Problem 182 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Br} \stackrel{\mathbf{H}_{2} \mathrm{O}}{\longrightarrow}\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{OH}\) is (a) addition reaction (b) substitution reaction (c) free radical reaction (d) elimination reaction

Step-by-Step Solution

Answer

(b) substitution reaction

Step 1: Identify the Reaction Components

Examine the reactants and the products. We start with \((\mathrm{CH}_{3})_{3}\mathrm{C}-\mathrm{Br}\) and end with \((\mathrm{CH}_{3})_{3}\mathrm{C}-\mathrm{OH}\). The bromine \((\mathrm{Br})\) is replaced with a hydroxyl group \((\mathrm{OH})\).

Step 2: Understand the Nature of the Reaction

Consider the change in the molecular structure: the bromine atom in \((\mathrm{CH}_{3})_{3}\mathrm{C}-\mathrm{Br}\) is replaced by a hydroxide group from water (\(\mathrm{H}_{2}\mathrm{O}\)), which indicates a substitution of functional groups.

Step 3: Classify the Type of Reaction

Since the bromine atom is replaced by an \(\mathrm{OH}\) group, this reaction is classified as a substitution reaction, specifically a nucleophilic substitution reaction, because the \(\mathrm{OH}\) group acts as a nucleophile.

Key Concepts

Organic Chemistry

Organic chemistry is a branch of chemistry that deals with the structure, properties, compositions, reactions, and synthesis of organic compounds containing carbon atoms. The backbone of organic chemistry lies in the ability of carbon atoms to form stable covalent bonds with each other, as well as with other elements such as hydrogen, oxygen, nitrogen, and halogens. This versatility leads to an immense variety of organic molecules with diverse chemical behaviors.

In organic chemistry, reactions often involve the making and breaking of these covalent bonds. Nucleophilic substitution reactions are a key type of reaction where a nucleophile, such as the hydroxide ion ( ext{OH}^-), replaces a leaving group in a molecule. This type of reaction is crucial for building more complex molecules from simpler ones. Understanding the nuanced differences between the various types of nucleophilic substitution reactions can provide insights into molecular design and synthesis in organic chemistry.

One of the fundamental principles in organic chemistry is the study of reaction mechanisms, which allows chemists to understand how and why reactions occur, paving the way for innovations in pharmaceuticals, materials science, and more.

Reaction Mechanisms

Reaction mechanisms detail the step-by-step process by which a chemical change occurs. They give a narrative of how reactants transform into products.

Knowing reaction mechanisms helps us predict the path and rate of a reaction, and control it to achieve desired outcomes. The description of a reaction mechanism includes intermediate stages, the sequence of bond-breaking and bond-forming events, and transition states.

In the context of nucleophilic substitution reactions, the mechanism can either be a single-step or multi-step process. When water replaces the bromine atom in the reaction \(( ext{CH}_3)_3 ext{C-Br} \) to form \(( ext{CH}_3)_3 ext{C-OH} \), the hydroxide group acts as a nucleophile. This replacement defines the nucleophilic substitution reaction. Mechanisms are further categorized into SN1 and SN2, each with distinct characteristics and different pathways through which the nucleophilic attack occurs.

SN1 and SN2 Reactions

SN1 and SN2 represent two general types of nucleophilic substitution reactions, each with unique pathways and conditions.

- **SN1 Reaction (Substitution Nucleophilic Unimolecular):** - Involves two steps. - The first step is the slowest and involves the dissociation of the leaving group, forming a carbocation intermediate. - The second step is fast, where the nucleophile quickly attaches to the positively charged carbocation. - The reaction rate depends solely on the concentration of the substrate. - Often occurs in polar protic solvents and is common in tertiary alkyl halides.
- **SN2 Reaction (Substitution Nucleophilic Bimolecular):** - Occurs in a single, concerted step, where the nucleophile attacks the substrate at the same time the leaving group leaves. - The reaction rate is influenced by both the substrate and the nucleophile concentrations. - Favored in polar aprotic solvents and is more common in primary and secondary alkyl halides.
Understanding these two pathways can help you predict the result of similar reactions and choose appropriate conditions for desired transformations in organic synthesis.