Problem 14 - Chemistry and Chemical Reactivity

Question

Isooctane, 2,2,4 -trimethylpentane, is one of the possible structural isomers with the formula \(\mathrm{C}_{8} \mathrm{H}_{18} .\) Draw the structure of this isomer, and draw and name structures of two other isomers of \(\mathrm{C}_{8} \mathrm{H}_{18}\) in which the longest carbon chain is five atoms.

Step-by-Step Solution

Answer

Isooctane and its two isomers are 2,3-dimethylhexane and 3-ethyl-2-methylpentane.

Step 1: Drawing Isooctane Structure

Isooctane, also known as 2,2,4-trimethylpentane, has the structural formula where you start with a five-carbon (pentane) backbone. To this chain, add three methyl groups (\(\text{CH}_3\)) at the 2, 2, and 4 positions along the chain. The structure looks like this: ``` CH3 |CH3-C-CH2-CH-CH3 | | CH3 CH3```This is the structure of isooctane.

Step 2: Drawing First Isomer - 2,3-Dimethylhexane

For the first isomer, maintain a five-carbon main chain, but reposition the methyl groups to the 2nd and 3rd carbons. The structure is: ``` CH3 | CH3-C-CH-CH2-CH2-CH3 | CH3 ``` This is called 2,3-dimethylhexane.

Step 3: Drawing Second Isomer - 3-Ethyl-2-methylpentane

For the second isomer, keep the longest chain with 5 carbons and place an ethyl group (\(\text{CH}_2\text{CH}_3\)) on the 3rd carbon, and a methyl group on the 2nd carbon. The structure is: ``` CH3 |CH3-C-CH2-CH-CH2-CH3 | CH2-CH3```This is 3-ethyl-2-methylpentane.

Key Concepts

isooctane

Isooctane is an important compound in organic chemistry, notable for being one of the several structural isomers of octane with the molecular formula \(\mathrm{C}_{8} \mathrm{H}_{18}\). Isooctane, scientifically known as 2,2,4-trimethylpentane, is widely used as a reference standard in octane rating for fuels.

The structure of isooctane consists of a chain of five carbon atoms, also called a pentane backbone.

• To form isooctane, three methyl groups \((\text{CH}_3)\) are attached at carbons 2, 2, and 4 of the pentane chain.
• The positioning of these groups adds complexity to the structure, allowing it to be distinguished from its isomers.

This particular arrangement of carbon and hydrogen not only defines the chemical personality of isooctane but also its physical properties, such as low density and high volatility.

2,3-dimethylhexane

2,3-Dimethylhexane is another intriguing structural isomer of octane. It features a rearranged set of carbon atoms compared to isooctane, yet it still retains the \(\mathrm{C}_{8} \mathrm{H}_{18}\) formula.

This compound involves shifting the substitute groups to create a different isomer:

• The structure maintains a backbone of five carbon atoms, similar to isooctane.
• However, the methyl groups \((\text{CH}_3)\) attach themselves to the 2nd and 3rd carbon atoms of the main chain, known as hexane.

By changing the positions of the methyl groups, we create a completely different compound with unique properties, even though its molecular formula doesn't change.Isomers like 2,3-dimethylhexane showcase the diversity of organic molecules that can exist with the same number of atoms but in different arrangements.

3-ethyl-2-methylpentane

3-Ethyl-2-methylpentane is yet another captivating structural isomer of the formula \(\mathrm{C}_{8} \mathrm{H}_{18}\). Each isomer presents its own structure and potential uses, making them important in industries and research.

This specific isomer features distinctive branching in its chain:

• The main chain consists of a pentane base, with five carbon atoms as the backbone.
• An ethyl group \((\text{CH}_2\text{CH}_3)\) is attached to the 3rd carbon in the chain.
• A methyl group \((\text{CH}_3)\) is added to the 2nd carbon, creating a unique structure.

This arrangement of substituents affects the compound's properties and its chemical behavior. Each isomer of \(\mathrm{C}_{8} \mathrm{H}_{18}\) plays a role in various chemical processes and industries by offering distinct characteristics while maintaining the same foundational formula.