Problem 5 - Chemistry and Chemical Reactivity

Question

One of the structural isomers with the formula \(\mathrm{C}_{9} \mathrm{H}_{20}\) 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 five-carbon chain.

Step-by-Step Solution

Answer

Draw 3-ethyl-2-methylhexane; another isomer could be 2,2-diethylpentane.

Step 1: Identify the Longest Carbon Chain

The name 3-ethyl-2-methylhexane tells us that the parent chain is hexane, consisting of six carbon atoms. The longest chain must have six carbon atoms.

Step 2: Add Substituents to the Parent Chain

According to the name, there is an ethyl group on the third carbon and a methyl group on the second carbon of the hexane chain. Position these substituents correctly along the hexane backbone to match the structure described by the name.

Step 3: Complete the Structure of 3-ethyl-2-methylhexane

Ensure all carbons have four bonds (consider hydrogen atoms to fulfill this requirement). The structure should be completed by attaching enough hydrogen atoms to satisfy this requirement. Your drawing should display a hexane chain with an ethyl group on the third carbon and a methyl group on the second carbon.

Step 4: Identify a Different Longest Carbon Chain for a New Isomer

For a structural isomer with five carbons as the longest chain, choose the base chain to be pentane (i.e., five carbon atoms in a straight line).

Step 5: Place the Remaining Carbons as Substituents

From the formula C₉H₂₀, 9 carbons are needed in total. Since 5 are in the main chain, the remaining 4 must be placed as substituents. You could place them as two ethyl groups or combinations of methyl and longer chains like propyl.

Step 6: Draw and Name One New Isomer

For instance, place two ethyl groups at carbon 2, resulting in the structure: 2,2-diethylpentane. This completes the structure and naming of a new isomer.

Key Concepts

Structural Isomers

Structural isomers are fascinating because they share the same molecular formula but exhibit different connectivity among the atoms. This means that while they contain the same number of each type of atom, the way these atoms are arranged differs in each isomer, leading to unique chemical and physical properties. For example, consider two structural isomers of the molecular formula \( \text{C}_9\text{H}_{20} \): 3-ethyl-2-methylhexane and 2,2-diethylpentane. Both contain nine carbon atoms and twenty hydrogen atoms, yet their structures and names vary due to the different configurations of their carbon backbones and substituents.

To identify structural isomers, follow these steps:

• Determine the longest carbon chain in the molecule, which serves as the parent hydrocarbon.
• Identify and arrange substituents (like methyl or ethyl groups) along the carbon backbone in different configurations that maintain the total number of carbons and hydrogens given by the molecular formula.

Recognizing structural isomerism is crucial for understanding the diversity of organic molecules and their specific interactions.

Chemical Nomenclature

Chemical nomenclature in organic chemistry is the systematic way we name structures based on international naming conventions, primarily set by IUPAC. It ensures each compound has a distinct and universally understood name. The name 3-ethyl-2-methylhexane reveals quite a bit about its structure:

• The base name 'hexane' indicates the longest continuous chain of carbon atoms, which is six in this case.
• The prefixes '3-ethyl' and '2-methyl' specify the types of substituents attached to the parent chain and their positions.

It's important to start numbering the carbon chain from the end nearest to any of the substituents to give them the lowest possible numbers. Understanding and utilizing chemical nomenclature correctly allows chemists to communicate complex molecular structures efficiently and effectively. This system demystifies molecules like isomers of \( \text{C}_9\text{H}_{20} \) without even having to draw them.

Hydrocarbons

Hydrocarbons are organic compounds consisting solely of carbon and hydrogen atoms. They are the foundational blocks of organic chemistry. Their vast structural variety is due to the different ways these atoms can bind together, forming chains or rings. The most basic groups of hydrocarbons are alkanes, alkenes, and alkynes, characterized by different types of carbon-carbon bonds.

Alkanes, like those in the formula \( \text{C}_9\text{H}_{20} \), are saturated hydrocarbons containing only single bonds. This gives them a maximum possible number of hydrogen atoms bonded to carbon, classifying them as the simplest form of hydrocarbons.

Characteristics of hydrocarbons include:

• Non-polarity, leading to low solubility in water.
• Varying boiling and melting points based on chain length and structure.

In the context of structural isomers such as 3-ethyl-2-methylhexane, the hydrocarbons can take many shapes, contributing distinctly to their interaction in chemical reactions and properties.