20.3 Isomers of Alkanes and IUPAC Nomenclature

Learning Objectives

By the end of this section, you will be able to:

  • Identify simple alkanes as straight-chain or branched-chain.
  • Describe and recognize structural and functional group isomers.
  • Name alkanes by the IUPAC system and write formulas for alkanes given IUPAC names

Isomers

Hydrocarbons with the same formula, including alkanes, can have different structures. For example, two alkanes have the formula C4H10: They are called n-butane and 2-methylpropane (or isobutane), and have the following structural formulas as shown in Figure 20.3a:

The figure illustrates three ways to represent molecules of n dash butane and 2 dash methlylpropane. In the first row of the figure, Lewis structural formulas show element symbols and bonds between atoms. The n dash butane molecule shows 4 carbon atoms represented by the letter C bonded in a straight horizontal chain with hydrogen atoms represented by the letter H bonded above and below all carbon atoms. H atoms are bonded at the ends to the left and right of the left-most and right-most C atoms. In the second row, ball-and-stick models are shown. In these representations, bonds are represented with sticks, and elements are represented with balls. Carbon atoms are black and hydrogen atoms are white in this image. In the third row, space-filling models are shown. In these models, atoms are enlarged and pushed together, without sticks to represent bonds. The molecule names are provided in the fourth row.
Figure 20.3a. Structural and ball-and-stick formulas for butane and 2-methylpropane (credit: General Chemistry 1 & 2 , CC BY 4.0).

The compounds n-butane and 2-methylpropane are structural isomers (the term constitutional isomers is also commonly used). Constitutional isomers have the same molecular formula but different spatial arrangements of the atoms in their molecules. The n-butane molecule contains an unbranched chain, meaning that no carbon atom is bonded to more than two other carbon atoms. We use the term normal, or the prefix n, to refer to a chain of carbon atoms without branching. The compound 2–methylpropane has a branched chain (the carbon atom in the center of the structural formula is bonded to three other carbon atoms).

Recall from section 20.2, Table 20.3a. which shows the molecular formula and condensed structural formulas for the first 10 straight-chain alkanes. Table 20.3a also shows the number of isomers for each alkane. The number of isomers increases rapidly as the number of carbon atoms increases.

Table 20.3a. The First 10 Straight-Chain Alkane Formulas and Isomers
Name Molecular Formula (CnH2n + 2) Condensed Structural Formula Number of Possible Isomers
methane CH4 CH4 —
ethane C2H6 CH3CH3 —
propane C3H8 CH3CH2CH3 —
butane C4H10 CH3CH2CH2CH3 2
pentane C5H12 CH3CH2CH2CH2CH3 3
hexane C6H14 CH3CH2CH2CH2CH2CH3 5
heptane C7H16 CH3CH2CH2CH2CH2CH2CH3 9
octane C8H18 CH3CH2CH2CH2CH2CH2CH2CH3 18
nonane C9H20 CH3CH2CH2CH2CH2CH2CH2CH2CH3 35
decane C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 75

Table source: “12.2: Structures and Names of Alkanes” In Basics of GOB Chemistry (Ball et al.), CC BY-NC-SA 4.0.

Identifying isomers from structural formulae is not as easy as it looks. Structural formulae that look different may actually represent the same isomers. For example, the three structures in Figure 20.3b all represent the same molecule, n-butane, and hence are not different isomers. They are identical because each contains an unbranched chain of four carbon atoms.

The figure illustrates three ways to represent molecules of n dash butane. In the first row of the figure, Lewis structural formulas show carbon and hydrogen element symbols and bonds between the atoms. The first structure in this row shows three of the linked C atoms in a horizontal row with a single C atom bonded above the left-most carbon. The left-most C atom has two H atoms bonded to it. The C atom bonded above the left-most C atom has three H atoms bonded to it. The C atom bonded to the right of the left-most C atom has two H atoms bonded to it. The right-most C atom has three H atoms bonded to it. The C atoms and the bonds connecting all the C atoms are red. The second structure in the row similarly shows the row of three linked C atoms with a single C atom bonded below the C atom to the left. The left-most C atom has two H atoms bonded to it. The C atom bonded below the left-most C atom has three H atoms bonded to it. The C atom bonded to the right of the left-most C atom has two H atoms bonded to it. The right-most atom has three H atoms bonded to it. All the C atoms and the bonds between them are red. The third structure has two C atoms bonded in a row with a third C atom bonded above the left C atom and the fourth C atom bonded below the right C atom. The C atom bonded above the left C atom has three H atoms bonded to it. The left C atom has two H atoms bonded to it. The right C atom has two H atoms bonded to it. The C atom bonded below the right C atom has three H atoms bonded to it. All the C atoms and the bonds between them are red. In the second row, ball-and-stick models for the structures are shown. In these representations, bonds are represented with sticks, and elements are represented with balls. Carbon atoms are black and hydrogen atoms are white in this image. In the third row, space-filling models are shown. In these models, atoms are enlarged and pushed together, without sticks to represent bonds.
Figure 20.3b. These three representations of the structure of n-butane are not isomers because they all contain the same arrangement of atoms and bonds (credit: General Chemistry 1 & 2 , CC BY 4.0.)

