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25.2: Hydrocarbon Rings

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Lesson Objectives

  • Describe and name cyclic hydrocarbons.
  • Explain how ring strain contributes to the shape of cyclic hydrocarbons. Describe the two primary conformations of cyclohexane.
  • Relate the concept of electron delocalization to the structure of benzene.
  • Name and write structural formulas for substituted benzene compounds.

Lesson Vocabulary

  • cyclic hydrocarbon
  • cycloalkane
  • cycloalkene
  • cycloalkyne
  • delocalized electrons

Check Your Understanding

Recalling Prior Knowledge

  • What are alkanes?
  • What is the difference between a sigma bond and a pi bond?

In the last lesson, you learned about straight-chain and branched hydrocarbons. In this lesson, you will see that hydrocarbons can also form cyclic structures.

Cyclic Hydrocarbons

A cyclic hydrocarbon is a hydrocarbon in which the carbon chain forms a ring. A cycloalkane is a cyclic hydrocarbon in which all of the carbon-carbon bonds are single bonds. Like other alkanes, cycloalkanes are saturated compounds. Cycloalkanes have the general formula of CnH2n. The simplest cycloalkane is cyclopropane, a three-carbon ring (Figure below).

Cyclopropane is the simplest cycloalkane. Its highly strained geometry makes it rather unstable and highly reactive.

The structural formulas of cyclic hydrocarbons can be represented in multiple ways, two of which are shown above. Each atom can be shown, as in the structure on the left. However, a convenient shorthand is to omit the element symbols and only show the shape, as in the triangle on the right. Carbon atoms are understood to be the vertices of the triangle.

The carbon atoms in cycloalkanes are still sp3 hybridized, with an ideal bond angle of 109.5°. However, due to the triangular structure of cyclopropane, the sum of the three C-C-C bond angles must be 180°, which means that each bond angle is 60°. This major deviation from the ideal angle is called ring strain, and it makes cyclopropane a fairly unstable and reactive molecule. Ring strain is decreased for cyclobutane, which has bond angles of 90°, but it is still significant. In contrast, cyclopentane has bond angles of about 108°. This minimal ring strain makes cyclopentane a much more stable compound.

Cyclohexane is a six-carbon cycloalkane, shown in Figure below.

Cyclohexane is one of the most stable cycloalkanes.

All three of these depictions of cyclohexane are somewhat misleading, because the molecule is not planar. For a planar hexagon, the internal bond angles would each be 120°. In order to reduce the ring strain and attain a bond angle of approximately 109.5°, the molecule is puckered. The puckering of the ring means that every other carbon atom is above and below the plane. Figure below shows two possibilities for the puckered cyclohexane molecule. Each of the structures is called a conformation. The conformation on the left is called a boat conformation, while the one on the right is called a chair conformation.

The cyclohexane molecule adopts various puckered conformations in order to reduce ring strain. The most stable one is called the chair conformation and is shown on the right. The boat conformation on the left is less stable than the chair, but it is still preferable to a planar molecule.

While both conformations reduce the ring strain compared to a planar molecule, the chair is preferred. This is because the chair conformation results in fewer repulsive interactions between the hydrogen atoms. However, interconversion readily occurs between the two conformations.

Larger cycloalkanes also exist, but five- and six-membered rings are among the most common structures found in naturally occurring organic compounds. Cyclic hydrocarbons may also be unsaturated. A cycloalkene is a cyclic hydrocarbon with at least one carbon-carbon double bond. A cycloalkyne is a cyclic hydrocarbon with at least one carbon-carbon triple bond. Figure below show the simplified structural formulas for cyclohexene and cyclooctyne.

Cyclohexene (left) and cyclooctyne (right).

Aromatic Hydrocarbons

Benzene is the parent compound of a large family of organic molecules known as aromatic compounds. Although benzene was discovered in 1826, it was many years before its structure was fully understood. In 1895, German chemist August Kekulé (1829-1896) realized that the benzene molecule could best be represented by a six-membered carbon ring with one hydrogen atom attached to each of the carbon atoms. Unlike cyclohexane, benzene is unsaturated; each carbon atom participates in one double bond. Two different resonance structures with alternating single and double bonds around the ring can be written for benzene.

Recall that when two valid Lewis structures can be drawn for a single molecule, the true structure is the average of those structures. In benzene, the true bonding between carbon atoms is neither a single nor a double bond. Rather, all of the bonds are a hybrid of a single and double bond. In benzene, the pi bonding electrons are free to move completely around the ring. Delocalized electrons are electrons that are not confined to the bond between two atoms but are instead able to move between three or more atoms. The delocalization of the electrons in benzene are often shown by drawing benzene with a ring inside the hexagon. The hydrogen atoms on each carbon are implied.

