Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following pair is correctly matched? List I List II (Reaction) \(\quad\) (Product) I. \(\mathrm{RX}+\mathrm{AgCN} \quad \mathrm{RNC}\) II. RX + KCN \(\quad\) RCN III. \(\mathrm{RX}+\mathrm{KNO}_{2} \mathrm{R}-\mathrm{N} \leqslant_{\mathrm{O}}^{\nearrow \mathrm{O}}\) IV. \(\mathrm{RX}+\mathrm{AgNO}_{2} \quad \mathrm{R}-\mathrm{O}-\mathrm{N}=\mathrm{O}\) Select the correct answer using the codes given bels (a) I and II (b) III and IV (c) I only (d) I, II, III and IV

Step-by-Step Solution

Answer

Option (a) I and II are correctly matched.

Step 1: Reviewing Reactions and Products

To solve this problem, we'll review each chemical reaction listed and their corresponding products. Our goal is to determine whether these reactions are correctly matched with the products given in List II.

Step 2: Analyzing Reaction I

The reaction is RX with AgCN. When organic halides like RX react with AgCN, the major product is often an isocyanide, RNC, due to the ambident nucleophile nature of CN−, where the Ag+ ion facilitates the formation of the isocyanide. Consequently, this pair, RX with AgCN forming RNC, is correctly matched.

Step 3: Analyzing Reaction II

The reaction involves RX with KCN. In this case, KCN provides the cyanide ion, CN−, which is a strong nucleophile that tends to attack the carbon atom, resulting in the formation of a nitrile, RCN. Therefore, this reaction is correctly matched, as RX with KCN indeed forms RCN.

Step 4: Analyzing Reaction III

The reaction specifies RX with KNO2. Typically, when RX reacts with KNO2, nitrite ions act as ambident nucleophiles that can attach either through the oxygen or nitrogen, often resulting in nitro-compounds. However, the structure given (R-N(O)-O) is inconsistent with typical nitro- or nitrito- structures. Therefore, this is not correctly matched.

Step 5: Analyzing Reaction IV

This reaction involves RX with AgNO2. In contrast to KNO2, AgNO2 tends to favor the formation of nitrito isomers (R-O-N=O) due to the weaker ionic nature of silver. Therefore, RX with AgNO2 forming R-O-N=O is correctly matched.

Step 6: Selecting the Correctly Matched Pairs

With the analysis completed: Reaction I is correctly matched with RNC, Reaction II is correctly matched with RCN, Reaction III is not correctly matched, and Reaction IV is correctly matched as R-O-N=O. Hence, the correctly matched pairs are I, II, and IV, making option (a) I and II or (d) I, II, III, and IV potentially correct. However, since III is not matched correctly, the correct choice is I and II.

Key Concepts

Nucleophile

In organic chemistry, a nucleophile is essentially a **chemical species that donates an electron pair to form a chemical bond**.
Imagine it as a chemical that is always looking for a friend to share its electrons with. Nucleophiles are rich in electrons and typically have either a negative charge or lone pairs of electrons.Nucleophiles are essential players in many organic reactions, where they attack electron-poor centers on other molecules, most commonly carbon atoms.
Some common features of nucleophiles include:

• A negative charge or neutral molecules with lone pairs, like ammonia (NH₃).
• They are often rich in electron density, ready to engage in bonding.
• They participate in reactions by targeting electron-deficient atoms, often marked by a positive charge or partial positive charge.

The power of a nucleophile, known as **nucleophilicity**, can be influenced by factors like the type of atom donating the electrons, the solvent, and the temperature. For example, a cyanide ion (\[\text{CN}^{-}\]) can act as a nucleophile in various reactions because it is willing to share its electrons with other carbon atoms. The ambident nature of CN- allows it to be both a strong nucleophile and readily attacking different carbon atoms within organic halides.

Organic Halides

**Organic halides** are simply organic compounds in which one or more hydrogen atoms have been replaced by halogen atoms like chlorine, bromine, or iodine.
These compounds are important in studying and executing various chemical reactions because they react in predictable ways due to the presence of the halide.The real charm of organic halides lies in their reactivity.
Here are a few things to remember about them:

• They often serve as substrates in many nucleophilic substitution reactions.
• The presence of a halogen makes the carbon atom bonded to it slightly positive, making it a **prime target for nucleophiles**.
• They are pivotal intermediates in organic synthesis, meaning they are used to construct more complex molecules.

Consider \(\text{RX}\), where *R* is a carbon-containing group and *X* is a halogen. When encountered by nucleophiles, these organic halides easily undergo substitution reactions, whereby the nucleophile takes the place of the halogen. It's like swapping dance partners at a party, where the nucleophile joins the compound, replacing the halogen.

Reaction Mechanisms

Understanding **reaction mechanisms** is like being given a backstage pass to chemistry world.
It's about knowing precisely how reactants transform into products, step by step, and what are the minute changes taking place in between.A reaction mechanism describes:

• The sequence of elementary steps leading to the overall transformation.
• The fate of electrons as bonds are broken and formed.
• The transitional states and intermediates that form during the process.

In the context of nucleophilic substitutions:- **SN1 and SN2** are the prime types of reaction mechanisms to understand.- **SN2 reactions** involve a single concerted step where the nucleophile attacks the substrate from the opposite side of the leaving group concurrently with the bond formation. \[\text{RX} + \text{Nucleophile} \rightarrow \text{Product}\]- **SN1 reactions** unfold in two steps where the first is often the slow rate-limiting step with the departure of the leaving group, leading to a carbocation. Subsequently, the nucleophile swoops in to form the final product.Recognizing how different factors, like the type of nucleophile and the nature of the substrate (i.e., organic halide involved), influence these mechanisms is crucial for predicting the product correctly. Think of these mechanisms as dance steps, where the tight choreography determines the final position of each molecule in the reaction.