Question

One of the structural isomers with the formula \(\mathrm{C}_{3} \mathrm{H}_{20}\) has the name 3 -ethyl- 2 -methylhexane. Draw its structure. Draw and name another structural isomer of \(\mathrm{C}_{9} \mathrm{H}_{20}\) in which there is a fivecarbon chain.

Short answer

The isomer with a five-carbon chain is 2,4-dimethyl-3-ethylpentane.

Step-by-step solution

Understand the Given Structure

The given structure is 3-ethyl-2-methylhexane. This name indicates that the main chain has six carbon atoms (hexane). An ethyl group is attached to the third carbon, and a methyl group is attached to the second carbon.

Draw 3-ethyl-2-methylhexane

Start by drawing the main hexane chain: C-C-C-C-C-C. Next, add a methyl group (CH₃) attached to the second carbon and an ethyl group (C₂H₅) attached to the third carbon. The structure should look like this: CH₃-CH(CH₃)-CH(C₂H₅)-CH₂-CH₂-CH₃.

Conceptualize Isomers for C₉H₂₀

Isomers have the same molecular formula but different arrangements. We need another structure of C₉H₂₀ that includes a five-carbon (pentane) chain.

Determine Substituents on the Pentane Chain

The remaining four carbons from C₉H₂₀ need to be added as substituents. Use possibilities like multiple methyl groups or larger groups. One option is to place two methyl groups on the second and fourth carbon of the main chain.

Draw the Isomer with a Five-carbon Chain and Name It

Draw a pentane chain (C-C-C-C-C) and add two methyl groups on the second carbon and two more methyl groups on the fourth carbon. A valid structure would be: CH₃-C(CH₃)₂-CH₂-CH(CH₃)₂-CH₃. The name for this is 2,4-dimethyl-3-ethylpentane.

Key concepts

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds and their properties. Carbon is a versatile element central to organic chemistry, bonding with elements like hydrogen, oxygen, and nitrogen to form an immense variety of compounds.
The study of organic chemistry helps us understand the composition and synthesis of these compounds. Structural isomers are an important concept within organic chemistry. They are compounds that have the same molecular formula but different structural arrangements. This means that even though isomers have the same number of each atom, their physical arrangement varies, leading to different chemical properties.
In the exercise, we are dealing with structural isomers of the formula \(C_9H_{20}\). By drawing and naming different structures, we grasp how structural variations affect molecular identity in organic chemistry.

Molecular Formula

The molecular formula represents the number and type of atoms in a molecule. In the exercise, \(C_9H_{20}\) is the molecular formula under consideration. This formula tells us that the compound consists of 9 carbon atoms and 20 hydrogen atoms.
Molecular formulas are crucial as they provide basic information about a compound's composition without specifying its structure. This is where structural isomers come into play, allowing the same molecular formula to form different structures.

• Understanding the molecular formula is the first step in identifying possible isomers.
• The challenge lies in rearranging these atoms to create diverse compounds.

When you familiarize yourself with the molecular formula, you start with essential information that guides structural determination in more complex scenarios.

Carbon Chains

In organic chemistry, carbon chains form the backbone of most organic compounds. These chains can vary from straight to branched configurations. The length and branching of carbon chains play a crucial role in determining the properties and names of organic molecules.
For instance, in the exercise, one isomer has a six-carbon chain (hexane), while another isomer with the same formula could include a five-carbon chain (pentane). The rest of the carbon atoms are adjusted as substituents like methyl or ethyl groups attached at different points in the chain.

• Straighter chains reduce complexity, while branching increases it and may alter the molecule's stability and reaction behavior.
• Carbon chains are named based on the number of carbon atoms they contain and their linear or branched nature.

Understanding carbon chains is fundamental for analyzing and synthesizing different structural isomers.

Isomer Naming

Naming isomers accurately is an essential skill in organic chemistry. It involves a systematic method known as IUPAC naming. This method specifies the precise structure of a molecule through a name.
Naming starts with identifying the main carbon chain, counting the carbon atoms in the longest continuous sequence, and naming it accordingly.
The position and nature of any substituents (such as methyl or ethyl groups) are then mentioned, using numbers to show where they attach to the main chain. For example, in the exercise, the name 2,4-dimethyl-3-ethylpentane indicates:

• The main chain is a five-carbon pentane.
• A methyl group is on the second and fourth carbons and an ethyl group on the third carbon.

Mastering the naming of isomers not only allows correct communication but also facilitates understanding the compound's structure purely from its name.