Problem 210 - A Complete Resource Book in Chemistry for JEE Main

Question

Which branched chain isomer of the hydrocarbon with molecular mass 72 u gives only one isomer of mono substituted alkyl halide? (a) Isohexane (b) Neopentane (c) Tertiary butyl chloride (d) Neohexane

Step-by-Step Solution

Answer

(b) Neopentane

Step 1: Understand the Concept

A branched-chain isomer of a hydrocarbon can have different isomers of mono-substituted alkyl halides. The question asks for the one that gives only one such isomer.

Step 2: Clarify the Problem

The molecular mass of the hydrocarbon is 72 u. We need to identify which given isomer will form only one isomer when substitution happens to create an alkyl halide.

Step 3: Calculate Molecular Formula

Calculate the molecular formula for a hydrocarbon with a molecular mass of 72 u. Since the average mass of a carbon atom is approximately 12 u and hydrogen is about 1 u, a likely formula is C5H12, which matches the molecular mass 72 u of pentane.

Step 4: Evaluate Each Option

Evaluate each given compound: - Isohexane (not C5H12) - Neopentane (option b), which is C5H12 and has the structure (CH3)4C and can give only one product when substituted. - Tertiary butyl chloride is already a substituted compound, not a hydrocarbon. - Neohexane (not C5H12).

Step 5: Determine the Correct Structure

Neopentane is the correct answer as it has a symmetrical structure with all hydrogen atoms equivalent due to rapid rotation, leading to only one mono-substituted product.

Key Concepts

Hydrocarbon Isomers

Isomers in organic chemistry are fascinating because they are compounds with the same molecular formula but a different arrangement of atoms. When we say 'hydrocarbon isomers,' we are specifically talking about molecules composed solely of carbon and hydrogen atoms that can exist in different structural forms.
For example, pentane ( C_5H_{12} ) has three isomers:

• Normal pentane: All carbon atoms are arranged in a single unbranched chain.
• Isopentane: The main chain is altered with a branching point.
• Neopentane: A highly branched structure with a central carbon atom bound to four methyl groups.

Neopentane is unique because it is so symmetrical. All hydrogen atoms are chemically equivalent, making it possible for only one type of mono-substituted alkyl halide to form during a substitution reaction. This is why neopentane fits the criteria given in the problem.

Molecular Mass Calculations

Understanding how to calculate molecular masses is essential for solving problems in organic chemistry. The molecular mass of a compound is the sum of the masses of all atoms in its molecular formula.
Each element has a standard atomic mass. For example, carbon has an approximate atomic mass of 12 unified atomic mass units (u), and hydrogen is about 1 u. To calculate the molecular mass of a hydrocarbon like pentane:

• Pentane (C_5H_{12}) consists of 5 carbon atoms and 12 hydrogen atoms.
• The molecular mass is calculated as: \(5 \times 12 \, \text{u} + 12 \times 1 \, \text{u} = 72 \, \text{u}\)

This approach helps in identifying the structure of the molecule and understanding its chemical behavior. By ensuring that the molecular mass fits the problem, we can be confident when identifying the correct isomer from a list of possibilities.

Substituted Alkyl Halides

Substituted alkyl halides are interesting compounds in organic chemistry, where one or more hydrogens in a hydrocarbon are replaced by halogens (like chlorine, bromine, etc.).
The properties and reaction types of alkyl halides depend significantly on the type and position of the substitution. In solving problems like the one in the exercise, it is crucial to understand that:

• Mono-substitution refers to replacing just one hydrogen atom with a halogen.
• The potential number of substituted products a hydrocarbon can form depends on its symmetry and branching.
  For instance, neopentane can only form one mono-substituted alkyl halide due to its symmetry.
  This is because its structure allows for indistinguishable substitution products, unlike non-symmetrical isomers with variable substitution sites that lead to multiple products.

These principles help predict the outcomes of halogen substitution reactions and contribute to understanding alkyl halide chemistry on a deeper level.