Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

Bottles containing \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{I}\) and \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{l}\) lost their orig- inal labels. They were labelled \(A\) and \(B\) 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) \(\mathrm{A}\) is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I}\) (b) \(\mathrm{A}\) is \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{I}\) (c) \(\mathrm{B}\) is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{I}\) (d) addition of \(\mathrm{HNO}_{3}\) was unnecessary

Step-by-Step Solution

Answer

(a) \( A \) is \( \mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{I} \)

Step 1: Understanding the Compounds

Two compounds, \( \mathrm{C}_{6} \mathrm{H}_{3} \mathrm{I} \) (iodobenzene) and \( \mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{I} \) (benzyl iodide), are tested. Iodobenzene is an aromatic compound, while benzyl iodide is aliphatic.

Step 2: Reaction with NaOH

Both compounds are boiled with \( \mathrm{NaOH} \) which facilitates the displacement of \( \mathrm{I} \) by \( \mathrm{OH}^- \). Aromatic compounds like iodobenzene do not undergo hydrolysis easily, whereas benzyl iodide reacts readily to form benzyl alcohol.

Step 3: Acidification and AgNO3 Test

After boiling with \( \mathrm{NaOH} \), the solutions are acidified with dilute \( \mathrm{HNO}_{3} \) to remove excess \( \mathrm{OH}^- \), then \( \mathrm{AgNO}_{3} \) is added. \(\mathrm{AgI} \) forms as a yellow precipitate in the presence of iodide ions.

Step 4: Analyze Precipitate Formation

Since solution \( B \) gave a yellow precipitate, it indicates the presence of free iodide ions, meaning that \( B \) is likely benzyl iodide, as it would readily form \( \mathrm{AgI} \) with \( \mathrm{AgNO}_{3} \).

Step 5: Conclusion

Since solution \( B \) forms a yellow precipitate, it confirms it was benzyl iodide due to the displacement reaction, so solution \( A \) must be iodobenzene. Aromatic iodobenzene does not release iodide ions easily and thus, does not form a yellow precipitate.

Key Concepts

Aromatic Compounds

Aromatic compounds are a fascinating and crucial part of organic chemistry. These molecules contain one or more planar rings with delocalized pi-electron clouds. The most common example is benzene, known for its stability and unique properties due to resonance. Iodobenzene is an aromatic compound where an iodine atom is bonded to the benzene ring. This configuration impacts its reactivity in chemical reactions. Aromatic compounds typically exhibit a lower reactivity compared to aliphatics, such as benzyl iodide, because their structure provides significant stability. Another interesting aspect of aromatic compounds is their exceptional resistance to reactions which might otherwise disrupt the ring's stability. When considering reactions, aromaticity plays a major role in determining both the pathway and the outcome. Recognizing if a compound is aromatic helps predict its behavior in various chemical processes.

Reaction Mechanism

Understanding reaction mechanisms is key to predicting and explaining chemical reactions. A reaction mechanism details each step of a chemical reaction, showing the pathway from reactants to products. In our exercise, the mechanism involves iodobenzene and benzyl iodide undergoing reactions with NaOH. Aromatic iodobenzene, owing to its stable delocalized electrons, does not readily participate in nucleophilic substitution reactions. Meanwhile, benzyl iodide's aliphatic nature makes it more susceptible to reaction with NaOH. In general, knowing the reaction mechanism helps chemists design and optimize processes for desired chemical synthesis, and also troubleshoot reactions when outcomes don’t match expectations.

Halogenation

Halogenation is the process of introducing a halogen atom, like iodine, into an organic compound. This can significantly alter the chemical properties of the molecule. For aromatic compounds, halogenation usually affects the electronic distribution within the ring. In iodobenzene, the iodine replacing a hydrogen on the benzene ring barely changes the apparent characteristics due to the compound's aromatic stability. In the case of benzyl iodide, the iodine is bonded to a benzylic (i.e., sp³ hybridized) carbon, which significantly influences its reactivity, particularly in nucleophilic substitution reactions. These reactions are a primary focus in organic synthesis, with specific halogenation processes carefully controlled to achieve the desired outcomes.

Chemical Reactions

Chemical reactions transform substances into different ones, and understanding their intricacies is essential for chemists. They can be classified into various types, such as substitution, addition, and elimination, among others. Our exercise involves a nucleophilic substitution reaction, where NaOH replaces the iodine in benzyl iodide, forming benzyl alcohol. The reaction does not readily occur with iodobenzene due to the stability conferred by its aromatic ring structure. The reduction in reactivity for aromatic halides when compared to aliphatic ones is an important consideration in synthetic chemistry, affecting how compounds are chosen for particular reactions.

Iodobenzene

Iodobenzene is an important compound in organic chemistry, often used as a reagent or precursor in various reactions. It features an iodine atom bonded to a benzene ring, which is characteristically more stable and less reactive due to the aromatic nature of benzene. However, iodobenzene can participate in certain reactions, like cross-coupling, under specific conditions. Its resistance to nucleophilic substitution underpins how it behaves differently compared to benzyl iodide in our exercise. This distinct characteristic makes iodobenzene less likely to yield iodide ions when tested with silver nitrate, as shown in the exercise where no yellow precipitate forms.

Benzyl Iodide

Benzyl iodide, an aliphatic compound with iodine attached to a benzylic carbon, is quite reactive. Its structure allows it to engage effectively in nucleophilic substitution reactions. When reacted with NaOH, the iodine is readily displaced, demonstrating the compound's reactivity compared to iodobenzene. This reaction forms benzyl alcohol and liberates iodide ions, which precipitate as silver iodide when AgNO₃ is added. Understanding the nature of benzyl iodide is crucial for predicting its behavior in chemical reactions, especially in laboratory settings like the one described in the exercise.