Problem 69 - Chemistry and Chemical Reactivity

Question

Draw Lewis structures and name three structural isomers with the formula \(\mathrm{C}_{6} \mathrm{H}_{12} .\) Are any of these isomers chiral?

Step-by-Step Solution

Answer

Cyclohexane, methylcyclopentane, and 1-hexene are the isomers, none are chiral.

Step 1: Understanding the problem

We need to draw Lewis structures for three different structural isomers of the molecular formula \(\mathrm{C}_{6}\mathrm{H}_{12}\). These isomers should have the same molecular formula but differing structures. We also need to identify if any of these structures are chiral.

Step 2: Drawing the first isomer: Cyclohexane

Cyclohexane is a cyclic structure consisting of six carbon atoms bonded in a ring, with each carbon also bonded to two hydrogen atoms. The formula for cyclohexane is still \(\mathrm{C}_{6}\mathrm{H}_{12}\), as a ring introduces one degree of saturation, meaning it has fewer hydrogens than an open chain alkane.

Step 3: Drawing the second isomer: Methylcyclopentane

Methylcyclopentane can be drawn by creating a cyclopentane ring, which is a five-carbon ring, and attaching a methyl group (\(\mathrm{CH}_3\)) to one of the carbon atoms in the ring. The structure maintains the formula \(\mathrm{C}_{6}\mathrm{H}_{12}\).

Step 4: Drawing the third isomer: Hexene isomer

A common acyclic structure is hexene, which includes a double bond between two of the carbon atoms. For example, 1-hexene has a double bond between the first and second carbons in a six-carbon chain. This introduces a degree of unsaturation that maintains the formula \(\mathrm{C}_{6}\mathrm{H}_{12}\).

Step 5: Determining chirality

For a molecule to be chiral, it must have a carbon atom (a stereocenter) with four different groups attached to it. In cyclohexane and methylcyclopentane, there are no stereocenters as each carbon atom either forms a symmetrical ring or is bonded to similar groups. In 1-hexene, there is no stereocenter as well because of similar groups attached due to symmetry or unsaturated bonds. Therefore, none of these isomers are chiral.

Key Concepts

Lewis structures

Lewis structures are a way of representing the arrangement of atoms within a molecule. They are useful for visualizing the bonding between atoms and the distribution of electrons. In drawing Lewis structures, the first step is to determine the number of valence electrons available. For example, carbon (\(\mathrm{C}\)) has four valence electrons, while hydrogen (\(\mathrm{H}\)) has one. These structures help us comprehend the molecular geometry and electron configuration.

• Count total valence electrons
• Draw skeleton structures using chemical symbols
• Distribute remaining electrons to satisfy the octet rule

Understanding and practicing Lewis structures can significantly aid in predicting the molecular behavior, reactivity, and polarity of different compounds.

chirality

Chirality refers to the property of a molecule that makes it non-superimposable on its mirror image. Imagine your hands; they are mirror images but cannot be perfectly placed over one another. This is what chirality is in chemistry. For a molecule to be chiral, it must contain an asymmetric carbon atom (stereocenter), which is a carbon bonded to four different groups or atoms.

• An asymmetric carbon creates two non-superimposable configurations: enantiomers.
• Chiral molecules often have different biological activities compared to their mirror images.

In our case, none of the structural isomers of \(\mathrm{C}_6\mathrm{H}_{12}\)– cyclohexane, methylcyclopentane, or hexene – exhibit chirality due to the absence of such asymmetric carbon atoms.

cyclohexane

Cyclohexane is a cyclic alkane with the chemical formula \(\mathrm{C}_6\mathrm{H}_{12}\). This structure forms a ring of six carbon atoms, each bonded to two hydrogen atoms. Since it is a saturated hydrocarbon, cyclohexane does not have any double or triple bonds.

• Each carbon atom forms single bonds with adjacent carbon atoms.
• Bond angles are nearly 109.5°, which is typical for sp3 hybridized carbons.

Cyclohexane’s chair conformation is most stable due to minimized torsional strain, making it prevalent in many chemical and industrial applications.

methylcyclopentane

Methylcyclopentane is an interesting structural isomer of \(\mathrm{C}_6\mathrm{H}_{12}\), characterized by a five-carbon ring with one carbon atom connected to a \(\mathrm{CH}_3\) group. The structure features a cyclic pentane ring and a methyl group, giving it its unique properties.

• Contains a five-carbon ring, similar to cyclopentane.
• Has one methyl (\(\mathrm{CH}_3\)) substituent.

This structure exemplifies how small changes, such as adding a methyl group, can significantly alter a molecule's chemical behavior and properties.

hexene

Hexene is an organic compound classified as an alkene, identifiable by its six-carbon chain featuring a double bond. The presence of this double bond is what differentiates hexene from other alkanes.

• Contains a double bond between two carbon atoms.
• 1-Hexene has a double bond between the first and second carbons.

The double bond introduces a degree of unsaturation and provides hexene with characteristics like geometric isomerism and reactivity that are unique to alkenes.