M12Q3: Alkenes: Naming, Geometric Isomers, Intermolecular Forces and Bond Properties; Optical Isomers

Learning Objectives

  • Draw structures and write chemical formulas for alkenes when given only the compound’s name.
    | Nomenclature |
  • Draw, recognize, and name geometric isomers (cis/trans) of alkenes and cycloalkanes.
    | Geometric Isomers |
  • Use strength of intermolecular forces in organic molecules to explain differences in physical properties such as boiling point and solubility.
    | Intermolecular Forces |
  • Determine whether a molecule has optical isomers (enantiomers) and identify chiral carbon atoms.
    | Optical Isomers |

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

Alkenes

Organic compounds that contain one or more double or triple bonds between carbon atoms are described as unsaturated. Unsaturated hydrocarbon molecules that contain one or more double bonds are called alkenes. Carbon atoms linked by a double bond are bound together with one σ bond and one π bond.

Ethene, C2H4, is the simplest alkene. Each carbon atom in ethene, commonly called ethylene, has a trigonal planar structure. The second member of the series is propene (propylene) (Figure 1); butene follows in the series. We will look more closely at butene later, learning about a new form of isomerism.

Lewis structural formulas show carbon and hydrogen element symbols and bonds between the atoms. The first structure in this row shows two bonded C atoms with a double bond between them. Each C atom has two H atoms bonded to it as well. The second structure in the row shows three bonded C atoms with a double bond up and to the right between the first and second C atoms moving left to right across the chain, and a single bond down and to the right between the second and third C atom. The first C atom has two H atoms bonded to it, the second C atom has one H atom bonded to it, and the third C atom has three H atoms bonded to it. The third structure shows four bonded C atoms with one bonded up and to the right to a C atom, down and to the right to a C atom, and double bonded up and to the right to a C atom. The first C atom, moving from left to right, has three H atoms bonded to it. The second C atom has two H atoms bonded to it. The third C atom has one H atom bonded to it, and the fourth C atom has two H atoms bonded to it. In the second row, ball-and-stick models for the structures are shown. In these representations, single bonds are represented with sticks, double bonds are represented with two parallel sticks, and elements are represented with balls. Carbon atoms are black and hydrogen atoms are white in this image. In the third row, space-filling models are shown. In these models, atoms are enlarged and pushed together, without sticks to represent bonds. In the final row, names are provided. The molecule with the double bond between two C atoms is named ethene. The molecule with the double bond between the first and second C atoms in the chain of three is named propene. The molecule with the double bond between the carbon atoms in the chain of four is named 1 dash butene.
Figure 1. Expanded structures and ball-and-stick structures for the alkenes ethene, propene, and 1-butene are shown.

Nomenclature of Alkenes

The name of an alkene is derived from the name of the alkane with the same number of carbon atoms. The presence of the double bond is signified by replacing the suffix -ane with the suffix -ene. The location of the double bond is identified by naming the smaller of the numbers of the carbon atoms participating in the double bond. When assigning numbers, the double bond takes priority over any substituents attached to the longest carbon chain and should be given the lowest possible number assignment. Some alkenes (like 2-butene) will also have a prefix associated with what type of isomer it is, which is described in the next section.

Four structural formulas and names are shown. The first shows two red C atoms connected by a red double bond illustrated with two parallel line segments. H atoms are bonded above and below to the left of the left-most C atom. Two more H atoms are similarly bonded to the right of the C atom on the right. Beneath this structure the name ethene and alternate name ethylene are shown. The second shows three C atoms bonded together with a red double bond between the red first and second C atoms moving left to right across the three-carbon chain. H atoms are bonded above and below to the left of the C atom to the left. A single H is bonded above the middle C atom. Three more H atoms are bonded above, below, and to the right of the third C atom. Beneath this structure the name propene and alternate name propylene is shown. The third shows four C atoms bonded together, numbered one through four moving left to right with a red double bond between the red first and second carbon in the chain. H atoms are bonded above and below to the left of the C atom to the left. A single H is bonded above the second C atom. H atoms are bonded above and below the third C atom. Three more H atoms are bonded above, below, and to the right of the fourth C atom. Beneath this structure the name 1 dash butene is shown. The fourth shows four C atoms bonded together, numbered one through four moving left to right with a red double bond between the red second and third C atoms in the chain. H atoms are bonded above, below, and to the left of the left-most C atom. A single H atom is bonded above the second C atom. A single H atom is bonded above the third C atom. Three more H atoms are bonded above, below, and to the right of the fourth C atom. Beneath this structure the name 2 dash butene is shown.

