Question

Draw the five structural isomers of hexane \(\left(\mathrm{C}_{6} \mathrm{H}_{14}\right).\)

Short answer

The five structural isomers of hexane are: 1. n-Hexane: H H H H H H | | | | | | C-C-C-C-C-C | | | | | | H H H H H H 2. 2-Methylpentane: H H H H H | | | | | C-C-C-C-C | | | | | H H-C-H H | | H H 3. 3-Methylpentane: H H H H H | | | | | C-C-C-C-C | | | | H H-C H H | | H H 4. 2,2-Dimethylbutane: H H H H | | | | C-C-C-C | | H-C-H | | H H H-C-H | | H H 5. 2,3-Dimethylbutane: H H H H | | | | C-C-C-C | | | | H-C H-C | | H H

Step-by-step solution

Draw the first isomer - n-Hexane

Pair all six carbon atoms in a straight chain and add hydrogen atoms to make sure each carbon atom forms four bonds. This is the most basic structural isomer of hexane, also known as n-Hexane: H H H H H H | | | | | | C-C-C-C-C-C | | | | | | H H H H H H

Draw the second isomer - 2-Methylpentane

Remove one carbon atom from the end of the straight chain and attach it to the second carbon atom in the chain (counting from either end). This will form a "branch" and create the second structural isomer called 2-Methylpentane: H H H H H | | | | | C-C-C-C-C | | | | | H H-C-H H | | H H

Draw the third isomer - 3-Methylpentane

Start with the same straight chain of five carbon atoms from the previous isomer and attach the branching carbon atom to the third carbon instead (counting from either end). This results in the third structural isomer called 3-Methylpentane: H H H H H | | | | | C-C-C-C-C | | | | H H-C H H | | H H

Draw the fourth isomer - 2,2-Dimethylbutane

Remove two carbon atoms from the end of the original straight chain, leaving four carbon atoms in a straight line. Then, attach the branching carbon atoms symmetrically to the second carbon atom in the chain. This results in the fourth structural isomer called 2,2-Dimethylbutane: H H H H | | | | C-C-C-C | | H-C-H | | H H H-C-H | | H H

Draw the fifth isomer - 2,3-Dimethylbutane

Start with the same four carbon atoms in a straight chain from the previous isomer and attach one branching carbon atom instead to the second carbon atom and the other to the third carbon atom. This gives us the fifth and final structural isomer called 2,3-Dimethylbutane: H H H H | | | | C-C-C-C | | | | H-C H-C | | H H These are the five structural isomers of hexane (C6H14).

Key concepts

n-Hexane

n-Hexane represents the simplest and most straightforward structure within the family of hexane isomers. Here, each carbon atom is bonded to the next in a linear or 'straight-chain' fashion. Its chemical structure can be verbalized as a chain of six carbon atoms with no branches. As the carbon atoms are connected in a sequential line, the hydrogens fill the remaining bonds to maintain carbon's tetravalency—meaning each carbon forms four bonds.

Consider visualizing n-Hexane as a straight line with no detours, which is often why it's referred to as 'normal' hexane. The absence of branching means that n-Hexane has the highest boiling point among its isomers due to the maximum surface area that allows for stronger dispersion forces. Aside from its use in laboratory settings to demonstrate structural isomerism, n-Hexane is also employed industrially as a solvent.

2-Methylpentane

2-Methylpentane is one of the structural isomers of hexane where the 'straight-chain' character of n-Hexane is altered. Think of it as taking one carbon from the end of the n-Hexane and placing it on the second carbon in the chain, hence the '2-Methyl' in its name.

Why is this important? Well, the branching reduces the molecule's surface area compared to n-Hexane, resulting in a lower boiling point. It's quite handy to remember that the position of the methyl group (a CH₃ group, in this case) can significantly affect the physical properties of the molecule. 2-Methylpentane can illustrate how isomerism can influence chemical behavior and reactivity, as well as boiling and melting points, solubility, and density.

3-Methylpentane

3-Methylpentane takes its place as another isomer of hexane when the methyl group chooses a different spot to park itself—on the third carbon of the chain. This little change from 2-Methylpentane slightly tweaks the molecule's shape and, subsequently, its physical properties.

If you were to compare 3-Methylpentane with its close relative, 2-Methylpentane, you'd note they have similar molecular formulas but different physical and chemical characteristics due to their varying structures. This highlights a crucial rule in chemistry: structure dictates function. So, although both molecules might look somewhat similar, their chemical behavior isn't identical.

2,2-Dimethylbutane

Enter 2,2-Dimethylbutane, an interesting character among the isomers of hexane. This molecule features a four-carbon backbone with two extra carbon atoms (the methyl groups) sticking out from the second carbon atom—imagine a caterpillar with two symmetrical legs. This specific arrangement is what gives it the name '2,2-' indicating that both methyl groups are on the second carbon.

This symmetry makes 2,2-Dimethylbutane unique in its physical properties among the isomers, including how it interacts with light and other chemicals. It's not just about the number of carbon atoms present but how they are arranged that creates diversity in the hexane isomers.

2,3-Dimethylbutane

Lastly, we meet 2,3-Dimethylbutane, where the branching takes on yet another pattern. This molecule has a four-carbon straight chain like its cousin, 2,2-Dimethylbutane, but the two additional methyl groups are at the second and third carbon atoms. This may seem like a small distinction, but it results in a different three-dimensional shape—and, as we've learned, shape affects everything in chemistry.

Because of the unique distribution of its branches, 2,3-Dimethylbutane will have different interaction forces between molecules, leading to different boiling points and other physical properties. It serves as a perfect example to show that even minimal changes in structural arrangements can lead to new isomers, each with their distinct characteristics.