11.6: IUPAC Nomenclature

Skills to Develop

• To name alkanes by the IUPAC system and write formulas for alkanes given IUPAC names

As noted in previously, the number of isomers increases rapidly as the number of carbon atoms increases. There are 3 pentanes, 5 hexanes, 9 heptanes, and 18 octanes. It would be difficult to assign unique individual names that we could remember. A systematic way of naming hydrocarbons and other organic compounds has been devised by the International Union of Pure and Applied Chemistry (IUPAC). These rules, used worldwide, are known as the IUPAC System of Nomenclature. (Some of the names we used earlier, such as isobutane, isopentane, and neopentane, do not follow these rules and are called common names.) A stem name (Table \(\PageIndex{1}\)) indicates the number of carbon atoms in the longest continuous chain (LCC). Atoms or groups attached to this carbon chain, called substituents, are then named, with their positions indicated by numbers. For now, we will consider only those substituents called alkyl groups.

Table \(\PageIndex{1}\): Stems That Indicate the Number of Carbon Atoms in Organic Molecules

Stem	Number
meth-	1
eth-	2
prop-	3
but-	4
pent-	5
hex-	6
hept-	7
oct-	8
non-	9
dec-	10

An alkyl group is a group of atoms that results when one hydrogen atom is removed from an alkane. The group is named by replacing the -ane suffix of the parent hydrocarbon with -yl. For example, the CH₃ group derived from methane (CH₄) results from subtracting one hydrogen atom and is called a methyl group. The alkyl groups we will use most frequently are listed in Table \(\PageIndex{2}\). Alkyl groups are not independent molecules; they are parts of molecules that we consider as a unit to name compounds systematically.

Table \(\PageIndex{2}\): Common Alkyl Groups

Parent Alkane		Alkyl Group		Condensed Structural Formula
methane		methyl		CH3–
ethane		ethyl		CH3CH2–
propane		propyl		CH3CH2CH2–
		isopropyl		(CH3)2CH–
butane		butyl*		CH3CH2CH2CH2–
*There are four butyl groups, two derived from butane and two from isobutane. We will introduce the other three where appropriate.

Naming Alkanes

Simplified IUPAC rules for naming alkanes are as follows (demonstrated in Example \(\PageIndex{1}\)).

1. Name alkanes according to the LCC of carbon atoms in the molecule (rather than the total number of carbon atoms). This LCC, considered the parent chain, determines the base name, to which we add the suffix -ane to indicate that the molecule is an alkane.

2. If the hydrocarbon is branched, number the carbon atoms of the LCC. Numbers are assigned in the direction that gives the lowest numbers to the carbon atoms with attached substituents. Hyphens are used to separate numbers from the names of substituents; commas separate numbers from each other. (The LCC need not be written in a straight line; for example, the LCC in the following has five carbon atoms.)

3. Place the names of the substituent groups in alphabetical order before the name of the parent compound. If the same alkyl group appears more than once, the numbers of all the carbon atoms to which it is attached are expressed. If the same group appears more than once on the same carbon atom, the number of that carbon atom is repeated as many times as the group appears. Moreover, the number of identical groups is indicated by the Greek prefixes di-, tri-, tetra-, and so on. These prefixes are not considered in determining the alphabetical order of the substituents. For example, ethyl is listed before dimethyl; the di- is simply ignored. The last alkyl group named is prefixed to the name of the parent alkane to form one word.

When these rules are followed, every unique compound receives its own exclusive name. The rules enable us to not only name a compound from a given structure but also draw a structure from a given name. The best way to learn how to use the IUPAC system is to put it to work, not just memorize the rules. It’s easier than it looks.

Example \(\PageIndex{1}\)

Name each compound.

1. 
2. 
3. 