Isomers of Alkenes

1-butene and 2-butene are structural isomers, since they have the same formula but a different arrangement of atoms. For example, the first carbon atom in 1-butene is bonded to two hydrogen atoms, but the first carbon atom in 2-butene is bonded to three hydrogen atoms.

The compound 2-butene and some other alkenes also form a second type of isomer called a geometric isomer. In a pair of geometric isomers, the atoms are attached in the same order, but the spatial arrangement of the two molecules differ. In order to be geometric isomers, each carbon of the C=C must have two different groups attached to it (for example, in 2-butene, each carbon of the C=C is attached to a hydrogen and a methyl group).

Recall that carbon atoms are free to rotate around a single bond, but not around a double bond; a double bond is rigid. This makes it possible to have two isomers of 2-butene, one with both methyl groups on the same side of the double bond (cis-) and one with the methyl groups on opposite sides (trans-) as seen in (Figure 2).

The figure illustrates three ways to represent isomers of butene. In the first row of the figure, Lewis structural formulas show carbon and hydrogen element symbols and bonds between the atoms. The first structure in this row shows a C atom with a double bond to another C atom which is bonded down and to the right to C H subscript 2 which, in turn, is bonded to C H subscript 3. The first C atom, moving from left to right, has two H atoms bonded to it and the second C atom has one H atom bonded to it. The second structure in the row shows a C atom with a double bond to another C atom. The first C atom is bonded to an H atom up and to the left and C H subscript 3 down and to the left. The second C atom is bonded to an H atom up and to the right and C H subscript 3 down and to the right. Both C H subscript 3 structures appear in red. The third structure shows a C atom with a double bond to another C atom. The first C atom from the left is bonded up to a the left to C H subscript 3 which appears and red. It is also bonded down and to the left to an H atom. The second C atom is bonded up and to the right to an H atom and down and to the left to C H subscript 3 which appears in red. In the second row, ball-and-stick models for the structures are shown. In these representations, single bonds are represented with sticks, double bonds are represented with two parallel sticks, and elements are represented with balls. C atoms are black and H atoms are white in this image. In the third row, space-filling models are shown. In these models, atoms are enlarged and pushed together, without sticks to represent bonds. In the final row, names are provided. The molecule with the double bond between the first and second carbons is named 1 dash butene. The two molecules with the double bond between the second and third carbon atoms is called 2 dash butene. The first model, which has both C H subscript 3 groups beneath the double bond is called the cis isomer. The second which has the C H subscript 3 groups on opposite sides of the double bond is named the trans isomer.
Figure 2. These molecular models show the structural and geometric isomers of butene.

Geometric isomers can also form in cycloalkanes when adjacent carbons have two different bonded groups. In 1,2-dimethylcyclohexane, two carbons in the ring each have a methyl and a hydrogen attached. The methyl groups can both be on the same side of the ring, forming a cis-isomer (see left structure below) or they can be on opposite sides of the ring, forming a trans-isomer (see right structure below).

Intermolecular Forces of Alkenes

The intermolecular forces of alkenes 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.

Geometric isomers may have different boiling and melting points, even though they have the same connectivity/branching. When there are polar bonds within cis/trans molecules, often the cis-isomer is more polar than the trans-isomer. A good example is 1,2-dichloroethene, shown in Figure 3. The cis-isomer has a molecular dipole moment but the trans-isomer does not have a molecular dipole moment, since the two C–Cl bond dipoles cancel each other out. Because the cis-isomer exhibits both dispersion and dipole-dipole forces but the trans-isomer only exhibits dispersion forces, the cis-isomer will have significantly higher boiling and melting points.

