Problem 65 - Chemistry The Molecular Science

Question

Draw structural formulas for the five constitutional isomers of \(\mathrm{C}_{6} \mathrm{H}_{14}\)

Step-by-Step Solution

Answer

The five isomers are n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane.

Step 1: Understand the Concept of Constitutional Isomers

Constitutional isomers are compounds that have the same molecular formula but different connectivity of their atoms. For the molecular formula \(\mathrm{C}_{6}\mathrm{H}_{14}\), we need to rearrange the six carbon atoms and fourteen hydrogen atoms into different structures.

Step 2: Draw the Longest Carbon Chain (n-Hexane)

Start with a straight chain of six carbon atoms, which is known as n-hexane. The carbon atoms are connected in a single unbranched line, so the structure is \(\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}_3\).

Step 3: Create a 5-Carbon Chain with One Methyl Branch (2-Methylpentane)

Reduce the main chain to five carbon atoms and add a methyl (\(\mathrm{CH}_3\)) group to the second carbon. This gives us 2-methylpentane with the structure: \(\mathrm{CH}_3-\mathrm{CH}(-\mathrm{CH}_3)-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}_3\).

Step 4: Create a 5-Carbon Chain with a Methyl Branch in Another Position (3-Methylpentane)

Keep the main chain of five carbons and move the methyl group to the third carbon. This structure is 3-methylpentane: \(\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}(-\mathrm{CH}_3)-\mathrm{CH}_2-\mathrm{CH}_3\).

Step 5: Create a 5-Carbon Chain with the Methyl Group at the 2-Position from the Opposite End (2,3-Dimethylbutane)

Convert the longest chain to four carbons and place methyl groups on the second and third carbons. Structure: \(\mathrm{CH}_3-\mathrm{CH}(-\mathrm{CH}_3)-\mathrm{CH}(-\mathrm{CH}_3)-\mathrm{CH}_3\).

Step 6: Create a Tertiary Structure with Three Methyl Branches (2,2-Dimethylbutane)

Construct the chain as four carbons with two methyl groups attached to the second carbon, creating 2,2-dimethylbutane: \(\mathrm{CH}_3-\mathrm{C}(-\mathrm{CH}_3)_2-\mathrm{CH}_2-\mathrm{CH}_3\).

Key Concepts

Structural Formulas

Structural formulas are critical in understanding the arrangement of atoms in a molecule. Unlike molecular formulas, which simply count atoms, structural formulas show how these atoms are connected.
They map out the bonds in a molecule, showing the precise arrangement of each carbon and hydrogen in hydrocarbons like alkanes. This is especially important when studying constitutional isomers, which have the same molecular formula but different structural formulas.
Understanding structural formulas helps us see at a glance why substances with the same molecular formula might behave differently chemically and physically. When creating these formulas, it's important to:

• Identify the longest chain of carbon atoms, which serves as the molecule's backbone.
• Place any additional atoms or groups along the chain to create different isomers.
• Acknowledge single, double, or triple lines between atoms that indicate the bonding type.

This visual representation ensures that we can distinguish the distinct arrangements seen in isomers.

Molecular Formula

A molecular formula is a simple way of representing the number of each type of atom in a molecule. For example, the molecular formula for hexane is \(\mathrm{C}_6\mathrm{H}_{14}\), showing it is composed of 6 carbon atoms and 14 hydrogen atoms.
Despite sharing the same molecular formula, molecules can be dramatically different in how these atoms are arranged. Understanding the molecular formula provides a quick insight into the elemental composition but falls short of showing connectivity or arrangement.
This is why the molecular formula is the starting point for identifying potential isomers. Once we know the types and numbers of atoms, we use this information to derive the various structural formulas. It is the exploration of rearrangements at the structural level that uncovers the diversity and uniqueness of each isomer.

Carbon Chain

The carbon chain is a fundamental concept in organic chemistry, especially when discussing alkanes such as hexane. It refers to a sequence of carbon atoms bonded together, forming the core structure of a molecule.
In hydrocarbons, the carbon chain can be straight or branched, leading to the existence of several isomers. The length and branching pattern impact the molecule's properties, such as boiling point and reactivity.

• A straight chain is one continuous sequence of carbon atoms without side branches.
• A branched chain includes side chains or alkyl groups, leading to different isomeric structures.
• The position of branches or functional groups along the chain can significantly alter the molecule's characteristics.

Understanding carbon chains helps in visualizing and predicting the behavior of different hydrocarbons. It's crucial in identifying different constitutional isomers which differ explicitly in these chains' structure or branching.