When identifying isomers, it is useful to trace the carbon backbone with your finger or a pencil and count carbons until you need to lift your hand or pencil to get the another carbon. Try this with each of the above arrangements of four carbons above in Figure 20.3b. Butane has a continuous chain of four carbons no matter how the bonds are rotated – you can connect the carbons in a line without lifting your finger from the page. In a later portion of this chapter, you will learn how to systematically name compounds by counting the number of carbons in the longest continuous chain and identifying any functional groups present.

Adding one more carbon to the butane chain gives pentane, which has the formula, C5H12. Pentane and its two branched-chain isomers are shown below in Figure 20.3c. The compound at the far left is pentane because it has all five carbon atoms in a continuous chain. The compound in the middle is isopentane; like isobutane, it has a one CH3 branch off the second carbon atom of the continuous chain. The compound at the far right, discovered after the other two, was named neopentane (from the Greek neos, meaning “new”). Although all three have the same molecular formula, they have different properties, including boiling points: pentane, 36.1°C; isopentane, 27.7°C; and neopentane, 9.5°C. The names isopentane and neopentane are common names for these molecules. As mentioned above, we will learn the systematic rules for naming compounds next.

Structural formulas representing 3 isomers of pentane. From the left side there is pentane, isopentane and neopetane (right)
Figure 20.3c. Structural formulas representing 3 isomers of pentane. (Credit: Introduction to Chemistry: GOB (V. 1.0). ,CC BY-NC-SA 3.0.)
A summary of isomers is illustrated in infographic 20.3a, which includes structural isomerism and stereoisomerism.

Infographic 20.3a.  Read more about “A Brief Guide to Types of Isomerism in Organic Chemistry” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 20.3a [New tab].

IUPAC System of Nomenclature for Alkanes

Looking at Table 20.3a., 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. A stem name (Table 20.3b.) 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 20.3b. 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

Table source: “12.5: IUPAC Nomenclature” In Basics of GOB Chemistry (Ball et al.), CC BY-NC-SA 4.0.

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 20.3c. Alkyl groups are not independent molecules; they are parts of molecules that we consider as a unit to name compounds systematically.

Table 20.3c. Common Alkyl Groups
Parent Alkane Alkyl Group Condensed Structural Formula
methane One carbon atom connected to four hydrogen atoms, CH4 methyl One carbon atom connected to three hydrogen atoms with opening for one more bond - a branch not a molecule CH3–
ethane One carbon atom connected to three hydrogen atoms. This carbon atom connected to another carbon with three more hydrogens, CH3CH3 ethyl One carbon atom connected to three hydrogen atoms connected to a carbon with two hydrogens with opening for one more bond - a branch not a molecule CH3CH2–
propane One carbon atom connected to three hydrogen atoms. This carbon atom connected to another carbon with two hydrogen atoms. This carbon connected to another carbon with three hydrogens, CH3CH2CH3 propyl One carbon atom connected to three hydrogen atoms connected to a carbon with two hydrogens connected to a carbon with two hydrogens with opening for one more bond - a branch not a molecule CH3CH2CH2–
isopropyl One carbon atom connected to three hydrogen atoms connected to a carbon with one hydrogen with opening for one more bond connected to a carbon with three hydrogens - a branch not a molecule (CH3)2CH–
butane One carbon atom connected to three hydrogen atoms. This carbon atom connected to another carbon with two hydrogen atoms. This carbon atom connected to another carbon with two hydrogen atoms. This carbon connected to another carbon with three hydrogens, CH3CH2CH2CH3 butyl* One carbon atom connected to three hydrogen atoms connected to a carbon with two hydrogens connected to a carbon with two hydrogens connected to a carbon with two hydrogens with opening for one more bond - a branch not a molecule CH3CH2CH2CH2–

Table 20.3c. note: *There are four butyl groups, two derived from butane and two from isobutane. We will introduce the other three where appropriate. (Image credits:  Introduction to Chemistry: GOB (V. 1.0). , CC BY-NC-SA 3.0).

Simplified IUPAC rules for naming alkanes are as follows (demonstrated in Example 20.3a.).

  1. Name alkanes according to the LCC (longest continuous chain) 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.)
    An image showing that the longest carbon chain does not need to be in a straight line. In this case, the chain is branched and the longest chain is 5 carbons as opposed to if you kept it in the straight line, the chain is only 4 carbons long.
    Figure 20.3d. An example showing that finding the longest carbon chain does not have to be in a straight line. (Credit: Introduction to Chemistry: GOB (V. 1.0). ,CC BY-NC-SA 3.0.)
  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 20.3a

Name the molecule whose structure is shown here:
A chain of six carbon atoms, numbered 6, 5, 4, 3, 2, and 1 is shown. Bonded above carbon 3, a chain of two carbons is shown, numbered 1 and 2 moving upward. H atoms are present directly above, below, left and right of all carbon atoms in positions not already taken up in bonding to other carbon atoms.