There are structures of two molecules. To the left, there is a carbon single bonded to a chlorine and a hydrogen, both to the left, and double bonded to another carbon, which is also single bonded to a chlorine and a hydrogen. The chlorines are both angled up and the hydrogens are both angled down. There is an arrow pointing up vertically through the double bond, representing the overall molecular dipole. Underneath the structure is cis 1 2 di chloro ethene. To the right is a similar structure. A central carbon is single bonded to a chlorine and a hydrogen, both to the left, and double bonded to a carbon to the right, which is also single bonded to a chlorine and a hydrogen. The chlorines are opposite each other, one up to the left and one down to the right. The hydrogens are also opposite, one down to the left and one up to the right. Underneath the structure is trans 1 2 di chloro ethene.
Figure 3: A comparison of cis-1,2-dichloroethene and trans-1,2-dichloroethene.

Optical Isomers and Chirality

Optical isomers have the same molecular formula and connectivity, but are mirror images of each other and not superimposable. Optical isomers have at least one chiral carbon. A chiral carbon is a sp3 hybridized carbon with four different groups attached (Figure 4). Optical isomers are extremely important in biological applications, such as pharmaceutical development. Optical isomers share the same intermolecular forces and thus have equivalent boiling and melting points. Separation of optical isomers is still possible, but requires special equipment and techniques that are beyond the scope of this text.

The figure has three structures. A pair to the left and one on the right. On the left are two structures that are mirror images of each other. Each structure has a central carbon single bonded to a hydrogen pointing up. The left structure has single bonds to chlorine and bromine to the left and fluorine to the right. The right structure has single bonds to chlorine and bromine to the right and a single bond to fluorine to the left. Under the two structures it says "mirror images". To the right, the two structures are placed on top of each other, one in red and the other in gray. This shows that the hydrogens, carbons, and chlorines overlap each other, but the fluorine and bromine do not. Under the structure is "non super imposable"
Figure 4: Optical isomers are mirror images of each other (left), but cannot be placed on top of each other (right).

Chemistry in Real Life: Recycling Plastics

Polymers (from Greek words poly meaning “many” and mer meaning “parts”) are large molecules made up of repeating units, referred to as monomers. Polymers can be natural (starch is a polymer of sugar residues and proteins are polymers of amino acids) or synthetic [like polyethylene, polyvinyl chloride (PVC), and polystyrene]. The variety of structures of polymers translates into a broad range of properties and uses that make them integral parts of our everyday lives. Adding functional groups to the structure of a polymer can result in significantly different properties (see the discussion about Kevlar later in this chapter).

Ethylene (the common industrial name for ethene) is a basic raw material in the production of polyethylene and other important compounds. Over 135 million tons of ethylene were produced worldwide in 2010 for use in the polymer, petrochemical, and plastic industries. Ethylene is produced industrially in a process called cracking, in which the long hydrocarbon chains in a petroleum mixture are broken into smaller molecules.

An example of a polymerization reaction is shown in Figure 5. The monomer ethylene (C2H4) is a gas at room temperature, but when polymerized, using a transition metal catalyst, it is transformed into a solid material made up of long chains of –CH2– units called polyethylene. Polyethylene is a commodity plastic used primarily for packaging (bags and films).

This diagram has three rows, showing ethylene reacting to form polyethylene. In the first row, Lewis structural formulas show three molecules of ethylene being added together, which are each composed of two doubly bonded C atoms, each with two bonded H atoms. Ellipses, or three dots, are present before and after the molecule structures, which in turn are followed by an arrow pointing right. On the right side of the arrow, the ellipses or dots again appear to the left of a dash that connects to a chain of 7 C atoms, each with H atoms connected above and below. A dash appears at the end of the chain, which in turn is followed by ellipses or dots. The reaction diagram is repeated in the second row using ball-and-stick models for the structures. In these representations, single bonds are represented with sticks, double bonds are represented with two parallel sticks, and elements are represented with balls. Carbon atoms are black and hydrogen atoms are white in this image. In the third row, space-filling models are shown. In these models, atoms are enlarged spheres which are pushed together, without sticks to represent bonds.
Figure 5. The reaction for the polymerization of ethylene to polyethylene is shown.

Polyethylene is a member of one subset of synthetic polymers classified as plastics. Plastics are synthetic organic solids that can be molded; they are typically organic polymers with high molecular masses. Most of the monomers that go into common plastics (ethylene, propylene, vinyl chloride, styrene, and ethylene terephthalate) are derived from petrochemicals and are not very biodegradable, making them candidate materials for recycling. Recycling plastics helps minimize the need for using more of the petrochemical supplies and also minimizes the environmental damage caused by throwing away these nonbiodegradable materials.

