12 Hydrocarbons – Alkynes and Aromatics

Learning Objectives

  • Draw structures and write chemical formulas for alkynes when given only the compound’s name.
    | Nomenclature |
  • Draw and interpret the different representations of benzene.
    | Aromatics |
  • Use strength of intermolecular forces in organic molecules to explain differences in physical properties such as boiling point and solubility.
    | Intermolecular Forces |
  • Apply knowledge of bond properties (length, strength, bond order) to observed physical and thermochemical properties.

| Key Concepts and Summary | Glossary | End of Section Exercises |

Alkynes

Hydrocarbon molecules with one or more triple bonds are called alkynes; they make up another series of unsaturated hydrocarbons. Two carbon atoms joined by a triple bond are bound together by one σ bond and two π bonds. The sp-hybridized carbons involved in the triple bond have bond angles of 180°, giving these types of bonds a linear, rod-like shape.

The simplest member of the alkyne series is ethyne, C2H2, commonly called acetylene. The Lewis structure for ethyne, a linear molecule, is:

The structural formula and name for ethyne, also known as acetylene, are shown. In red, two C atoms are shown with a triple bond illustrated by three horizontal line segments between them. Shown in black at each end of the structure, a single H atom is bonded.

Nomenclature of Alkynes

The IUPAC nomenclature for alkynes is similar to that for alkenes except that the suffix -yne is used to indicate a triple bond in the chain. For example, CH3CH2C≡CH is called 1-butyne.

Example 1

Structure of Alkynes
Describe the geometry and hybridization of the carbon atoms in the following molecule:

A structural formula is shown with C H subscript 3 bonded to a C atom which is triple bonded to another C atom which is bonded to C H subscript 3. Each C atom is labeled 1, 2, 3, and 4 from left to right.

Solution
Carbon atoms 1 and 4 have four single bonds and are thus tetrahedral with sp3 hybridization. Carbon atoms 2 and 3 are involved in the triple bond, so they have linear geometries and would be classified as sp hybrids.

Check Your Learning
Identify the hybridization and bond angles at the carbon atoms in the molecule shown:
A structural formula is shown with an H atom bonded to a C atom. The C atom has a triple bond with another C atom which is also bonded to C H. The C H has a double bond with another C H which is also bonded up and to the right to C H subscript 3. Each C atom is labeled 1, 2, 3, 4, or 5 from left to right.

Answer:

carbon 1: sp, 180°; carbon 2: sp, 180°; carbon 3: sp2, 120°; carbon 4: sp2, 120°; carbon 5: sp3, 109.5°

Aromatic Hydrocarbons

Benzene, C6H6, is the simplest member of a large family of hydrocarbons, called aromatic hydrocarbons. These compounds can be drawn with alternating single and double bonds in a ring, but a more accurate depiction involves the use of resonance hybrid concept of valence bond theory. To illustrate this concept, consider the resonance structures for benzene, C6H6, below:

This figure shows a six carbon hydrocarbon ring in a variety of ways. There are three rings in the top row and three rings in the bottom row. On the top row, the left and middle structure have alternating single and double bonds between the carbon atoms. In the left, there is a double bond between the top and top right carbon, the a single bond between the top right and bottom right carbons, a double bond between the bottom right and bottom carbons, a single bond between the bottom and bottom left carbons, a double bond between the bottom left and top left carbons, and a single bond between the top left and top carbons. Each carbon also a hydrogen coming off of it, for a total of six hydrogens. There is a double facing arrow between the left and center structures. The center structure has single and double bonds in opposite spaces, showing how the single and double bonds can alternate. There is then an equals sign to the structure on the right, which has the same 6 carbon ring with single bonds between each carbon. There is also a circle inside the hydrocarbon ring, representing the ability of the double bonds to be averaged between all the bonds. In the second row, the same structures are shown without explicitly labeling the carbons and hydrogens. These are known as line structures.

Valence bond theory describes the benzene molecule as a hexagonal ring of sp2-hybridized carbon atoms with the unhybridized p orbital of each carbon atom perpendicular to the plane of the ring (coming straight out and behind the computer screen). Benzene does not, however, exhibit the characteristics expected for a cyclic alkene with alternating single and double bonds. Each of the six C–C bonds is equivalent and exhibits properties that are intermediate between those of a C–C single bond and a C=C double bond (closer to a “1.5” C-C bond). To represent this unique bonding, structural formulas for benzene and its derivatives are typically drawn with single bonds between the carbon atoms and a circle within the ring as shown above.

Example 2

Structure of Aromatic Hydrocarbons
An aromatic compound with one methyl- and one chloro- substituent is shown below. Draw two constitutional isomers of this molecule that are still aromatic hydrocarbons:

Two structural formulas are shown. The first has a six carbon hydrocarbon ring in which four of the carbon atoms are each bonded to only one H atom. At the upper right of the ring, the carbon that does not have a bonded H atom has a C H subscript 3 group attached. The C to the lower right has a C l atom attached. A circle is at the center of the ring. The second molecule has a hexagon with a circle inside. From a vertex of the hexagon at the upper right a C H subscript 3 group is attached. From the vertex at the lower right, a C l atom is attached.