Delocalization of the electrons makes for a more stable molecule than a similar molecule that does not have delocalized electrons. Benzene is a more stable and less reactive compound than straight-chain hexenes. Because the carbons are sp2 hybridized, the ideal C-C-C bond angles are 120°, which is equal to the internal bond angles of a planar hexagon. As a result, benzene is completely planar and does not pucker like cyclohexane. Benzene rings are common in a great many natural substances and biomolecules. Figure below shows the structural formulas for vanillin, which is the primary component of vanilla extract, and naphthalene, which is commonly used in mothballs.

Nomenclature of Aromatic Compounds

The simplest aromatic compounds are benzene rings with one substituent replacing one of the hydrogen atoms. If this substituent is an alkyl group, it is named first, followed by “benzene.” For example, the molecule shown in Figure below is called ethylbenzene.

Ethylbenzene.

Substituents do not need to be alkyl groups (see Figure below). If a chlorine atom were substituted for a hydrogen, the name would be chlorobenzene. An –NH2 group is called an amino group, so the corresponding molecule is called aminobenzene, often referred to as aniline. An –NO2 group is called a nitro group, so the third example below is nitrobenzene.

If more than one substituent is present, their location relative to each other can be indicated by numbering the positions on the benzene ring.

The number of the carbon then precedes the name of the substituent in the name, with the numbers separated by a comma. As with branched alkanes, the lowest possible numbers are used, and additional prefixes are used if more than one of the same substituent is present. If there are different substituents, the first in alphabetical order is given the lower number and listed first. The structures below are called 1,2-dimethylbenzene and 1-ethyl-4-methylbenzene.

An alternate system for naming di-substituted benzene rings uses three different prefixes: ortho, meta, and para. If two groups are ortho to one another, they are on adjacent carbon atoms. Meta positioning refers to a 1,3 arrangement, and para substituents are in a 1,4 arrangement. Shown below are the three possibilities for dimethylbenzene.

Lastly, a benzene ring missing one hydrogen atom (−C6H5) can itself be considered the substituent on a longer chain of carbon atoms. That group is called a phenyl group, so the molecule below is called 2-phenylbutane.

Lesson Summary

  • Hydrocarbons can form ringed structures called cyclic hydrocarbons. Cyclopentane and cyclohexane are the most common cycloalkanes because they have the least ring strain.
  • Cyclohexane exists in puckered confirmations, such as the chair and boat conformations.
  • Benzene is the base structure of aromatic compounds. Benzene has a ring of delocalized electrons, which provides additional stability to the structure.
  • Other aromatic compounds can be formed by substituting various groups for one or more of the hydrogen atoms on benzene. Polysubstituted benzene compounds are named by numbering the carbons in the ring.

Lesson Review Questions

Reviewing Concepts

  1. Name and give the molecular formula for the simple cycloalkane containing a ring of eight carbon atoms.
  2. What is the ideal C−C−C bond angle in a cycloalkane? Which two cycloalkanes come closest to the ideal bond angles?
  3. Which molecule is more reactive: cyclopropane or cyclopentane? Explain.
  4. Both benzene and cyclohexane are six-membered rings. Why is benzene planar while cyclohexane is not?
  5. What makes benzene a stable and fairly unreactive molecule?

Problems

  1. Name the following molecules.
  2. Write structural formulas for the following compounds.
    1. methylcyclopentane
    2. 1,3,5-trichlorocyclohexane
    3. cyclooctene
  3. Name the following aromatic molecules.

    Note: In a zigzag line as in C and D above, each vertex is understood to be a carbon atom. These are called skeletal diagrams. For example, butane would be represented as: .

  4. Draw structural formulas for the following compounds.
    1. 1,3-diaminobenzene
    2. 3,4-diphenylhexane
    3. para-dichlorobenzene
  5. Trinitrotoluene, abbreviated TNT, is commonly used as an explosive. The IUPAC name is 2-methyl-1,3,5-trinitrobenzene. Draw the structural formula for TNT.

Further Reading / Supplemental Links

Points to Consider

There is a wide variation in the reactivity of organic compounds due to specific groups in the molecule, most of which contain atoms other than hydrogen and carbon. These are called functional groups.

  • What are some of the most common functional groups?
  • How are functional groups indicated in the name of an organic compound?

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