13.2: Naming Alkenes and Alkynes

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Learning Objectives

Objective 1
Objective 2

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

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.
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).
Substituent groups are named as with alkanes, and their position is indicated by a number. Thus, the structure below 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 \(\PageIndex{1}\)

Name each compound.

Solution

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.
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.

Exercise \(\PageIndex{1}\)

Name each compound.

CH₃CH₂CH₂CH₂CH₂CH=CHCH₃

Just as there are cycloalkanes, there are cycloalkenes. These compounds are named like alkenes, but with the prefix cyclo- attached to the beginning of the parent alkene name.

Example \(\PageIndex{2}\)

Draw the structure for each compound.

3-methyl-2-pentene
cyclohexene

Solution

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.

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.

Exercise \(\PageIndex{2}\)

Draw the structure for each compound.

2-ethyl-1-hexene
cyclopentene

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, \(\mathrm{CH_3CH_2C≡CH}\) is called 1-butyne.

Example \(\PageIndex{6}\): 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.

Exercise \(\PageIndex{6}\)

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°