Problem 15 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The compound which has one isopropyl group is : (a) \(2,2,3,3\)-tetramethylpentane (b) 2,2 -dimethylpentane (c) \(2,2,3\)-trimethylpentane (d) 2-methylpentane

Step-by-Step Solution

Answer

Option (c) - 2,2,3-trimethylpentane contains one isopropyl group.

Step 1: Understand the Structure of an Isopropyl Group

An isopropyl group consists of a propane molecule that has one hydrogen atom removed, represented as \(-C_3H_7\). The structure looks like a Y shape, with the central carbon bonded to two other carbon atoms.

Step 2: Analyze Option (a) - 2,2,3,3-tetramethylpentane

The name indicates a pentane chain with four methyl groups attached at the 2nd and 3rd positions. This creates many branched groups, but no single isopropyl group is formed. Instead, the branching here is symmetrical with identical groups.

Step 3: Analyze Option (b) - 2,2-dimethylpentane

This compound has a pentane main chain with two methyl groups attached to the 2nd carbon atom. While this causes branching, it doesn't form an isopropyl group (a three-carbon branch). The branching here is symmetrical and doesn't resemble a Y shape.

Step 4: Analyze Option (c) - 2,2,3-trimethylpentane

This compound has a pentane chain with three methyl groups: two on the second carbon and one on the third carbon in the main chain. The two methyl groups on the 2nd carbon create an isopropyl group with the third carbon in the main chain. Thus, this compound contains one isopropyl group.

Step 5: Analyze Option (d) - 2-methylpentane

This compound has a simple addition of one methyl group to a pentane chain, positioned at the 2nd carbon. While it creates a branch, it doesn't form an isopropyl group, as only one extra carbon branch is attached, failing to form the necessary Y shape of an isopropyl group.

Key Concepts

Isopropyl Group

The isopropyl group is a common structural feature found in organic compounds. It's derived from propane, a three-carbon alkane, by the removal of one hydrogen atom, resulting in the molecular formula \(-C_3H_7\).

Its unique structure closely resembles a Y shape, where the central carbon atom is linked to two other carbon atoms.

• This setup makes the isopropyl group a distinctive type of hydrocarbon branch.
• It is known for adding complexity and diversity to molecular structures.

To identify an isopropyl group in a compound, look for this specific configuration in the molecule's branching.
Consider analyzing the Y-shaped structure, which helps you quickly recognize an isopropyl group on an organic molecule.

Molecular Structure

In organic chemistry, understanding the molecular structure is crucial for identifying different functional groups, including isopropyl groups.

Compounds can have linear or branched structures, which influence their physical and chemical properties.

• Molecular structures are defined by how atoms are bonded together.
• The naming convention in organic chemistry often hints at the structure.

For example, the compound name provides clues about its carbon backbone and substituents. For instance, the name "2,2,3-trimethylpentane" reveals a pentane chain with methyl groups on the 2nd and 3rd carbon atoms. Analyzing the name helps predict the molecule’s structure.

Hydrocarbon Branching

Hydrocarbon branching occurs when carbon chains in a molecule are not strictly linear, introducing branches with additional carbon groups.

This branching affects a compound's properties, such as boiling and melting points.

• Isopropyl is an example of a branched group, as it attaches by branching off from the main carbon chain.
• Branches can significantly alter a molecule's chemical behavior, often making them more complex.

Analyzing named compounds can provide information on how branching occurs. For instance, in "2,2,3-trimethylpentane," two methyls on one carbon and another on an adjacent carbon create branching, forming isopropyl. This distinct branching pattern leads to the formation of an isopropyl group in this example.