Plastic recycling is the process of recovering waste, scrap, or used plastics, and reprocessing the material into useful products. For example, polyethylene terephthalate (soft drink bottles) can be melted down and used for plastic furniture, in carpets, or for other applications. Other plastics, like polyethylene (bags) and polypropylene (cups, plastic food containers), can be recycled or reprocessed to be used again. Many areas of the country have recycling programs that focus on one or more of the commodity plastics that have been assigned a recycling code (see Figure 6). These operations have been in effect since the 1970s and have made the production of some plastics among the most efficient industrial operations today.

This table shows recycling symbols, names, and uses of various types of plastics. Symbols are shown with three arrows in a triangular shape surrounding a number. Number 1 is labeled P E T E. The related plastic, polyethylene terephthalate (P E T E), is used in soda bottles and oven-ready food trays. Number 2 is labeled H D P E. The related plastic is high-density polyethylene (H D P E), which is used in bottles for milk and dishwashing liquids. Number 3 is labeled V. The related plastic is polyvinyl chloride or (P V C). This plastic is used in food trays, plastic wrap, and bottles for mineral water and shampoo. Number 4 is labeled L D P E. This plastic is low density polyethylene (L D P E). It is used in shopping bags and garbage bags. Number 5 is labeled P P. The related plastic is polypropylene (P P). It is used in margarine tubs and microwaveable food trays. Number 6 is labeled P S. The related plastic is polystyrene (P S). It is used in yogurt tubs, foam meat trays, egg cartons, vending cups, plastic cutlery, and packaging for electronics and toys. Number 7 is labeled other for any other plastics. Items in this category include those plastic materials that do not fit any other category. Melamine used in plastic plates and cups is an example.
Figure 6. Each type of recyclable plastic is imprinted with a code for easy identification.

Key Concepts and Summary

Alkenes are unsaturated molecules that contain at least one double bond. The nomenclature of alkenes is very similar to alkanes, except that the double bond is always given the smallest number in the longest carbon chain and the ending changes to –ene from –ane. Alkenes have the potential to form geometric isomers, where two molecules with the same molecular formula and connectivity, but have different spatial arrangements (and often different physical properties).

Although alkenes do not form optical isomers, optical isomers are similar to a geometric isomer in that they have the same molecular formula and connectivity, but they are mirror images of each other. Optical isomers always have a chiral center, or a sp3 hybridized carbon with four different groups attached. It is important to look more holistically at these four groups. For example, all four of these groups could be alkyl chains which begin with carbon. The difference is the length of the alkyl group rather than the fact that each begin with a carbon atom.

Glossary

alkene
a molecule that contains one or more double bond

chiral carbon
a sp3 hybridized carbon with four different groups attached

geometric isomer
molecules that have the same molecular formula and connectivity, but the spatial arrangement of the two molecules differs. These are most often present in molecules with a C=C or cycloalkanes.

optical isomers
molecules that have the same molecular formula and connectivity, but are mirror images of each other caused by a chiral center and are not superimposable

unsaturated
a molecule that contains one or more double or triple bonds between carbon atoms

Chemistry End of Section Exercises

  1. What is the difference between a saturated and an unsaturated hydrocarbon?
  2. What is the hybridization of the carbon atoms’ valence orbitals in ethane and ethene?
  3. Explain why the carbon-carbon bond in ethane rotates freely, while the carbon-carbon bond in ethene does not.
  4. Explain why these two molecules are not isomers:
    Two structural formulas are shown. In the first, a chain of six carbon atoms with a single double bond between carbons two and three counting right to left across the molecule is shown with twelve total H atoms bonded. H atoms are bonded at each end of the molecule as well as above. H atoms are also bonded below all C atoms except those involved in the double bond. In the second structure, a hydrocarbon chain of five C atoms connected by single bonds is shown. A single C with three attached H atoms is bonded beneath the second carbon counting right to left across the molecule.
  5. Are these two molecules isomers? Explain why or why not:
  6. Write the Lewis structure and molecular formula for each of the following hydrocarbons:
    1. hexane
    2. 3-methylpentane
    3. cis-3-hexene
    4. 4-methyl-1-pentene
  7. Write Lewis structures for the cis and trans isomers of CH3CH=CHCl.
  8. Consider the two isomers of 1,2-dibromoethene. Which one will have the higher boiling point?
  9. How many chiral carbons can be found in Vitamin E:

