22.1 Alkenes and Alkynes – Structure and Naming

Learning Objectives

By the end of this section, you will be able to:

  • Identify the difference between saturated and unsaturated hydrocarbons.
  • Describe the functional groups, alkenes and alkynes.
  • Properly name alkene and alkynes using the IUPAC naming system

As noted before, alkenes are hydrocarbons with carbon-to-carbon double bonds (R2C=CR2) and alkynes are hydrocarbons with carbon-to-carbon triple bonds (R–C≡C–R). Collectively, they are called unsaturated hydrocarbons because they have fewer hydrogen atoms than does an alkane with the same number of carbon atoms, as is indicated in the following general formulas in Figure 22.1a.

Three images. On the left shows a saturated hydrocarbon giving the alkane formula C subscript n H subscript 2n+2. On the right shows the general formulas for unsaturated hydrocarbons: an alkene C subscript n H subscript 2n and for an alkyne C subscript n H subscript 2n-2
Figure 22.1a. The general formulas of alkanes, alkenes and alkynes (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

You have likely heard of unsaturated fats. These are complex organic molecules with long chains of carbon atoms, which contain at least one double bond between carbon atoms. Double and triple bonds give rise to a different geometry around the carbon atom that participates in them, leading to important differences in molecular shape and properties. The differing geometries are responsible for the different properties of unsaturated versus saturated fats.

Alkenes

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 22.1b.); the butene isomers follow in the series. Four carbon atoms in the chain of butene allows for the formation of isomers based on the position of the double bond, as well as 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 22.1b. Expanded structures, ball-and-stick structures, and space-filling models for the alkenes ethene, propene, and 1-butene are shown. (credit: Chemistry (OpenStax), CC BY 4.0)

Some representative alkenes—their names, structures, and physical properties—are given in Table 22.1a. In general, as the chain length increases, the melting and boiling points increase. In comparison to alkanes, alkenes have higher melting points but lower boiling points. For instance, looking at propane versus propene, propane has a melting point of -190oC and a boiling point of -42oC, whereas propene has a melting point of -185oC and a boiling point of -47oC. When comparing alkenes to alkanes, refer to Table 20.1b Properties of Some Alkanes.

Table 22.1a. Physical Properties of Some Selected Alkenes
IUPAC Name Molecular Formula Condensed Structural Formula Melting Point (°C) Boiling Point (°C)
ethene C2H4 CH2=CH2 –169 –104
propene C3H6 CH2=CHCH3 –185 –47
1-butene C4H8 CH2=CHCH2CH3 –185 –6
1-pentene C5H10 CH2=CH(CH2)2CH3 –138 30
1-hexene C6H12 CH2=CH(CH2)3CH3 –140 63
1-heptene C7H14 CH2=CH(CH2)4CH3 –119 94
1-octene C8H16 CH2=CH(CH2)5CH3 –102 121

Table source: “13.1: Alkenes: Structures and Names” In Basics of GOB Chemistry (Ball et al.), CC BY-NC-SA 4.0.

We used only condensed structural formulas in Table 1. Thus, CH2=CH2 is illustrated in Figure 22.1c.

A two carbon double bonded chain with four hydrogens attached representing ethene.
Figure 22.1c. The structural representation of CH2=CH2 (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

The double bond is shared by the two carbons and does not involve the hydrogen atoms, although the condensed formula does not make this point obvious. Note that the molecular formula for ethene is C2H4, whereas that for ethane is C2H6.

The first two alkenes in Table 22.1a., ethene and propene, are most often called by their common names—ethylene and propylene, respectively (Figure 22.1d.). Ethylene is a major commercial chemical. The US chemical industry produces about 25 billion kilograms of ethylene annually, more than any other synthetic organic chemical. More than half of this ethylene goes into the manufacture of polyethylene, one of the most familiar plastics. Propylene is also an important industrial chemical. It is converted to plastics, isopropyl alcohol, and a variety of other products.

The ball and spring models of ethene on the left and propene on the right. The carbons are shown as red balls, the hydrogens are shown as blue balls and that bonds connecting the carbons are shown as springs.
Figure 22.1d. Ethene and Propene. The ball-and-spring models of ethene/ethylene (a) and propene/propylene (b) show their respective shapes, especially bond angles. (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Naming Alkenes

Alkenes are named using a series of rules similar to those for alkanes, with the suffix -ene used instead of -ane to identify the functional group. There are three steps to this process.

1. Name the parent hydrocarbon. Find the longest carbon chain containing the double bond, and name the compound accordingly, using the suffix -ene as in Figure 22.1e.

