Problem 110 - A Complete Resource Book in Chemistry for JEE Main

Question

A hydrocarbon of molecular formula, \(\mathrm{C}_{\mathrm{s}} \mathrm{H}_{18}\) gives only one monochloro derivative. The hydrocarbon is (a) 2 -methyl heptane (b) octane (c) \(2,2,3,3\)-tetramethyl butane (d) \(2,2,4\)-trimethyl pentane

Step-by-Step Solution

Answer

The hydrocarbon is \(2,2,3,3\)-tetramethyl butane.

Step 1: Understanding the Problem

We are asked to identify a hydrocarbon that forms only one monochloro derivative when it reacts with chlorine. The compound's molecular formula is given as \(\mathrm{C}_{8}\mathrm{H}_{18}\). The choices provided are different structural isomers based on this molecular formula.

Step 2: Defining Monochloro Derivative

A monochloro derivative results when one hydrogen atom in the hydrocarbon is replaced by a chlorine atom. If a compound gives only one such derivative, then all hydrogen atoms must be equivalent for substitution, indicating equivalent carbon environments.

Step 3: Analyzing the Options

Examine each option: (a) 2-methyl heptane: Has different types of hydrogens (primary, secondary, tertiary) leading to multiple chlorination sites.(b) Octane: Contains different carbon environments resulting in multiple derivatives.(c) \(2,2,3,3\)-tetramethyl butane: All hydrogens are equivalent.(d) \(2,2,4\)-trimethyl pentane: Has hydrogens in different positions, leading to multiple derivatives.

Step 4: Identifying the Correct Hydrocarbon

From Step 3, \(2,2,3,3\)-tetramethyl butane is the only structure where all hydrogen atoms are identical due to its symmetry, resulting in only one possible monochloro derivative.

Key Concepts

Hydrocarbon Isomers

Hydrocarbon isomers are different compounds that have the same molecular formula but different arrangements of atoms. This concept is crucial in organic chemistry because it illustrates how the same set of atoms can create substances with vastly different properties.

• Molecular formulae like \(C_nH_{2n+2}\) apply to alkanes, showcasing the diversity among isomers.
• Isomers can be further categorized into structural isomers, where the connectivity of carbon atoms differs, and stereoisomers, which differ only in spatial orientation.

For instance, in the given exercise, all the options are isomers of \(C_8H_{18}\). Each one has the same number of carbon and hydrogen atoms but varies structurally, influencing their chemical behavior, notably in reactions such as chlorination.

Monochloro Derivative

A monochloro derivative in organic chemistry is a compound formed when one hydrogen atom in a hydrocarbon is substituted with a chlorine atom. This change can significantly alter the properties of the original hydrocarbon. Key points about monoclorination:

• It results in a single chlorine atom being added.
• The position where substitution occurs is crucial, as it affects the compound's chemical properties.

In the case of determining which hydrocarbon only forms one monochloro derivative, all hydrogens in this compound must be equivalent. This means each hydrogen is equally likely to be replaced by chlorine, leading to a single substitution pattern.

Structural Isomers

Structural isomers are compounds with the same molecular formula but differing in how their atoms are connected. These variations generate different physical and chemical properties even though they share the same components. To identify structural isomers:

• Examine the arrangement of carbon skeletons.
• Consider how they branch and differ in carbon-hydrogen bonding patterns.
• Understand that each unique configuration can lead to distinct reactions.

In the exercise, options like 2-methyl heptane and octane are structural isomers of \(C_8H_{18}\) but differ in the way carbon and hydrogen are organized, affecting their potential to form monochloro derivatives.

Chemical Symmetry

Chemical symmetry in molecules refers to the distribution of similar or equivalent parts, impacting how the molecule interacts with other substances. When a molecule is highly symmetrical, its chemical behavior can be more predictable, particularly in reactions like substitution.

• Symmetry leads to equivalent environments for atoms, meaning all equivalent hydrogens react the same way.
• In symmetrical structures, substitution reactions result in fewer derivatives due to identical carbon environments.

The exercise highlighted that \(2,2,3,3\)-tetramethyl butane is symmetrical, making it the only compound that forms a single monochloro derivative, as all its hydrogen atoms are identical. This concept underlines the importance of symmetry for simplifying reaction pathways in organic compounds.