The compound formed in the positive test for nitrogen with the Lassaigne solution of an organic comp

Q: Why do we use sodium metal specifically for fusion?

Sodium is highly reactive and its melting point is low. It effectively breaks the covalent bonds in the organic compound and converts elements like nitrogen, sulphur, and halogens into their corresponding water-soluble ionic sodium salts (like NaCN, Na2S, NaX), which are easy to detect.

Q: Why must the ferrous sulphate solution be freshly prepared?

Ferrous ions ($\text{Fe}^{2+}$) are easily oxidized to ferric ions ($\text{Fe}^{3+}$) by atmospheric oxygen. If we use an old solution, we won't have enough $\text{Fe}^{2+}$ to form the required intermediate hexacyanoferrate(II) complex.

Q: What if the organic compound contains both nitrogen and sulphur?

In that case, sodium thiocyanate (NaSCN) is formed instead of NaCN. When $\text{Fe}^{3+}$ is added, it forms a blood-red complex $[\text{Fe}(\text{SCN})]^{2+}$ instead of Prussian blue.

Q: Why is the solution acidified before adding ferric chloride?

The solution is initially alkaline due to excess sodium reacting with water to form NaOH. If we add $\text{Fe}^{3+}$ directly to an alkaline solution, it will precipitate as brown ferric hydroxide ($\text{Fe(OH)}_3$), which would mask the Prussian blue color.

Q: What is the IUPAC name of Prussian blue?

The IUPAC name is Iron(III) hexacyanoferrate(II). The Roman numerals indicate the oxidation states: the iron outside the coordination sphere is +3, and the iron inside is +2.

Question

The compound formed in the positive test for nitrogen with the Lassaigne solution of an organic compound is

Options:

(A) $	ext{Fe}_4[	ext{Fe}(	ext{CN})_6]_3$

(B) $	ext{Na}_3[	ext{Fe}(	ext{CN})_6]$

(C) $	ext{Fe}(	ext{CN})_3$

(D) $	ext{Na}_4[	ext{Fe}(	ext{CN})_5	ext{NOS}]$

Sigma Percentile · Accepted Answer

The detection of extra elements like nitrogen, sulphur, and halogens in organic compounds is a fundamental aspect of qualitative analysis. The most reliable method for this is **Lassaigne's Test**.

But why do we need a special test? Organic compounds are primarily covalent. The elements within them are tightly bound by sharing electrons. To detect these elements using standard inorganic reagents, we must first convert them into an ionic form. This is where the magic of sodium fusion comes in.

### Step 1: The Sodium Fusion

Imagine taking a small piece of highly reactive sodium metal and heating it intensely with your organic compound. This is a violent, transformative process. The sodium attacks the covalent bonds. If nitrogen is present in the compound (along with carbon, which is always there), they combine with the sodium to form **sodium cyanide**.

$$ 	ext{Na} + 	ext{C} + 	ext{N} \xrightarrow{\Delta} 	ext{NaCN} $$

This simple reaction is the heart of Lassaigne's extract. We have successfully transformed the hidden, covalently bonded nitrogen into a free, water-soluble cyanide ion ($	ext{CN}^-$).

### Step 2: Building the Complex

Now that we have our cyanide ions in solution, we need to prove they are there. We do this by adding freshly prepared **ferrous sulphate** ($	ext{FeSO}_4$).

Why freshly prepared? Because ferrous ions ($	ext{Fe}^{2+}$) love to oxidize to ferric ions ($	ext{Fe}^{3+}$) in the presence of air. We need the $	ext{Fe}^{2+}$ for this specific step. The ferrous ions react with the cyanide ions to form a stable coordination complex called **sodium hexacyanoferrate(II)**.

$$ 	ext{FeSO}_4 + 6	ext{NaCN} ightarrow 	ext{Na}_4[	ext{Fe}(	ext{CN})_6] + 	ext{Na}_2	ext{SO}_4 $$

At this stage, the solution might look pale, but the trap is set. The nitrogen is now locked inside this complex ion.

### Step 3: The Grand Finale - Prussian Blue

To reveal the presence of our complex, we first acidify the solution (usually with dilute sulphuric acid) to neutralize any excess alkali. Then, we add a few drops of **ferric chloride** ($	ext{FeCl}_3$).

The ferric ions ($	ext{Fe}^{3+}$) react instantly with the hexacyanoferrate(II) complex. The result is a breathtaking, deep blue precipitate known as **Prussian blue**.

$$ 3	ext{Na}_4[	ext{Fe}(	ext{CN})_6] + 4	ext{FeCl}_3 ightarrow 	ext{Fe}_4[	ext{Fe}(	ext{CN})_6]_3 + 12	ext{NaCl} $$

The chemical name for this magnificent blue compound is **Iron(III) hexacyanoferrate(II)**. Its formula is $	ext{Fe}_4[	ext{Fe}(	ext{CN})_6]_3$.

Seeing this Prussian blue color is the definitive, undeniable proof that nitrogen was present in the original organic compound. It is one of the most visually satisfying tests in all of chemistry!

Animated Solution for Chemistry - Organic Chemistry: The compound formed in the positive test for nitrogen with the Lassaigne solution of an organic compound is

Select Answer:

Visualized Solution

$Lassaigne’s Extract Preparation$

$Addition of Ferrous Sulphate$

$Formation of Prussian Blue$

$Conclusion$

The Sigma Insight: Nomenclature and Characterisation

Step 1

Step 2

Step 3

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