Problem 31 - Chemistry: An Atoms-Focused Approach

Question

Draw and name all the constitutional isomers of \(\mathrm{C}_{5} \mathrm{H}_{12}\)

Step-by-Step Solution

Answer

Answer: There are three constitutional isomers for the formula \(\mathrm{C}_{5}\mathrm{H}_{12}\): Pentane, Isopentane (2-methylbutane), and Neopentane (2,2-dimethylpropane).

Step 1: Draw the parent chain for each isomer

First, we need to draw the longest continuous chain of carbon atoms for each isomer. In this case, since we have 5 carbon atoms to work with, we can have chains of 5, 4 or 3 carbon atoms.

Step 2: Add the remaining hydrogen atoms

Once we have the parent chain, we need to add the remaining hydrogen atoms. In order to maintain proper valency, each carbon must have a total of 4 bonds (including bonds to other carbon atoms).

Step 3: Identify the different constitutional isomers

After drawing the different possible structures, we need to identify which ones are actually different isomers. It is important to note that if you can rotate or move a molecule in any way to make it look like another, they are considered the same isomer.

Step 4: Isomer 1: Pentane

A continuous chain of 5 carbon atoms with the remaining hydrogen atoms bonded to make the structure: \(\displaystyle \mathrm{CH}_{3} -\mathrm{CH}_{2} -\mathrm{CH}_{2} -\mathrm{CH}_{2} -\mathrm{CH}_{3}\)

Step 5: Isomer 2: Isopentane (2-methylbutane)

A continuous chain of 4 carbon atoms, with one carbon branching off at the second position, to form: \(\displaystyle \mathrm{CH}_{3} -\mathrm{CH}(\mathrm{CH}_{3}) -\mathrm{CH}_{2} -\mathrm{CH}_{3}\)

Step 6: Isomer 3: Neopentane (2,2-dimethylpropane)

A continuous chain of 3 carbon atoms, with two carbon atoms branching off at the second position, to form: \(\displaystyle \mathrm{CH}_{3} -\mathrm{C}(\mathrm{CH}_{3})_{2} -\mathrm{CH}_{3}\) To conclude, there are three constitutional isomers for the formula \(\mathrm{C}_{5}\mathrm{H}_{12}\): Pentane, Isopentane (2-methylbutane), and Neopentane (2,2-dimethylpropane).

Key Concepts

Pentane

Pentane, one of the simplest alkanes, is a straight-chain hydrocarbon with the molecular formula \( \mathrm{C}_5\mathrm{H}_{12} \). In the structure of pentane, you'll find five carbon atoms connected in a continuous chain. Each carbon is bonded to enough hydrogen atoms to make sure that each carbon atom forms four covalent bonds, adhering to the tetravalency rule. This gives us the formula:

• \( \mathrm{CH}_3 - \mathrm{CH}_2 - \mathrm{CH}_2 - \mathrm{CH}_2 - \mathrm{CH}_3 \)

In this linear configuration, the molecule is fully extended, making it the longest possible configuration for five carbon atoms. As a straightforward alkane, pentane is found in natural gas and petroleum, and it is often used as a solvent in the chemical industry. Its simple structure provides a foundation for understanding more complex constitutional isomers.

Isopentane

Isopentane, also known as 2-methylbutane, is the first branched-chain isomer of pentane with the same formula \( \mathrm{C}_5\mathrm{H}_{12} \). Breaking from the straight chain pattern, isopentane has a four-carbon chain with a methyl group (\( \mathrm{CH}_3 \)) branching off from the second carbon:

• \( \mathrm{CH}_3 - \mathrm{CH} (\mathrm{CH}_3) - \mathrm{CH}_2 - \mathrm{CH}_3 \)

This branching gives it a more compact shape compared to pentane. This structural difference not only changes its shape but also affects its physical properties like boiling and melting points. Isopentane, due to its branched nature, typically has a slightly lower boiling point than its straight-chain counterpart, making it valuable in refining processes to enhance gasoline octane levels.

Neopentane

Neopentane, chemically known as 2,2-dimethylpropane, is an interesting isomer of pentane with a high degree of branching. Its structure is more compact than either pentane or isopentane. In neopentane, the three-carbon backbone has two additional methyl groups (\( \mathrm{CH}_3 \)) attached to the central carbon atom, forming:

• \( \mathrm{CH}_3 - \mathrm{C} (\mathrm{CH}_3)_2 - \mathrm{CH}_3 \)

This highly symmetric, compact structure imparts unique properties to neopentane, such as a relatively high melting point compared to its isomers. It is also less soluble in water due to its compact shape minimizing interactions with water molecules. Understanding neopentane is crucial for grasping how even subtle changes in atomic arrangement can affect the chemical and physical properties of molecules.