Two structures of 2-ethylpent-1-ene highlighted differently to justify that the molecule is named as a pentene, not a hexane, since the double bond is not contained in the six-carbon chain.
Figure 22.1e. Finding the longest chain in an alkene using pentene as the example. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0)

2. Number the carbon atoms in the chain. Begin at the end nearer the double bond or, if the double bond is equidistant from the two ends, begin at the end nearer the first branch point. This rule ensures that the double-bond carbons receive the lowest possible numbers such as those examples shown in Figure 22.1f.

The first structure has a 6-carbon chain with a double bond between C2-C3. The second compound has a 6-carbon chain with methyl group at C2 and double bond between C3-C4.
Figure 22.1f. Numbering the carbon atoms in the chain to ensure the double-bond carbons receive the lowest possible numbers. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0)

3. Write the full name. Number the substituents according to their positions in the chain, and list them alphabetically. Indicate the position of the double bond by giving the number of the first alkene carbon and placing that number directly before the parent name. If more than one double bond is present, indicate the position of each and use one of the suffixes -diene, -triene, and so on.

The structures of 2-hexene, 2-methyl-3-hexene, 2-ethyl-1-pentene, and 2-methyl-1 3-butadiene. The carbon atoms are numbered in all structures.
Figure 22.1g. Examples when numbering substituents within an alkene chain or when you have more than one double-bond present. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0)

Example 22.1a

Name each compound.

a) on the left shows a 5 carbon chain with a double bond at the 2nd carbon and a methyl group at the 4th carbon. b) on the right shows a 5 carbon chain with a double bond at the 2nd carbon and a methyl group at the 3rd carbon.
(Credit: Introduction to Chemistry: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

Solution

  1. The longest chain containing the double bond has five carbon atoms, so the compound is a pentene (rule 1). To give the first carbon atom of the double bond the lowest number (rule 2), we number from the left, so the compound is a 2-pentene. There is a methyl group on the fourth carbon atom (rule 3), so the compound’s name is 4-methyl-2-pentene.
  2. The longest chain containing the double bond has five carbon atoms, so the parent compound is a pentene (rule 1). To give the first carbon atom of the double bond the lowest number (rule 2), we number from the left, so the compound is a 2-pentene. There is a methyl group on the third carbon atom (rule 3), so the compound’s name is 3-methyl-2-pentene.

Example 22.1b

Name this compound.

a 5 carbon chain with a double bond at the 1st carbon and an ethyl group at the second carbon and a methyl group at the 4th carbon.

Solution:

First, find the longest chain that contains the C=C bond.  Notice that is it not the six-carbon chain across the page.  The longest chain is 5 carbons.  The C=C bond gets the lowest possible number.  This compound is 2-ethyl-4-methylpent-1-ene.

Activity source: Exercise 22.1b is created by Samantha Sullivan Sauer, using images from Biovia Draw, licensed under CC BY-NC 4.0

Beta-carotene (found in spinach), capsanthin and capsorubin (both found in peppers), consist of long chained alkenes. Check out the infographics 22.1a. and 22.1b. for more details.

Spotlight on Everyday Chemistry: Spinach and Bell Peppers

The vitamin A precursor found in both spinach and bell peppers is beta-carotene, an alkene. Beta-carotene consists of a long carbon chain with multiple double bonds with a substituted cyclohexene at each end. In addition, pigments and scents within bell peppers are alkenes. Specifically, chlorophyll contributes to the green pigment found in bell peppers and many other green plants. For more information see infographic 22.1a. and 22.1b.

Infographic 22.1a. Read more about “The Chemistry of Spinach” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 22.1a [New tab].

 

Infographic 22.1b.  Read more about “The Chemistry of Bell Peppers” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 22.1b [New tab].

Naming Cycloalkenes

Cycloalkenes are named similarly, but because there is no chain end to begin from, we number the cycloalkene so that the double bond is between C1 and C2 and the first substituent has as low a number as possible. It’s not necessary to indicate the position of the double bond in the name because it’s always between C1 and C2. As with open-chain alkenes, the newer but not yet widely accepted naming rules place the locant immediately before the suffix in a cyclic alkene.

The structures of 1-methylcyclohexene, 1,4-cyclohexadiene (new name: cyclohexa-1,4-diene) and 1,5-dimethylcyclopentene. The carbon atoms are numbered in all structures.
Figure 22.1h. Examples of naming cycloalkenes. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0)

Example 22.1c

Draw the structure for each compound.

  1. 3-methyl-2-pentene
  2. cyclohexene

Solution

  1. First write the parent chain of five carbon atoms: C–C–C–C–C. Then add the double bond between the second and third carbon atoms:
    A 5 carbon chain with a double bond at the 2nd carbon
    (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Now place the methyl group on the third carbon atom and add enough hydrogen atoms to give each carbon atom a total of four bonds.

a 5 carbon chain with a double bond at the 2nd carbon and a methyl group at the 3rd carbon.
(credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).
  • First, consider what each of the three parts of the name means. Cyclo means a ring compound, hex means 6 carbon atoms, and –ene means a double bond.
    a molecular structure of cyclohexene
    (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Exercise 22.1a

Name each compound.