Answers to Chemistry End of Section Exercises

  1. A saturated hydrocarbon is a molecule containing only single bonds between carbon atoms. An unsaturated hydrocarbon is a molecule with at least one multiple bond (double or triple) between carbon atoms.
  2. Ethane has a single bond between the two carbons, with three hydrogens bonded to each carbon, resulting in four electron domains and sp3 hybridization. Ethene has a double bond between the two carbons, with two hydrogens bonded to each carbon, resulting in three electron domains and sp2 hybridization.
  3. The carbon-carbon bond in ethane is a single bond, consisting of a freely-rotating σ bond. The carbon-carbon bond in ethene is a double bond, containing both a σ and π bond. Rotations around the carbon-carbon bond requires breaking and reforming the π bond. It is this lack of free rotation that can give rise to geometric isomers in alkenes.
  4. The molecules do not have the same molecular formula and so are different molecules, not isomers. The first molecule is C6H12 and the second is C6H14.
  5. Even though the first molecule is a cycloalkane and the second is an alkene, they are structural isomers because they each have the chemical formula C6H12.
    1. C6H14This figure shows a horizontal hydrocarbon chain consisting of six singly bonded carbon atoms. Each C atom has an H atom bonded above and below it. The two C atoms on either end of the chain each of a third H atom bonded to it.
    2. C6H14This figure shows five C atoms bonded together with a sixth C atom bonded below the chain. The first C atom (from left to right) has three H atoms bonded to it and is also bonded to another C atom. The second C atom has two H atoms bonded above and below it and is also bonded to another C atom. The third C atom has an H atom bonded above it and a C atom bonded below it. The C atom bonded below the third C atom in the chain has three H atoms bonded to it. The third C atom is also bonded to another C atom. The fourth C atom in the chain has two H atoms bonded above and below it and is bonded to another C atom. The fifth C atom has three H atoms bonded to it.
    3. C6H12This figure shows a C atom with three H atoms bonded to it. This C atom is bonded to another C atom with two H atoms bonded above and below it. The second C atom is also bonded to another C atom down and to the right. This C atom is bonded to one H atom and has a double bond to a fourth C atom. The fourth C atom is also bonded to one H atom. The fourth C atom has a bond up and to the right to another C atom. This C atom has two H atoms bonded above and below it. This C atom also bonds to another C atom which is bonded to three H atoms.
    4. C6H12This figure shows a hydrocarbon chain with a length of five C atoms. The first C atom (from left to right) is bonded to two H atoms and also forms a double bond with the second C atom. The second C atom is bonded to one H atom above it and is also bonded to a third C atom. The third C atom is bonded to two H atoms and also bonded to a fourth C atom. The fourth C atom is bonded to one H atom above it and a C atom below it. The C atom bonded to the fourth C atom in the chain has three H atoms bonded to it. The fourth C atom is also bonded to a fifth C atom which is bonded to three H atoms.
  6. The cis isomer will have the CH3 and Cl groups oriented along the same side of the π bond. The trans isomer will have the CH3 and Cl groups oriented on the same side of the π bond.This figure includes two structural formulas. The first structure shows two double bounded C atoms with C l bonded to the upper right, C H subscript 3 bonded to the upper left, and H atoms attached to the lower right and lower left in the structure. This structure is labeled cis dash. The second structure shows two double bounded carbon atoms with C l attached to the lower right, C H subscript 3 attached to the upper left, and H atoms attached to the upper right and lower left in the structure. This structure is labeled trans dash.
  7. The cis-1,2-dibromoethene is a polar molecule and the trans-1,2-dibromoethene is a nonpolar molecule. The polar isomer has dipole-dipole intermolecular forces, which are stronger than the dispersion forces of the trans-isomer. The polar isomer’s stronger intermolecular forces gives the cis-isomer a higher boiling point.
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