a) \(\mathrm{Fe}(\mathrm{CN})_{3}\) (b) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) (c) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NOS}\right]\) (d) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\)

Short answer

The compound formed is (b) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\).

Step-by-step solution

Understanding Lassaigne's Test

Lassaigne's test is used for the detection of nitrogen in an organic compound. The compound is fused with sodium to form sodium cyanide if nitrogen is present.

Formation of Prussian Blue

The sodium cyanide formed reacts with ferrous sulfate in the presence of an acid to ultimately form a complex compound known as Prussian Blue, which confirms the presence of nitrogen.

Identifying Prussian Blue Formula

Prussian Blue is chemically represented as the complex compound \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\). This corresponds to the molecular formula of Prussian Blue, which is produced during Lassaigne's test.

Matching with Choices

From the provided options, match the compound \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) with the given answer choices. It corresponds to option (b).

Key concepts

Prussian Blue formation

When performing Lassaigne's test for nitrogen in organic compounds, one of the most fascinating outcomes is the formation of Prussian Blue. This is a deep blue colored compound that vividly signals the presence of nitrogen. The process begins when the organic compound is fused with sodium, resulting in the formation of sodium cyanide (NaCN).

Next, the sodium cyanide reacts with ferrous sulfate ( FeSO 4) in an acidic environment, typically with hydrochloric acid. This combination undergoes a series of chemical interactions that finally lead to the creation of Prussian Blue. The complete chemical story involves a few intermediate steps but ultimately results in the complex Fe 4[ Fe( CN ) 6] 3, known for its distinct blue color. This reaction is not only a beautiful demonstration of chemistry but also a reliable indicator of nitrogen's presence in a compound.

Detection of nitrogen in organic compounds

Lassaigne's test provides an elegant method to detect nitrogen in organic compounds. This test utilizes the principle that when an organic sample is heated with metallic sodium, nitrogen present in the compound is converted into sodium cyanide. This reaction happens because the heat facilitates the fusion of sodium with the organic compound, allowing any nitrogen atoms present to form bonds with sodium.

The presence of sodium cyanide can then be confirmed by the subsequent formation of Prussian Blue upon reacting with ferrous sulfate under acidic conditions. If nitrogen is not present, this reaction does not occur, confirming the absence of nitrogen. Thus, Lassaigne's test is a classic qualitative analysis technique, helping chemists to efficiently confirm if an organic compound contains nitrogen.

Iron(II) cyanide complex

In the process of forming Prussian Blue, we encounter an important intermediate: the iron(II) cyanide complex. After the initial creation of sodium cyanide from the fusion of the organic compound with sodium, the cyanide ions are ready to react.

These cyanide ions combine with iron(II) ions from ferrous sulfate to form an intermediate complex. Under the acidic conditions of the reaction, this complex further reacts to yield the iron(II) cyanide complex, which is an essential precursor to the formation of Prussian Blue. The ability of cyanide ions to stabilize iron by forming strong chemical bonds is crucial in this step. This intermediate acts as a critical bridge in the transformation leading to the vivid blue color that signifies the presence of nitrogen.