Question

On monochlorination of \(\mathrm{n}\) -pentane, the number of isomers formed is are (a) 4 (b) 3 (c) 2 (d) 1

Short answer

The number of isomers formed is (b) 3.

Step-by-step solution

Understanding the Chlorination Process

When n-pentane undergoes monochlorination, hydrogen atoms can be replaced by chlorine to form different products. The key to solving the problem is identifying how many unique positions chlorine can be attached to on n-pentane.

Identifying Unique Hydrogen Positions

n-Pentane is a straight-chain alkane with the molecular formula \\(\text{C}_5\text{H}_{12}\). The carbon chain can be represented as \(\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3\). There are three types of hydrogen on these carbon positions: those on the terminal carbons (1°), those on the second and fourth carbons (2°), and those on the third carbon (3°). Each type can potentially form different isomers when chlorinated.

Counting Chlorinated Isomers

Chlorinating at the terminal carbon positions (1°) forms one type of isomer, the isomer at the second or fourth carbon (2°) forms another, and chlorinating at the middle carbon (3°) forms a third type of isomer. Thus, there are three unique monochlorination products possible.

Key concepts

n-Pentane

n-Pentane is an organic compound from the alkane family, which is the simplest group of hydrocarbons consisting only of carbon and hydrogen atoms. With the molecular formula \(\text{C}_5\text{H}_{12}\), n-Pentane is a straight-chain alkane that features five carbon atoms connected by single bonds, forming a linear structure. Such a linear or unbranched arrangement distinguishes n-Pentane from its isomers, which have the same molecular formula but different structural arrangements.
An n-Pentane molecule is represented as \(\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3\) showing its chain-like format. Each vertex or carbon atom in this chain has enough hydrogen atoms attached to satisfy the bonding arrangement of carbon, which typically forms four bonds.

• Terminal Carbons (Primary, 1°): These are the first and fifth carbon atoms to which three hydrogen atoms are attached.
• Internal Carbons (Secondary, 2°): The second and fourth carbon atoms, each bonded to two hydrogens and two other carbons.
• Central Carbon (Tertiary, 3°): The middle or third carbon attached to only one hydrogen.

Understanding the positions of hydrogen atoms in n-Pentane is crucial when considering chemical reactions like chlorination, where hydrogen atoms are replaced by chlorine atoms.

Isomer Formation

Isomer formation is an intriguing concept in organic chemistry. It refers to molecules with the same formula but different structures or spatial arrangements. In the context of monochlorination of n-Pentane, the reaction leads to the formation of different isomers based on where the chlorine atom attaches to the carbon chain.
During chlorination:

• The substitution can happen at three distinct types of hydrogen positions in n-Pentane.
• Chlorination at terminal positions (1°) results in a unique isomer, different from those formed at secondary (2°) and tertiary (3°) positions.

This reaction process creates three unique isomers because each chlorinated compound has a distinct attachment point for the chlorine atom, altering the molecular structure subtly but significantly enough to constitute different compounds, even though they all share the same molecular formula. Understanding isomerism is crucial because isomers can exhibit different chemical and physical properties.

Organic Chemistry

Organic chemistry is the branch of chemistry that focuses on the study of carbon-based compounds. This field explores a vast multitude of substances from simple hydrocarbons like alkanes to more complex, diverse organic molecules.
When delving into the monochlorination of n-Pentane, we focus on a specific type of chemical reaction under organic reactions where halogen atoms like chlorine replace hydrogen atoms in hydrocarbons. This process is known as substitution and it's a common practice in organic synthesis.
Why is this important?

• Such reactions help modify molecules to tailor properties suitable for specific applications, such as creating intermediates for further chemical reactions in synthetic chemistry.
• They are fundamental in understanding reactivity patterns, mechanisms, and structural variations of organic molecules.

Overall, organic chemistry serves as the foundation for developing a multitude of products essential in everyday life, from fuels to pharmaceuticals, showcasing the versatility and importance of carbon compound transformations.