Solution

The longest carbon chain runs horizontally across the page and contains six carbon atoms (this makes the base of the name hexane, but we will also need to incorporate the name of the branch). In this case, we want to number from right to left (as shown by the blue numbers) so the branch is connected to carbon 3 (imagine the numbers from left to right—this would put the branch on carbon 4, violating our rules). The branch attached to position 3 of our chain contains two carbon atoms (numbered in red)—so we take our name for two carbons eth- and attach -yl at the end to signify we are describing a branch. Putting all the pieces together, this molecule is 3-ethylhexane.

Example & image source: General Chemistry 1 & 2 , CC BY 4.0.

Exercise 20.3a

Name the following molecule:
This figure shows a C atom bonded to three H atoms and another C atom. This C atom is bonded to two H atoms and third C atom. The third C atom is bonded to two H atoms and a fourth C atom. The fourth C atom is bonded to two H atoms and a fifth C atom. This C atom is bonded to an H atom, a sixth C atom in the chain, and another C atom which appears to branch off the chain. The C atom in the branch is bonded to two H atoms and another C atom. This C atom is bonded to two H atoms and another C atom. This third C atom appears to the left of the second and is bonded to three H atoms. The sixth C atom in the chain is bonded to two H atoms and a seventh C atom. The seventh C atom is bonded to two H atoms and an eighth C atom. The eighth C atom is bonded to three H atoms.

Check Your Answer[1]

Exercise & image source: General Chemistry 1 & 2 , CC BY 4.0.

Example 20.3b

Name each compound:

There are 3 chemical structures. From left to right: A 5 carbon chain with a methyl group at the 2nd carbon; a 6 carbon chain with a methyl group at the 2nd and 4th carbon; and a 8 carbon chain with a ethyl and a methyl at the 4th carbon.

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.

Example & image source: Introduction to Chemistry: GOB (V. 1.0).,  CC BY-NC-SA 3.0

Exercise 20.3b

Name each compound.

There are 3 chemical structures. From left to right: a 5 carbon chain with a methyl group at the 3rd carbon; a 5 carbon chain with two methyl groups at the 2nd carbon; and lastly, a 9 carbon chain with two methyl groups at the 3rd carbon.

Check your answer[2]

Exercise & image source: Introduction to Chemistry: GOB (V. 1.0).,  CC BY-NC-SA 3.0

Example 20.3c

Draw the structure for each compound.

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

Solution

In drawing structures, always start with the parent chain.

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

a carbon chain labelled 1 to 4

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.

a carbon chain labelled 1 to 4. At the 2nd and 3rd carbon there is a methyl group.

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

A 4 carbon chain with a methyl group at the 2nd and 3rd carbon.

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

Adding the groups at their proper positions gives

a 7 carbon chain with a methyl group at the 2nd carbon and a isopropyl group at the 4th carbon.

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

Two chemical structures. On the left there is a 7 carbon chain with a methyl group at the 2nd carbon and an isopropyl group at the 4th carbon. On the right there is an 8 carbon chain with a methyl group at the 2nd carbon and a isopropyl group at the 4th carbon.

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

Example & image source: Introduction to Chemistry: GOB (V. 1.0).,  CC BY-NC-SA 3.0

Exercise 20.3c

Draw the structure for each compound.

4-ethyloctane

  1. 3-ethyl-2-methylpentane
  2. 3,3,5-trimethylheptane

Check your answer [3]

 

Want more practice naming alkanes?

Watch the video tutorial Naming simple alkanes (10 mins) on YouTube to review the nomenclature process.

Video source: Khan Academy. (2010, July 21) Naming simple alkanes [Video]. YouTube.

For a summary on naming organic compounds, infographic 20.3b looks at the rules for decoding the types of organic compounds and how to name them.

Infographic 20.3b.  Read more about “A Basic Guide to Decoding Organic Compound Names” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary [New tab].

Links to Enhanced Learning

For a general introduction to organic chemistry naming beyond the basic alkane naming, watch The Basics of Organic Nomenclature: Crash Course Organic Chemistry #2 – YouTube.

For interactive practice questions on isomers link to Organic Chemistry Practice from eCampusOntario H5P Studio.

Attribution & References

Except where otherwise noted, this page is adapted by Adrienne Richards from the following sources

  1. 4-propyloctane ↵
    1. 3-methylpentane
    2. 2, 2-dimethylpentane
    3. 2-ethyl 2-methyloctane

    ↵

    1. See the image: 4-Ethyloctane | C10H22 | CID 85925 - PubChem (nih.gov)
    2. See the image: 3-Ethyl-2-methylpentane | C8H18 | CID 11863 - PubChem (nih.gov)
    3. See the image: 3,3,5-Trimethylheptane | C10H22 | CID 23544 - PubChem (nih.gov) ↵
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Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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