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Naming Alkanes: IUPAC, Common Names

 

Naming Alkanes: IUPAC, Common Names

The IUPAC System

Ideally, every organic compound should have a name from which its structural formula can be drawn. For this purpose, chemists have adopted a set of rules established by an organization called the International Union of Pure and Applied Chemistry (IUPAC)

.

The IUPAC name of an alkane with an unbranched chain of carbon atoms consists of two parts:

(1) a prefix that indicates the number of carbon atoms in the chain and

(2) the ending -ane to show that the compound is a saturated hydrocarbon.

 

Table 1 gives the prefixes used to show the presence of 1 to 20 carbon atoms.

The first four prefixes listed in Table 2 were chosen by the IUPAC because they were well established in the language of organic chemistry.

In fact, they were well established even before there were hints of the structural theory underlying the discipline.

For example, the prefix but- appears in the name butyric acid, a compound of four carbon atoms formed by the air oxidation of butter fat (Latin: butyrum, butter). Prefixes to show five or more carbons are derived from Greek or Latin numbers.

 

The IUPAC name of an alkane with a branched chain consists of a parent name that indicates the longest chain of carbon atoms in the compound and substituent names that indicate the groups bonded to the parent chain.


Naming Alkanes: IUPAC, Common Names


A substituent group derived from an alkane by the removal of a hydrogen atom is called an alkyl group and is commonly represented by the symbol R-. We name alkyl groups by drop- ping the -ane from the name of the parent alkane and adding the suffixyl. Table 2 gives the names and structural formulas for eight of the most common alkyl groups. The prefix sec- is an abbreviation for secondary, meaning a carbon bonded to two other carbons.

 

Alkyl group: a group derived by removing a hydrogen from an alkane; given the symbol R-.

 

R-: a symbol used to represent an alkyl group.

 

Table 1: Prefixes Used in the IUPAC System to Show the Presence of 1 to 20 Carbons in an Unbranched Chain

Table 1: Prefixes Used in the IUPAC System to Show the Presence of 1 to 20 Carbons in an Unbranched Chain

 


Table 2: Names of the Most Common Alkyl Groups

Table 2: Names of the Most Common Alkyl Groups

 


The prefix tert- is an abbreviation for tertiary, meaning a carbon bonded to three other carbons. Note that when these two prefixes are part of a name, they are always italicized.

 


The rules of the IUPAC system for naming alkanes

1.      The name for an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms in the chain and the ending -ane.

2.     For branched-chain alkanes, take the longest chain of carbon atoms as the parent chain, and its name becomes the root name.

3.     Give each substituent on the parent chain a name and a number. The number shows the carbon atom of the parent chain to which the substituent is bonded. Use a hyphen to connect the number to the name:


The rules of the IUPAC system for naming alkanes


4.     If there is one substituent, number the parent chain from the end that gives it the lower number: 


The rules of the IUPAC system for naming alkanes

5.     If there are two or more identical substituents, number the parent chain from the end that gives the lower number to the substituent encountered first. The number of times the substituent occurs is indicated by the prefix di-, tri-, tetra-, penta-, hexa-, and so on. A comma is used to separate position numbers:


The rules of the IUPAC system for naming alkanes


6.     If there are two or more different substituents, list them in alphabetical order, and number the chain from the end that gives the lower number to the substituent en- countered first. If there are different substituents in equivalent positions on opposite ends of the parent chain, the substituent of lower alphabetical order is given the lower number:


The rules of the IUPAC system for naming alkanes


7.      The prefixes di-, tri-, tetra-, and so on are not included in alphabetizing. Neither are the hyphenated prefixes sec- and tert-. “Iso,” as in isopropyl, is included in alphabetizing. Alphabetize the names of the substituents first, and then insert the prefix. In the following example, the alphabetizing parts are ethyl and methyl, not ethyl and dimethyl:


The rules of the IUPAC system for naming alkanes


 

Example 1

Write IUPAC names for these alkanes:


Write IUPAC names for these alkanes:


Strategy

First determine the root name of the alkane. Then name the substituents and place them in alphabetical order. Number the parent  chain so as  to give the lower number to the substituents encountered first. If substituents have equivalent positions, the lower number is assigned to the substituents with the lower alphabetical order.

 

Solution

Write IUPAC names for these alkanes:


 

Common Names

In the older system of common nomenclature, the total number of carbon atoms in an alkane, regardless of their arrangement, determines the name. The first three alkanes are methane, ethane, and propane. All alkanes with the molecular formula C4H10 are called butanes, all those with the molecular formula C5H12 are called pentanes, and all those with the molecular formula C6H14 are called hexanes. For alkanes beyond propane, iso indicates that one end of an otherwise unbranched chain terminates in a (CH3)2CH group. Following are examples of common names:


Naming Alkanes: IUPAC, Common Names


This system of common names has no good way of handling other branching patterns, so, for more complex alkanes, it is necessary to use the more flexible IUPAC system of nomenclature.

In this text, we concentrate on IUPAC names. However, we also use common names, especially when the common name is used almost exclusively in the everyday discussions of chemists and biochemists. When both IUPAC and common names are given in the text, we always give the IUPAC name first, followed by the common name in parentheses. In this way, you should have no doubt about which name is which.

 


Classification of Carbon and Hydrogen Atoms

We classify a carbon atom as primary (1°), secondary (2°), tertiary (3°), or quaternary (4°), depending on the number of carbon atoms bonded to it. A carbon bonded to one carbon atom is a primary carbon; a carbon bonded to two carbon atoms is a secondary carbon, and so forth.

For example, propane contains two primary carbons and one sec- ondary carbon, 2-methylpropane contains three primary carbons and one tertiary car- bon, and 2,2,4-trimethylpentane contains five primary carbons, one secondary carbon, one tertiary carbon, and one quaternary carbon:


Classification of Carbon and Hydrogen Atoms


Similarly, hydrogens are also classified as primary, secondary, or tertiary, depending on the type of carbon to which each is bonded. Those bonded to a primary carbon are classified as primary hydrogens, those on a secondary carbon are secondary hydrogens, and those on a tertiary carbon are tertiary hydrogens.

 

 

Example 2

Classify each carbon atom in the following compounds as

1°, 2°, 3°, or 4°.


Classify each carbon atom in the following compounds as  1°, 2°, 3°, or 4°.


 

Strategy

To classify carbons, determine whether each is bonded to 1 carbon (1°), 2 carbons (2°), 3 carbons (3°), or 4 carbons (4°).

 

Solution

To classify carbons, determine whether each is bonded to 1 carbon (1°), 2 carbons (2°), 3 carbons (3°), or 4 carbons (4°).


 

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