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

Question

The reaction \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Br} \stackrel{\mathrm{H}, \mathrm{O}}{\longrightarrow}\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{OH}\) is \([2003 \mid\) (a) addition reaction (b) substitution reaction (c) free radical reaction (d) elimination reaction

Step-by-Step Solution

Answer

It's a substitution reaction.

Step 1: Understand Reaction Participants

In this reaction, we observe that tert-butyl bromide \((\mathrm{(CH}_3)_3\mathrm{C-Br})\) reacts with water \((\mathrm{H}_2\mathrm{O})\) to produce tert-butyl alcohol \((\mathrm{(CH}_3)_3\mathrm{C-OH})\). This transformation involves changing the bromine atom (-Br) to a hydroxyl group (-OH).

Step 2: Classify the Reaction Type

A substitution reaction is one where an atom or a group of atoms in a molecule is replaced by another atom or group. Here, the -Br group in tert-butyl bromide is replaced by an -OH group, which fits the definition of a substitution reaction.

Step 3: Eliminate Incorrect Options

- **Addition Reaction**: Involves adding atoms to a molecule without removing anything. Not applicable here as -Br is replaced. - **Free Radical Reaction**: Involves free radicals, which are not seen in the given reaction. - **Elimination Reaction**: Involves removing elements without replacing, which is not the case here. Therefore, option (b) is the correct answer.

Key Concepts

Tert-butyl bromide

Tert-butyl bromide is an organic compound that plays an important role in substitution reactions. Its chemical structure is \((\mathrm{(CH}_3)_3\mathrm{C-Br})\), where a carbon atom is bonded to three methyl groups and a bromine atom. This specific arrangement makes tert-butyl bromide a primary candidate for nucleophilic substitution reactions due to the polarized C-Br bond.
The bromine atom, being more electronegative, pulls the electron density towards itself, creating a partial positive charge on the carbon atom. This makes the carbon atom susceptible to attack by nucleophiles, which are electron-rich species. In the context of the given reaction, water acts as the nucleophile, replacing the bromine atom.

Tertiary alcohol

A tertiary alcohol, like tert-butyl alcohol \((\mathrm{(CH}_3)_3\mathrm{C-OH})\), is a type of alcohol where the hydroxyl group (-OH) is attached to a carbon atom that is connected to three other carbon atoms.
The formation of a tertiary alcohol through a substitution reaction is significant because it typically occurs under specific conditions where the carbon cage provides steric hindrance. This hinders direct nucleophilic attack, making the nucleophile approach the carbocation intermediate instead of directly attacking the substrate, a hallmark of S\(N\)1 reactions.
Understanding tertiary alcohols is important for recognizing how substitution reactions can modify molecules to create valuable organic compounds.

Reaction classification

Classifying reactions in organic chemistry is essential for understanding how various transformations occur.
In the context of the provided exercise, the reaction is classified as a substitution reaction. This is because one group of atoms (bromine) is replaced by another group (hydroxyl) within the same molecular framework. Substitution reactions differ from other reaction types, such as addition, elimination, or free radical reactions, as they involve the replacement rather than the addition or removal of atoms.
Knowing how to correctly classify reactions allows chemists to predict reaction outcomes and optimize conditions for desired transformations.

Organic chemistry

Organic chemistry is the study of carbon-containing compounds, fundamental to understanding the behavior and transformation of various molecules in this field. Its principles are applied to decipher how complex reactions occur and how different compounds interact.

• This discipline involves a vast array of reactions, including substitution reactions, which are pivotal in forming new compounds.
• Organic chemistry also helps identify reaction mechanisms, predict product formation, and involves the study of reaction energetics to increase efficiency.

Learning organic chemistry is key for students wanting to excel in fields like pharmaceuticals, environmental science, and chemical engineering.

Reaction mechanism

A reaction mechanism presents the step-by-step process by which a reaction occurs. It illustrates how bonds are broken and formed, and what intermediates are generated.
In the example of tert-butyl bromide reacting with water, the mechanism is typically an S\(N\)1 process. This involves:

• Formation of a carbocation intermediate where the C-Br bond breaks, with bromine leaving as a bromide ion.
• Water, the nucleophile, then attacks this positively charged carbocation.
• Formation of a stable tert-butyl alcohol as the final product occurs after the water molecule attaches, forming \(\mathrm{(CH}_3)_3\mathrm{C-OH})\).

Understanding reaction mechanisms is crucial as it helps us predict the behavior of different compounds, enabling us to tailor reactions for specific outcomes.