Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Bottles containing \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{I}\) and \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I}\) lost their original labels. They were labeled \(\mathrm{A}\) and \(\mathrm{B}\) for testing. A and B were separately taken in test tubes and boiled with \(\mathrm{NaOH}\) solution. The end solution in each tube was made acidic with dilute \(\mathrm{HNO}_{3}\) and some \(\mathrm{AgNO}_{3}\) solution added. Solution B gave a yellow precipitate. Which one of the following statements is true for the experiment? (a) A was \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I}\) (b) A was \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{I}\) (c) \(\mathrm{B}\) was \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{I}\) (d) addition of \(\mathrm{HNO}_{3}\) was unnecessary

Step-by-Step Solution

Answer

(a) A was \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I} \).

Step 1: Reactivity of Iodo Compounds with NaOH

When organic iodides like \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{I} \) (iodobenzene) and \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I} \) (benzyl iodide) are boiled with \( \mathrm{NaOH} \), they undergo a substitution reaction. The iodide is replaced by a hydroxyl group, forming \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH} \) (phenol) and \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OH} \) (benzyl alcohol), respectively. This step prepares the iodide ions for the next reaction.

Step 2: Reaction with AgNO3

After the boiling with \( \mathrm{NaOH} \), the solutions are made acidic using dilute \( \mathrm{HNO}_{3} \). This ensures that the medium is acidic for the \( \mathrm{AgNO}_{3} \) test. When \( \mathrm{AgNO}_{3} \) is added, silver ions (\( \mathrm{Ag}^+ \)) react with the free iodide ions (\( \mathrm{I}^- \)) in the solution to form silver iodide (\( \mathrm{AgI} \)), a yellow precipitate. This precipitate confirms the presence of free iodide ions.

Step 3: Analyzing the Results

The yellow precipitate of \( \mathrm{AgI} \) is indicative of a successful reaction with free iodide ions. Benzyl iodide ( \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I} \)) produces free iodide ions upon reaction with \( \mathrm{NaOH} \), while iodobenzene (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{I} \)) does not, due to the stable nature of the aromatic compound that prevents the release of iodide ions. Thus, \( \mathrm{B} \), which gives a yellow precipitate, must be \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I} \).

Key Concepts

Silver Nitrate Test

The Silver Nitrate Test is a classic method used in chemistry to detect the presence of halide ions like iodide. It involves adding silver nitrate (\( \mathrm{AgNO}_3 \)) to a solution containing halides. When iodide ions are present, they react with \( \mathrm{Ag}^+ \) ions to produce a yellow precipitate of silver iodide (\( \mathrm{AgI} \)).

• This test is useful in distinguishing substances that produce free halide ions from those that do not.
• The reaction is sensitive, making it a reliable indicator of iodide presence.

The formation of the yellow solid confirms reactions involving iodide ions, as seen in the experiment where one solution forms this precipitate.

Substitution Reaction

A substitution reaction occurs when an atom or a group of atoms in a molecule is replaced by a different atom or group. In organic chemistry, it often involves halogens like iodine.
When organic iodides such as benzyl iodide (\(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{I}\)) are heated with sodium hydroxide (\(\mathrm{NaOH}\)), a substitution reaction takes place. The iodine atom is replaced by a hydroxyl group, converting benzyl iodide into benzyl alcohol (\(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{OH}\)).

• This reaction releases free iodide ions into the solution, priming it for further testing.

It's this substitution that lets us perform the Silver Nitrate Test to determine the presence of iodide.

Benzyl Iodide

Benzyl iodide (\(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{I}\)) is a reactive organic compound notable for its susceptibility to substitution reactions due to the presence of the benzyl group. This reactivity is due to the \(\mathrm{-CH}_2\) group, which is more prone to nucleophilic attack compared to stable aromatic rings.
Benzyl iodide readily undergoes a substitution with hydroxide ions from \(\mathrm{NaOH}\), releasing iodide ions into the solution.

• This property makes it detectable by the Silver Nitrate Test.

Its reactivity is utilized in the experiment to distinguish it from other less reactive compounds like iodobenzene.

Iodobenzene

Iodobenzene (\(\mathrm{C}_6\mathrm{H}_5\mathrm{I}\)) displays stability due to its aromatic ring structure. This stability makes it less reactive compared to benzyl compounds. In a substitution reaction, iodobenzene is resistant to releasing the iodide ion because the aromatic ring stabilizes the compound and prevents nucleophilic attack.
In the context of the Silver Nitrate Test, this means iodobenzene won't form a yellow precipitate, helping distinguish it from more reactive compounds like benzyl iodide.

• Its inertness under test conditions prevents iodide ion release.

This characteristic makes iodobenzene a non-reactive participant in the tested substitution scenario.

Precipitation Reaction

A precipitation reaction is a process where dissolved ions in a solution react to form an insoluble solid, or precipitate. This type of reaction is at the core of the Silver Nitrate Test.
In the experiment, when silver nitrate is added to a solution with free iodide ions, \(\mathrm{Ag}^+ \) and \(\mathrm{I}^- \) ions combine to form silver iodide (\(\mathrm{AgI}\)), a yellow precipitate.

• This reaction is immediate and visible, serving as a strong indicator of iodide presence.
• The distinct yellow color of \(\mathrm{AgI}\) confirms the completion of the precipitation reaction.

Precipitation reactions like this one are fundamental to analytical chemistry and are used to detect and confirm the presence of specific ions in a solution.