Question

Sodium fusion extract, obtained from aniline, on treatment with iron(II) sulphate and \(\mathrm{H}_{2} \mathrm{SO}_{4}\) in presence of air gives a Prussian blue precipitate. The blue colour is due to the formation of (A) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) (B) \(\mathrm{Fe}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{2}\) (C) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{2}\) (D) \(\mathrm{Fe}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\)

Short answer

The blue colour precipitate formed is due to the compound (A) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\).

Step-by-step solution

Understand the Chemistry behind the Test

Aniline, when fused with sodium, forms sodium phenylamide. This fusion extract when treated with iron(II) sulfate and sulfuric acid in the presence of air produces a Prussian blue precipitate. This reaction occurs due to the transformation of some of the iron(II) sulfate into iron(III) ions which then react with the cyanide ions to form the compound responsible for the blue color

Recognize the Prussian Blue Compound

Prussian blue is a dark blue pigment that is chemically known as iron hexacyanoferrate. The correct formula for Prussian blue is Fe4[Fe(CN)6]3, which contains iron in both +2 and +3 oxidation states.

Match the Correct Option

Based on the known chemical formula for Prussian blue, we can match the appropriate option. The correct answer is the formula that contains both the Fe2+ and Fe3+ ions in the structure of Prussian blue.

Key concepts

Understanding Sodium Fusion Extract

The term 'sodium fusion extract' might sound technical, but it's actually a method employed in organic chemistry to identify the presence of certain elements in an organic compound. To create this extract, the organic compound, in this case, aniline, is fused i.e., melted together, with sodium metal. This process results in the breakdown of complex organic molecules into simpler inorganic ions.

When aniline is fused with sodium, it forms sodium phenylamide. This is an essential step in forming the intended Prussian blue compound later on. This process is particularly useful in converting the nitrogen present in aniline into a form that can react with iron to eventually lead to the iconic blue compound. It's also interesting to note that this extract can be tested for various elements like nitrogen, sulfur, and halogens - elements often found in organic compounds.

Deciphering the Iron Hexacyanoferrate Formula

Iron hexacyanoferrate is commonly known as Prussian blue, which is not only historically significant as one of the first synthetic pigments but also is intriguing because of its chemical structure. The correct formula for Prussian blue is \( \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \), and here's the exciting bit - it has both iron(II) ions (\( \mathrm{Fe}^{2+} \) ) and iron(III) ions (\( \mathrm{Fe}^{3+} \) ) within its crystal lattice.

Apart from being a pigment, this compound has applications in medicine as an antidote for certain types of poisoning. It functions due to its unique ability to bind to certain substances, a property imparted by its complex structure. Understanding the formula is crucial not only when learning about color formation but also for grasping the concept of coordination chemistry, where metals and ligands join hands to form extraordinary structures like Prussian blue.

Aniline's Reaction with Iron - From Simple to Complex

The chemistry behind the reaction between aniline and iron, especially when aiming to synthesize Prussian blue, is a fascinating example of how organic and inorganic chemistry interplay. Initially, aniline reacts with iron(II) sulfate and sulfuric acid in the presence of air. The iron in the iron(II) sulfate is in the +2 oxidation state, but when it's exposed to air (oxygen), some of it oxidizes to the +3 state.

Why does this matter? Because the iron(III) ion is what eventually reacts with the cyanide ions that were introduced into the system through the sodium fusion process. It's this specific reaction between iron(III) ions and cyanide ions that leads to the formation of the Prussian blue pigment. This is a fantastic example of a redox reaction, where both reduction and oxidation are taking place, leading to a significant change in both the valency of iron and the color of the solution.