Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which one of the following species is not a pseudohalide? [1997 - 1 Mark] (a) \(\mathrm{CNO}^{-}\) (b) \(\mathrm{RCOO}^{-}\) (c) \(\mathrm{OCN}\) (d) \(\mathrm{NNN}\)

Step-by-Step Solution

Answer

Option (b) \\( \mathrm{RCOO}^{-} \\\) is not a pseudohalide.

Step 1: Understanding Pseudohalides

Pseudohalides are polyatomic inorganic ions that mimic the properties of halide ions (like \( ext{Cl}^- \) or \( ext{Br}^- \)). They form compounds similar to halides and often show similar reactivity, including forming salts and acids.

Step 2: Analyzing Option (a): \\( \mathrm{CNO}^{-} \\\)

The ion \( \mathrm{CNO}^{-} \), known as cyanate, is a well-known pseudohalide in chemistry. It behaves similarly to monohalide ions due to its structure and reactivity.

Step 3: Analyzing Option (b): \\( \mathrm{RCOO}^{-} \\\)

The ion \( \mathrm{RCOO}^{-} \) is the carboxylate ion and is not traditionally considered a pseudohalide. It comes from carboxylic acids and forms esters and salts, but has different characteristics from traditional pseudohalides.

Step 4: Analyzing Option (c): \\( \mathrm{OCN} \\\)

The ion \( \mathrm{OCN} \), known as fulminate, is similar to other pseudohalides and forms salts and complexes like pseudohalides do.

Step 5: Analyzing Option (d): \\( \mathrm{NNN} \\\)

The ion \( \mathrm{NNN} \), known as azide, is a typical pseudohalide due to its linear structure and reactivity similar to halides. It forms salts like \( \text{NaN}_3 \) (sodium azide).

Step 6: Identifying the Non-Pseudohalide

Among the given options, \( \mathrm{RCOO}^{-} \) (the carboxylate ion) is not considered a pseudohalide because it derives from carboxylic acids with different properties compared to pseudohalide ions. Therefore, the answer is option (b).

Key Concepts

Cyanate Ion

The cyanate ion, represented as \( \mathrm{CNO}^{-} \), is a notable example of a pseudohalide. Pseudohalides are groups of polyatomic ions that resemble halides both structurally and in terms of their chemical behaviors. The cyanate ion is of particular interest because it mimics halide ions like chloride \( \mathrm{Cl}^{-} \) or bromide \( \mathrm{Br}^{-} \) through its ability to form similar types of compounds, such as salts and acids.

In terms of structure, the cyanate ion consists of a carbon atom triple-bonded to nitrogen, with an additional oxygen atom. This arrangement allows it to exhibit properties akin to halides, making it reactive and versatile in chemical synthesis.

Some key features of cyanate ions include:

• Ability to form stable ionic compounds.
• Participation in substitution reactions.
• Formation of complex compounds with transition metals.

Because of these characteristics, cyanate ions are classified within the pseudohalides family, showcasing their functional similarities to traditional halides.

Carboxylate Ion

The carboxylate ion \( \mathrm{RCOO}^{-} \) stands apart from pseudohalides, primarily due to its origin and chemical behavior. Derived from carboxylic acids, carboxylate ions have unique properties that differentiate them from typical pseudohalides.

Unlike the cyanate ion, carboxylate ions do not share the same structural resemblance to halide ions. They comprise a carbon atom bonded to two oxygen atoms, one of which bears a negative charge. This structural composition offers distinctive properties:

• The ability to form salts, like sodium acetate.
• Participation in esterification, unlike halides and pseudohalides.
• Less reactivity in forming complexes with metals as compared to halides or pseudohalides.

Carboxylate ions do not mimic halides in the formation of salts and complexes or in reactivity, thus excluding them from being considered pseudohalides.

Azide Ion

The azide ion \( \mathrm{NNN} \) is a classic example of a pseudohalide, famous for its linear structure and resemblance to traditional halide ions in both form and function. It consists of three nitrogen atoms, wherein the terminal nitrogens are negatively charged often, giving it significant reactivity.

Azide ions mimic the reactivity of halides by:

• Readily forming salts, such as sodium azide \( \text{NaN}_3 \).
• Undergoing substitution reactions.
• Forming covalent azides, akin to halogen derivates.

With their capacity to partake in explosions (as seen in airbags) and formation of complexes, azides are well-regarded within the pseudohalide category. Their behavior, particularly in forming stable ionic compounds, positions them as intriguing objects of study in the realm of inorganic chemistry.