Solution
Since a six-carbon ring with alternating single and double bonds is necessary for the molecule to be classified as aromatic, appropriate isomers can be produced only by changing the positions of the chloro-substituent relative to the methyl-substituent:

Two pairs of structural formulas are shown. The first has a six carbon hydrocarbon ring in which four of the C atoms are each bonded to only one H atom. At the upper right of the ring, the C atom that does not have a bonded H atom has a C H subscript 3 group attached. The C atom to the right has a C l atom attached. A circle is at the center of the ring. The second molecule in the first pair has a hexagon with a circle inside. From a vertex of the hexagon at the upper right a C H subscript 3 group is attached. From the vertex at the right, a C l atom is attached. The second pair first shows a six carbon hydrocarbon ring in which four of the C atoms are each bonded to only one H atom. A C l atom is attached to the left-most C atom and a C H subscript 3 group is attached to the right-most C atom. A circle is at the center of the ring. The second molecule in the pair has a hexagon with a circle inside. A C H subscript 3 group is attached to a vertex on the right side of the hexagon and to a vertex on the left side, a C l atom is bonded.

Check Your Learning
Draw three isomers of a six-membered aromatic ring compound substituted with two bromines.

Answer:
Three pairs of structural formulas are shown. The first has a six carbon hydrocarbon ring in which four of the C atoms are each bonded to only one H atom. At the upper right and right of the ring, the two C atoms that do not have bonded H atoms have one B r atom bonded each. A circle is at the center of the ring. Beneath this structure, a similar structure is shown which has a hexagon with a circle inside. From vertices of the hexagon at the upper right and right single B r atoms are attached. The second has a six carbon hydrocarbon ring in which four of the C atoms are each bonded to only one H atom. At the upper right and lower right of the ring, the two C atoms that do not have bonded H atoms have a single B r atom bonded each. A circle is at the center of the ring. Beneath this structure, a similar structure is shown which has a hexagon with a circle inside. From vertices of the hexagon at the upper right and lower right single B r atoms are attached. The third has a six carbon hydrocarbon ring in which four of the C atoms are each bonded to only one H atom. At the upper right and lower left of the ring, the two C atoms that do not have bonded H atoms have B r atoms bonded. A circle is at the center of the ring. Beneath this structure, a similar structure is shown which has a hexagon with a circle inside. From vertices of the hexagon at the upper right and lower left, single B r atoms are attached.

Intermolecular Forces of Alkynes and Aromatics

The intermolecular forces of alkynes are very similar to that of alkanes. Increasing the length of the longest carbon chain will increase the strength of the dispersion forces. As the molar mass of the molecules increases, the boiling and melting points will also increase. Additionally, the less branched an isomer is, the greater surface area the molecule will have to interact with other molecules, leading to stronger dispersion forces and higher boiling and melting points.

Intermolecular forces for aromatic compounds depend greatly on the identity and positioning of the substituents on the ring. For example, long hydrocarbon chain substituents would increase dispersion forces and raise boiling and melting points. The addition of electronegative atoms on the ring would introduce bond dipoles. These dipoles could cancel each other out or create a molecular dipole.

Key Concepts and Summary

Alkynes are unsaturated molecules that contain at least one triple bond. The nomenclature of alkynes is very similar to alkanes, except that the triple bond is always given the smallest number in the longest carbon chain and the ending changes to -yne from –ane. Alkynes tend to only form structural isomers with other alkynes or alkenes with similar levels of unsaturation. Aromatic molecules contain alternate single and double bonds that leads to resonance, or delocalization of electrons.

Glossary

alkyne
a molecule that contains one or more triple bond

aromatic hydrocarbon
a molecule that has alternating single and double bonds creating resonance, often in a 6-membered ring. (Note: there are robust rules for determining if a molecule is aromatic that is outside the scope of this course)

Chemistry End of Section Exercises

  1. What is the hybridization of each carbon in ethyne? How many σ bonds are found in ethyne? How many π bonds?
  2. What part of the name ethyne indicates a triple carbon-carbon bond is present in the molecule?
  3. Draw a bond-line structure for 2-methyl-3-hexyne. Are geometric isomers possible with this molecule?

Answers to Chemistry End of Section Exercises

  1. C1 (two electron domains): sp;
    C2 (two electron domains): sp;
    C3 (four electron domains): sp3
    6 σ bonds and 2 π bonds
  2. The -yne suffix
  3. While the carbon-carbon triple bond is a rigid bond and will not rotate, the geometry around the triple bond is linear and will not allow geometric isomers.

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