Problem 169 - 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 orig- inal labels. They were labelled \(\mathrm{A}\) 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? [2003] (a) \(\mathrm{A}\) is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I}\) (b) \(\mathrm{A}\) is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{I}\) (c) B is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{I}\) (d) addition of \(\mathrm{HNO}_{3}\) was unnecessary

Step-by-Step Solution

Answer

(a) \( \mathrm{A} \) is \( \mathrm{C_{6}H_{5}CH_{2}I} \). Compound B gave yellow precipitate.

Step 1: Understanding the Reaction with NaOH

When compounds like \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{I} \) or \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I} \) react with NaOH, they undergo nucleophilic substitution. \( \mathrm{I}^{-} \) ions are released in the process, which will later react with \( \mathrm{AgNO}_{3} \).

Step 2: Reaction with AgNO3

Adding \( \mathrm{AgNO}_{3} \) to the solution will cause silver halides to precipitate. \( \mathrm{AgI} \) forms a yellow precipitate, which is characterized by its insolubility in dilute acids and appears when \( \mathrm{I}^{-} \) ions are present.

Step 3: Analyze Compound B

Since compound B gives a yellow precipitate, this confirms the presence of \( \mathrm{I}^{-} \) ions after boiling with NaOH and reacting with \( \mathrm{AgNO}_{3} \). This indicates that \( \mathrm{B} \) is \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I} \) as benzylic iodine (as in \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I} \)) is more reactive and tends to form \( \mathrm{AgI} \).

Step 4: Conclusion about the Identity of A

Since \( \mathrm{B} \) is \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{I} \), compound \( \mathrm{A} \) must be \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{I} \). Compound \( \mathrm{A} \) would not react as readily to form \( \mathrm{AgI} \) due to the lack of reactivity of the aryl halide.

Key Concepts

Silver halide precipitation

Silver halide precipitation is a fascinating process often used to test for halide ions in a given compound. When a halide ion, such as iodide \(\mathrm{I}^-\), is present in a solution and comes into contact with \(\mathrm{AgNO_3}\), it forms an insoluble silver halide compound such as silver iodide \(\mathrm{AgI}\). This reaction leads to the appearance of a characteristic yellow precipitate. It's particularly important to note that the yellow color of \(\mathrm{AgI}\) is due to its specific properties among the silver halides, as others, like silver chloride \(\mathrm{AgCl}\), may form white precipitates.

The precipitation of silver halides is a significant indicator in nucleophilic substitution reactions, especially when working with organic iodides. These reactions require careful addition of nitric acid \(\mathrm{HNO_3}\) to prevent false precipitations by ensuring that other soluble salts do not interfere. The accuracy and clarity of silver halide tests make them an essential tool in organic chemistry for identifying and confirming the presence of specific halide ions.

Aryl and benzylic halides

Aryl and benzylic halides are distinct types of organic compounds that react differently in substitution reactions due to their structures. An aryl halide involves a halogen attached directly to a benzene ring, represented by compounds like \(\mathrm{C_6H_5I}\). On the other hand, a benzylic halide has the halogen attached to a carbon atom adjacent to the benzene ring, as seen in \(\mathrm{C_6H_5CH_2I}\).

These structural differences significantly influence their reactivity. Aryl halides, such as \(\mathrm{C_6H_5I}\), are generally less reactive in nucleophilic substitution reactions. This lack of reactivity is due to the strong bond between the iodine atom and the benzene ring, which is stabilized by resonance structures within the aromatic system.

In contrast, benzylic halides like \(\mathrm{C_6H_5CH_2I}\) are significantly more reactive. The benzylic position allows for a more facilitated release of the iodide ion, primarily due to the position's ability to stabilize the transition state through resonance with the benzene ring. This increased reactivity explains why \(\mathrm{B}\), in our exercise, formed the yellow \(\mathrm{AgI}\) precipitate readily with silver nitrate.

Reactivity of organic iodides

Organic iodides play a pivotal role in nucleophilic substitution reactions due to their ability to form iodide ions efficiently. The reactivity of organic iodides in these reactions can greatly vary, depending on whether the iodide is attached to an aryl or benzylic position.

As discussed, the benzyl position, such as in \(\mathrm{C_6H_5CH_2I}\), facilitates easier release of \(\mathrm{I}^-\) ions, leading to swift formation of \(\mathrm{AgI}\) upon reaction with \(\mathrm{AgNO_3}\). This increased reactivity is primarily due to the resonance stabilization provided by the benzene ring, which aids in overcoming the activation energy required for the substitution reaction.

Conversely, iodides bonded directly to a benzene ring (aryl iodides) exhibit considerably reduced reactivity. The \(\mathrm{C_6H_5I}\) bond is particularly robust due to its resonance-stabilized nature, making nucleophilic substitutions less favorable and, therefore, \(\mathrm{AgI}\) less likely to precipitate from it.

• Organic iodides like \(\mathrm{C_6H_5CH_2I}\) are favored in conditions that support nucleophilic substitution, especially when testing for iodide ions using silver nitrate.
• Knowing the reactivity differences helps in correctly identifying compounds during chemical analysis experiments.

Understanding these nuances is essential for accurately predicting and observing the outcomes in lab-based nucleophilic substitution reactions, as exemplified by the exercise studied.