Some representative alkenes—their names, structures, and physical properties—are given in Table 13.1 “Physical Properties of Some Selected Alkenes”.

We used only condensed structural formulas in Table 13.1 “Physical Properties of Some Selected Alkenes”. Thus, CH₂=CH₂ stands for

The double bond is shared by the two carbon atoms 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 13.1 “Physical Properties of Some Selected Alkenes”—ethene and propene (Figure 13.1 “Ethene and Propene”)—are most often called by their common names—ethylene and propylene, respectively. 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. (For more information about polymers and plastics, see Section 13.5 “Polymers”.) Propylene is also an important industrial chemical. It is converted to plastics, isopropyl alcohol, and a variety of other products. (For more information about alcohols, see Chapter 14 “Organic Compounds of Oxygen”, Section 14.2 “Alcohols: Nomenclature and Classification”.)

Figure 13.1 Ethene and Propene. The ball-and-spring models of ethene/ethylene (a) and propene/propylene (b) show their respective shapes, especially bond angles.

Here are some basic rules for naming alkenes from the International Union of Pure and Applied Chemistry (IUPAC):

1. The longest chain of carbon atoms containing the double bond is considered the parent chain. It is named using the same stem as the alkane having the same number of carbon atoms but ends in –ene to identify it as an alkene. Thus the compound CH₂=CHCH₃ is propene.
2. If there are four or more carbon atoms in a chain, we must indicate the position of the double bond. The carbons atoms are numbered so that the first of the two that are doubly bonded is given the lower of the two possible numbers. The compound CH₃CH=CHCH₂CH₃, for example, has the double bond between the second and third carbon atoms. Its name is 2-pentene (not 3-pentene).

3. Substituent groups are named as with alkanes, and their position is indicated by a number. Thus,

   

   is 5-methyl-2-hexene. Note that the numbering of the parent chain is always done in such a way as to give the double bond the lowest number, even if that causes a substituent to have a higher number. The double bond always has priority in numbering.

Example 2

Draw the structure for each compound.

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

Show Answer

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:

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.

2. 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.

Skill-Building Exercise

Draw the structure for each compound.

1. 2-ethyl-1-hexene

2. cyclopentene

Concept Review Exercises

1. Briefly identify the important distinctions between a saturated hydrocarbon and an unsaturated hydrocarbon.

2. Briefly identify the important distinctions between an alkene and an alkane.

3. Classify each compound as saturated or unsaturated. Identify each as an alkane, an alkene, or an alkyne.

a. 

b. CH₃CH₂C≡CCH₃

c. 

Show Answer

1. Unsaturated hydrocarbons have double or triple bonds and are quite reactive; saturated hydrocarbons have only single bonds and are rather unreactive.

2. An alkene has a double bond; an alkane has single bonds only.

3. a. saturated; alkane  b. unsaturated; alkyne  c. unsaturated; alkene

Exercises

1. Draw the structure for each compound.

   1. 2-methyl-2-pentene
   2. 2,3-dimethyl-1-butene
   3. cyclohexene

2. Draw the structure for each compound.

   1. 5-methyl-1-hexene
   2. 3-ethyl-2-pentene
   3. 4-methyl-2-hexene

3. Name each compound according to the IUPAC system.

   1. 
   2. 
   3. 

4. Name each compound according to the IUPAC system.

   1. 
   2. 
   3. 

Answers

Show Answer

1.a.b. c. 3. a. 2-methyl-2-pentene b. 3-methyl-2-heptene c. 2,5-dimethyl-2-hexene