Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Carbon Functional Groups and Organic Nomenclature
2.7 Structure and Nomenclature of Alkenes

2.7.1 Nomenclature of Branched Alkenes

For the nomenclature of alkenes, there are only minor changes to rules 1 through 3 that were used for the nomenclature of alkanes.

Rule #1 (in the naming of alkenes): Determine the longest continuous chain that contains the alkene functionality. Note that there may be other chains that are longer than the one that has the carbon—carbon double bond, but the longest chain that contains the carbon—carbon double bond is used to derive the names of alkenes.

Rule #2 Assign numbers to each carbon of this longest continuous chain that contains the double bond. Start the assignment so that the end closest to the double bond gets number 1.

Rule #3 Use Table 2.3 to determine the name of the parent or root name and specify the position of the double bond with a number. Based on the numbering of carbons present in the compound, a root or parent name is derived by changing the -ane of the corresponding alkane to -ene. Thus, a compound that has six carbons and contains a carbon—carbon double bond is a hexene, and a number is used to indicate the position of the double bond. The rest of the rules are the same as those discussed for the naming of alkanes. The IUPAC name for an alkene is shown below.

For alkenes that have the carbon—carbon double bond in position 1, the use of 1 is optional.

An isomer of pentene, however, that has the double bond in a position other than number 1, the position of the double bond must be specified so that the position of the double bond gets the lowest possible number.

Problem 2.13

1. Give the line-angle structures of the following molecules

1. 2-Methyl-2-pentene

2. 2,5-Dimethyl-3-hexene

3. 2,3-Dimethyl-2-butene

4. 3,4,4-Trimethyl-1-pentene

2. Give IUPAC names for the following molecules.

2.7.2 Nomenclature of Polyenes

Compounds that have two double bonds are named as dienes, and the position of each double bond must be specified with appropriate numbers as shown in the example below.

A great majority of organic compounds found in nature have alternating double bonds and such compounds are referred to as having conjugated double bonds. Isoprene is one of the simplest conjugated systems, but conjugated systems that have extended conjugation are typically highly colored compounds. Examples of conjugate polyenes, along with their IUPAC names, are shown below.

As shown in the examples above, numbers are used to describe the locations of the double bonds of polyenes.

Problem 2.14

i. Give the structures of the following molecules.

1. 2,4-Dimethyl-1,4-pentadiene

2. 4-Chloro-2,6-dimethyl-1,3,5-heptatriene

ii. Give the IUPAC names for the following molecules.

2.7.3 Nomenclature of Cyclic Alkenes

Cyclic compounds that have a double bond are named as a cycloalkenes. In naming cycloalkenes, one of the carbons of the double bond is assigned position 1 and the other carbon of the double bond is assigned position 2. In other words, number in the direction of the double bond as shown in the example below.

For cycloalkenes that have branches, number the cycloalkene in the direction of the double bond and also in the direction so that the branch gets the lowest possible number as shown in the example below.

The correct IUPAC name for the compound above is:

The number 1 for the double bond is optional, and often not used, in naming organic compounds.

Problem 2.15

i. Give the IUPAC names for the following cyclic alkenes.

ii. Give line-angle structures for the following cyclic alkenes.

1. 1,2-Dimethylcyclohexene

2. 2-Bromo-3-methylcyclopentene