Animated Solution for Chemistry - Organic Chemistry: Nitrogen can be estimated by Kjeldahl's method for which of the following compound ?

Let's dive into one of the most classic and widely used techniques in analytical chemistry: Kjeldahl's method. This method is a cornerstone for estimating the amount of nitrogen in an organic compound.

The core idea is elegantly simple yet chemically aggressive. You take your organic compound and heat it strongly with concentrated sulfuric acid ($H_{2}S{O}_4$). This process, known as digestion, aims to completely break down the organic framework and convert all the organic nitrogen into a stable inorganic salt: ammonium sulfate, $(N{H}_4{)}_{2}S{O}_4$. Once the nitrogen is safely trapped as ammonium ions, it can be easily quantified using standard titration techniques.

But here is the catch—Kjeldahl's method is not a universal magic wand. It has a well-known Achilles heel. It fails for certain types of nitrogen environments.

If the nitrogen atom is trapped inside a highly stable aromatic ring, or if it exists in a highly oxidized state (like a nitro group), or if it is part of an azo or diazonium group, the concentrated sulfuric acid simply cannot break it down into ammonium sulfate. The bonds are either too strong, or the required reduction to the $-3$ oxidation state of $N{H}_4^{+}$ is thermodynamically unfavorable under these specific digestion conditions.

Let's look closely at the options provided in the question to see which ones fall into these trap categories.

Take Pyridine (Option C). In pyridine, the nitrogen atom is a fundamental part of the aromatic ring itself. Because of the immense resonance stability of the aromatic system, the ring strongly resists digestion by sulfuric acid. The nitrogen refuses to leave the ring to form ammonium sulfate. So, Kjeldahl's method will absolutely not work here.

Now, let's check Nitrobenzene (Option D) and Benzenediazonium chloride (Option A). Nitrobenzene contains a nitro group ($-N{O}_2$), where nitrogen is in a high oxidation state. Benzenediazonium chloride contains a diazonium group ($-N_2^{+}C{l}^{-}$). As we just discussed, these groups are highly resistant to the specific reduction process occurring during Kjeldahl digestion. They will not yield ammonium sulfate either.

Benzylamine

Finally, focus on Benzylamine (Option B). Here, the structure is a benzene ring attached to a $-C{H}_{2}N{H}_2$ group.

Notice where the nitrogen is located. It is not part of the aromatic ring; it is part of an aliphatic amine side chain. This nitrogen is "free" to react. It is already in a favorable oxidation state and is not protected by the extreme stability of the aromatic core. Upon heating with concentrated sulfuric acid, this aliphatic amine easily and quantitatively gets converted into ammonium sulfate.

So, by eliminating the compounds that resist digestion, we can confidently conclude that Benzylamine is the only suitable candidate among these for nitrogen estimation using Kjeldahl's method.

Dumas Method

Before we wrap up, think about this: what if we absolutely needed to find the nitrogen content in Pyridine or Nitrobenzene? Are we just out of luck?

Not at all. When Kjeldahl fails, chemists turn to the Dumas method. In the Dumas method, the organic compound is combusted at high temperatures in the presence of copper oxide, converting all nitrogen into nitrogen gas ($N_2$), which is then measured volumetrically. Unlike Kjeldahl's, the Dumas method works universally for all nitrogen-containing organic compounds. Keep that distinction in mind for your exams!