  1. CH3CH2CH2CH2CH2CH=CHCH3

  2. a 7 carbon chain with a double bond at the 3rd carbon and a methyl group at the 5th carbon.

Check Your Answers:[1]

Exercise source: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0

Spotlight on Everyday Chemistry: Ethiopian Rooster Doro Wat

Doro wat is a common dish served at Christmas in Ethiopia. It consists of a spicy chicken stew with the flavours arising from alkenes and cycloalkenes. Three spices in particular are sotolon (flavour in fenugreek), linalool (flavour in coriander seeds) and 1,8-cineole (flavour in cardamon seeds). Linalool is shown below in Figure 22.1i.

An 8 carbon long straight chained double alkene. A double bond at the 1st carbon and the 6th carbon. An OH group at the 3rd carbon and a methyl group at the 3rd and 7th carbon.
Figure 22.1i. The molecular structure of linalool (credit: Image by Capaccio, CC BY-SA 3.0).

 

For more information on this dish refer to the infographic from Compound Interest: The Chemistry Advent Calendar 2023 (compoundchem.com).

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 have bond angles of 180°, giving these types of bonds a linear shape.

The simplest member of the alkyne series is ethyne, C2H2, commonly called acetylene as shown in Figure 22.1j. 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.
Figure 22.1j. The molecular structure of ethyne. (credit: Chemistry (OpenStax), CC BY 4.0)

Acetylene is used in oxyacetylene torches for cutting and welding metals. The flame from such a torch can be very hot. Most acetylene, however, is converted to chemical intermediates that are used to make vinyl and acrylic plastics, fibers, resins, and a variety of other products.

Alkynes are similar to alkenes in both physical and chemical properties. For example, alkynes undergo many of the typical addition reactions of alkenes. Again, when comparing alkynes to alkanes, refer to Table 20.1b. Properties of Some Alkanes.

Naming Alkynes

The International Union of Pure and Applied Chemistry (IUPAC) names for alkynes parallel those of alkenes, except that the family ending is –yne rather than –ene.

Alkynes follow the same naming rules as alkenes, using the same stem as alkanes, however they end in -yne to identify it as an alkyne.

Alkyne nomenclature follows the general rules for hydrocarbons replacing with the suffix -yne, and the position of the triple bond is indicated by giving the number of the first alkyne carbon in the chain.

  1. Numbering the main chain begins at the end nearer the triple bond so that the triple bond receives as low a number as possible as illustrated in Figure 22.1k.
The structure of 6-methyl-3-octyne or 6-methyloct-3-yne (new) with the numbering of groups starting from the carbon on the right side.
Figure 22.1k. Numbering the alkyne at the end nearer the triple bond. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0)

Compounds with more than one triple bond are called diynes, triynes, and so forth; compounds containing both double and triple bonds are called enynes (not ynenes).

  1. Numbering of an enyne chain starts from the end nearer the first multiple bond, whether double or triple. When there is a choice in numbering, double bonds receive lower numbers than triple bonds as shown in Figure 22.1l.
The structure of 1-hepten-6-yne or hept-1-en-6-yne (new) and 4-methyl-7-nonen-1-yne or 4-methylnon-7-en-1-yne (new). The double bond is shown in blue and the triple bond in pink color.
Figure 22.1l. Examples of naming alkynes when numbering the double bond receives lower numbers than triple bonds. (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0)

For a summary on how to name alkyne, watch Naming Alkynes shown below.

Watch Naming Alkynes – IUPAC Nomenclature & Common Names – Youtube (13 min).

Video Source: The Organic Chemistry Tutor. (2018, April 28). Naming Alkynes – IUPAC Nomenclature & Common Names [Video]. YouTube.

Exercise 22.1b

Draw the structure for each compound.

  1. acetylene
  2. 3-methyl-1-hexyne

Check Your Answers:[2]

Exercise source: Introduction to Chemistry: GOB (v. 1.0), CC BY-NC 3.0

Exercise 22.1c

Name each alkyne.

  1. CH3CH2CH2C≡CH
  2. CH3CH2CH2C≡CCH3

Check Your Answers:[3]

Exercise source: Intro Chem: GOB (v. 1.0), CC BY-NC 3.0

Attribution & References

Except where otherwise noted, this page is adapted by David Wegman from

  1. 1) 2-octene 2) 5-methyl-3-heptene ↵
  2.  a) H–C≡C–H b)A 6 carbon chain with a triple bond at the 1st carbon and a methyl group at the 3rd carbon.

    ↵

  3. a) 1-pentyne b) 2-hexyne ↵
definition

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book