Question

Cyclohexane and benzene each have six carbon atoms. How are they different?

Short answer

Cyclohexane is a saturated ring with single bonds and 12 hydrogens. Benzene has alternating double and single bonds, is planar, and has 6 hydrogens.

Step-by-step solution

Identify the Structure of Cyclohexane

Cyclohexane is an alicyclic hydrocarbon with the chemical formula \( C_6H_{12} \). It consists of a six-carbon ring where each carbon atom is bonded to two hydrogen atoms. This forms a saturated ring structure.

Identify the Structure of Benzene

Benzene is an aromatic hydrocarbon with the chemical formula \( C_6H_6 \). It consists of a six-carbon ring with alternating double bonds between the carbon atoms. This results in a planar ring structure with delocalized π-electrons.

Compare the Hydrogen Atoms

Cyclohexane has 12 hydrogen atoms, whereas benzene has only 6 hydrogen atoms. The difference arises due to the nature of bonding in each molecule.

Compare the Types of Bonds

In cyclohexane, all bonds between carbon atoms are single bonds (sigma bonds). In benzene, there are alternating double and single bonds between the carbon atoms, leading to a resonance structure with delocalized π-electrons.

Compare the Shape and Stability

Cyclohexane adopts a non-planar chair conformation to minimize steric strain and is more flexible. Benzene is planar and has a special stability known as aromaticity due to the delocalized π-electrons.

Key concepts

Alicyclic Hydrocarbon

Cyclohexane is a prime example of an alicyclic hydrocarbon. These are compounds that contain carbon atoms arranged in a ring but do not exhibit aromaticity. Alicyclic hydrocarbons can have single or multiple rings, and they can also have saturated or unsaturated rings. Cyclohexane specifically has a saturated ring structure, meaning all carbon-carbon bonds are single bonds, and each carbon is bonded to two hydrogen atoms. This gives cyclohexane a chemical formula of \( C_6H_{12} \). The ring structure of cyclohexane is non-planar, typically adopting a chair conformation to minimize steric strain.

Aromatic Hydrocarbon

Benzene is a classic example of an aromatic hydrocarbon. Aromatic hydrocarbons contain one or more planar rings with a specific arrangement of alternating single and double bonds, known as conjugation. Benzene's chemical formula is \( C_6H_6 \), indicating it has equal numbers of carbon and hydrogen atoms. The unique stability of benzene and other aromatic hydrocarbons comes from the delocalization of π-electrons across the ring structure, giving rise to a phenomenon called aromaticity. This makes benzene much less reactive than other unsaturated hydrocarbons under normal conditions.

Resonance Structure

In the context of benzene, resonance refers to the way in which its electrons are distributed across multiple contributing structures. While benzene is often depicted with alternating double and single bonds, in reality, the electrons are evenly spread out, a concept known as resonance. In diagrams, benzene is sometimes represented by a hexagon with a circle inside to indicate the delocalized electrons. This resonance structure is key to the chemical stability and aromaticity of benzene. It allows the π-electrons to be shared evenly across the ring, lowering the overall energy of the molecule.

Delocalized π-Electrons

The concept of delocalized π-electrons is crucial to understanding benzene's chemistry. In benzene, the six π-electrons from the double bonds are not fixed between individual carbon atoms. Instead, they are free to move around the entire ring structure. This delocalization forms a cloud of electrons above and below the plane of the ring, providing extra stability to the molecule. It's this electron cloud that gives benzene its characteristic low reactivity and high stability. Cyclohexane, however, does not have delocalized π-electrons because it only contains single bonds.

Saturated Ring Structure

Cyclohexane's structure is described as a saturated ring because it contains only single bonds between its carbon atoms. A saturated hydrocarbon means all carbon atoms are 'saturated' with hydrogen atoms, having the maximum number of hydrogen atoms possible (in this case, 12, with the formula \( C_6H_{12} \)). This contrasts with benzene, which is unsaturated due to its alternating double bonds (formula \( C_6H_6 \)). The saturated ring structure of cyclohexane allows it to adopt various conformations, with the chair conformation being the most stable due to minimized steric hindrance.