SOLUTION

1. The LCC has five carbon atoms, and so the parent compound is pentane (rule 1). There is a methyl group (rule 2) attached to the second carbon atom of the pentane chain. The name is therefore 2-methylpentane.
2. The LCC has six carbon atoms, so the parent compound is hexane (rule 1). Methyl groups (rule 2) are attached to the second and fifth carbon atoms. The name is 2,5-dimethylhexane.
3. The LCC has eight carbon atoms, so the parent compound is octane (rule 1). There are methyl and ethyl groups (rule 2), both attached to the fourth carbon atom (counting from the right gives this carbon atom a lower number; rule 3). The correct name is thus 4-ethyl-4-methyloctane.

Exercise \(\PageIndex{1}\)

Name each compound.

1. 
2. 
3. 

Example \(\PageIndex{2}\)

Draw the structure for each compound.

1. 2,3-dimethylbutane
2. 4-isopropyl-2-methylheptane

SOLUTION

In drawing structures, always start with the parent chain.

1. The parent chain is butane, indicating four carbon atoms in the LCC.

   

   Then add the groups at their proper positions. You can number the parent chain from either direction as long as you are consistent; just don’t change directions before the structure is done. The name indicates two methyl (CH₃) groups, one on the second carbon atom and one on the third.

   

   Finally, fill in all the hydrogen atoms, keeping in mind that each carbon atom must have four bonds.

   

2. The parent chain is heptane in this case, indicating seven carbon atoms in the LCC.

   –C–C–C–C–C–C–C–

   Methyl is a one-carbon substituent. Isopropyl is a three-carbon substituent, with the middle of the three carbons attached. (Propyl would be a three-carbon substituent, attached at the end.) Adding the groups at their proper positions gives

   

   Filling in all the hydrogen atoms gives the following condensed structural formulas:

   

   Note that the bonds (dashes) can be shown or not; sometimes they are needed for spacing.

If a halogen substituent replaces a hydrogen, then "-gen" ending is left off and it becomes "halo" at the beginning of the name. So a chlorine substituent on the first carbon of propane (CH₃CH₂CH₂Cl) would be 1-chloropropane. Two bromine substituents, both on the second carbon of butane (CH₃CBr₂CH₂CH₃) would be 2,2-dibromobutane.

Naming Cycloalkanes

Naming cycloalkanes is essentially the same as alkanes, but the ring is the parent instead of the longest chain. If there is only one substituent, no number is required because it is always assumed to be #1 (e.g. 1-methylcyclopentane is often just methylcyclopentane). If there are multiple substituents, one of them is always #1 and you count around the ring in whichever direction gives the lowest number. 

In the structure below, there are two one-carbon methyl substituents on a six-carbon cyclohexane ring. The lowest possible numbers in this example would be "1,3" so it would be named 1,3-dimethylcyclohexane. (There are many other incorrect ways to number the carbons here; two incorrect examples are shown.)

 

Naming Alkenes

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 \(\PageIndex{1}\)

Name each compound.

3. F₂CH₂CH₂CH₂CH=CHCH₃

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 four carbon atoms, so the parent compound is a butene (rule 1). (The longest chain overall has five carbon atoms, but it does not contain the double bond, so the parent name is not pentene.) 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 1-butene. There is an ethyl group on the second carbon atom (rule 3), so the compound’s name is 2-ethyl-1-butene.
3. This is a six-carbon chain so the parent is hexene (rule 1). Counting from the right (rule 2), gives the the fist carbon of the double bond the lower number (#2 < #4). Both fluorine substituents are located on the sixth carbon (rule 3), so the name is 6,6-difluoro-2-hexene.

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.

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:

   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. (This could be named 1-cylohexene, but no number is necessary because the double bond is assumed to start on the first carbon.)

Key Takeaway

• Organic compounds have both common names and systematic names, specified by IUPAC.

Exercises

1. Draw the structure for each compound.

   1. 2-methyl-2-pentene
   2. 2,3-dimethyl-1-butene
   3. 3-bromocyclopentene
   4. cis-1-chloro-1-propene

2. Name each compound according to the IUPAC system.

   5. (Include stereochemistry, since shown in structure.)

 

Answers

1.  

   3. 
   4. 

 

2.

1. 2-methyl-2-pentene
2. 3-methyl-2-heptene
3. 2,5-dimethyl-2-hexene
4. cis-1,3-difluorocyclooctane