Question

Draw the structural formulas for at least five isomeric alkanes having the formula \(\mathrm{C}_{8} \mathrm{H}_{18}\).

Short answer

Five isomeric alkanes of C8H18 are: 1. \(n\)-octane: \(\mathrm{CH_3 - CH_2 - CH_2 - CH_2 - CH_2 - CH_2 - CH_2 - CH_3}\) 2. 2-methylheptane: \(\mathrm{CH_3 - CH(CH_3) - CH_2 - CH_2 - CH_2 - CH_2 - CH_3}\) 3. 3-methylheptane: \(\mathrm{CH_3 - CH_2 - CH(CH_3) - CH_2 - CH_2 - CH_2 - CH_3}\) 4. 2,2-dimethylhexane: \(\mathrm{CH_3 - C(CH_3)_2 - CH_2 - CH_2 - CH_2 - CH_3}\) 5. 2,3-dimethylhexane: \(\mathrm{CH_3 - CH(CH_3) - CH(CH_3) - CH_2 - CH_2 - CH_3}\)

Step-by-step solution

1. Simple straight-chain:\(n\)-octane

Starting with the simplest alkane, draw a straight chain of 8 carbon atoms with all hydrogens satisfied. This represents \(n\)-octane. \[ \mathrm{CH_3 - CH_2 - CH_2 - CH_2}\mathrm{-} CH_2\mathrm{-} CH_2\mathrm{-} CH_2\mathrm{-} CH_3 \]

2. 2-methylheptane

The next isomer will have a 7-carbon-long chain and one branch. Start with a 7-carbon-long chain and add a methyl (CH3) group to the second carbon. This creates 2-methylheptane. \[ \mathrm{CH_3 - CH(CH_3) - CH_2 - CH_2}\mathrm{-} CH_2\mathrm{-} CH_2\mathrm{-} CH_3 \]

3. 3-methylheptane

Now create another isomer with a 7-carbon-long chain but with the methyl group at the third position. This creates 3-methylheptane. \[ \mathrm{CH_3 - CH_2 - CH(CH_3) - CH_2}\mathrm{-} CH_2\mathrm{-} CH_2\mathrm{-} CH_3 \]

4. 2,2-dimethylhexane

Now create an isomer with a 6-carbon-long chain and two branches at the second carbon. This creates 2,2-dimethylhexane. \[ \mathrm{CH_3 - C(CH_3)_2 - CH_2 - CH_2}\mathrm{-} CH_2\mathrm{-} CH_3 \]

5. 2,3-dimethylhexane

Finally, create another isomer with a 6-carbon-long chain. But this time, place the branches at the second and third carbons. This creates 2,3-dimethylhexane. \[ \mathrm{CH_3 - CH(CH_3) - CH(CH_3) - CH_2}\mathrm{-} CH_2\mathrm{-} CH_3 \] These are five isomeric alkanes with a molecular formula of C8H18: \(n\)-octane, 2-methylheptane, 3-methylheptane, 2,2-dimethylhexane, and 2,3-dimethylhexane.

Key concepts

Structural Formulas

Structural formulas are a detailed way to represent molecules. They show which atoms are bonded to which, and sometimes how they are arranged in space. This is particularly important for hydrocarbons like alkanes since they can have multiple isomers. Isomers have the same molecular formula but different structural formulas.

• Atoms and Bonds: Each carbon atom can form four single covalent bonds with other atoms, while hydrogen atoms form just one bond.
• Chains and Branches: Alkanes can be straight chains or contain branches, leading to different isomers.
• Variations: The structural formula can show variations like the position of branches or functional groups.

Understanding structural formulas is key when exploring isomeric alkanes like those with the molecular formula \( \mathrm{C}_{8} \mathrm{H}_{18} \). It also helps decipher complex organic molecules and their properties.

n-Octane

n-Octane is the simplest isomer of \( \mathrm{C}_{8} \mathrm{H}_{18} \), as it features a straight-chain organization. In n-octane, all eight carbon atoms are linked in a straight line with hydrogens completing the structure. This arrangement maximizes symmetry and straightforwardness.

• Straight Chain: The carbon atoms in n-octane connect in a linear sequence.
• Maximum Length: n-Octane represents the highest possible straight-chain length for \( \mathrm{C}_{8} \mathrm{H}_{18} \).

Due to the lack of branching, n-octane often serves as a basic reference for comparing other isomers. The straight-chain contributes to a higher boiling point relative to its branched isomers.

Methylheptane Isomers

Methylheptane isomers are versions of \( \mathrm{C}_{8} \mathrm{H}_{18} \) where a 7-carbon chain is altered by adding a single methyl group. - **Positions and Naming:** - In 2-methylheptane, the methyl group is attached to the second carbon, creating a slightly branched structure. - In 3-methylheptane, the methyl group moves to the third carbon, maintaining the seven-carbon backbone but changing the overall shape.

• Structural Impact: The position of the methyl group affects the isomer’s shape and properties greatly.
• Boiling Points: More branching typically leads to lower boiling points compared to n-octane.

These isomers display how small changes in structure can result in various physical and chemical attributes. Each has distinct implications in fields like fuel, where differing properties may be advantageous.

Dimethylhexane Isomers

Dimethylhexane isomers incorporate two branches into a 6-carbon chain, both characterized by where these branches are positioned. - **Different Configurations:** - 2,2-dimethylhexane: Features two methyl groups attached to the second carbon, creating a symmetric branch. - 2,3-dimethylhexane: Offers a slightly different scenario with one methyl on the second and another on the third carbon.

• Branching Complexity: The number and location of branches affect the strength and type of intermolecular interactions.
• Physical Properties: Variability in structure influences boiling and melting points differently compared to less branched molecules.

The concept of dimethylhexane highlights how even a small molecular change, like adding a methyl group, can lead to diverse chemical behaviors. This is crucial for applications requiring tailored chemical characteristics.