Alkane Nomenclature
Introduction
The elements on the periodic table can be used to create an infinite number of possible molecules. Even
with a relatively small number of given atoms, there can be many different ways of assembling those
atoms into actual molecules. Scientists need a way of identifying molecules uniquely. One way is to
draw the structure, but it is also useful to give each molecule a unique name. In Unit 4/5, you learned
how to write the formulas and names for simple ionic, covalent and acid compounds. In this activity,
we will investigate the rules for naming a group of organic compounds called alkanes, the simplest
hydrocarbons. Hydrocarbons are compounds made of only carbon and hydrogen and include many of
the molecules that you have heard of, like methane, butane and propane.
Information
The simplest hydrocarbons are called alkanes. They contain only single bonds. The simplest alkane is
methane, CH4. Some other simple alkanes are represented in the line drawings below. Line drawings
are easier to draw than Lewis dot diagrams for hydrocarbons, because they can get very long!
The first ten “straight-chain” alkanes are listed in the table below. They are called “straight-chain”
alkanes, because all the carbons are bonded in a row.
Formula
Alkane
Radical
CH4
methane
methyl
C2H6
ethane
ethyl
C3H8
propane
propyl
C4H10
butane
butyl
C5H12
pentane
pentyl
C6H14
hexane
hexyl
C7H16
heptane
heptyl
C8H18
octane
octyl
C9H20
nonane
nonyl
C10H22
decane
decyl
The line drawing tells you that there is a carbon atom at the end of each line and each carbon atom has
the number of hydrogen atoms needed to give a total of four bonds. Use a model kit to build pentane.
What is the difference between the bold lines and dashed lines in the structural formula?
Copied with permission from Foundations of Chemistry: Applying POGIL Principles, by David Hanson
Pacific Crest Publishing
If you think about it for a little while, you can see a general pattern. All the carbons in a “straightchain” alkane have two hydrogen atoms attached, except the two end atoms. Those two atoms have
three hydrogen atoms. Thus if an alkane was n carbons long, it would have 2n hydrogen atoms plus 2
more for the end atoms. We can say that the general formula for an alkane is CnH2n + 2. All of the
formulas in the table above follow this rule.
Exercises
1. Name the following hydrocarbons and write their formulas.
2. Draw a line drawing of a “straight-chain” alkane with 12 carbon atoms. Use the general formula to
determine the number of hydrogen atoms that it has.
Information
Straight-chain alkanes are the simplest ones, but most alkanes have branched structures. They are
isomers of the straight-chain alkanes. Isomers are molecules with the same chemical formula and often
with the same kinds of bonds between atoms, but in which the atoms are arranged differently. Isomers
have different structural formulas. For example, butane has an isomer called 2-methyl propane. The
compound 3-methylheptane is an isomer of octane.
Since they are isomers of each other, the compounds have the same formulas, C4H10 for the first pair
and C8H18 for the second pair (you should verify this). In fact, all alkanes follow the CnH2n + 2 formula.
To name a branched hydrocarbon we need to use the concept of a radical. A radical is a fragment of a
hydrocarbon missing a hydrogen atom. For example, a methyl radical is a methane molecule minus a
hydrogen atom.
Copied with permission from Foundations of Chemistry: Applying POGIL Principles, by David Hanson
Pacific Crest Publishing
The methyl radical has a free electron and wants to form another bond. It can take the place of a
hydrogen atom of some alkane and make the molecule bigger.
Any alkane can be turned into a radical by removing a hydrogen atom. The table on page 2 gives the
names of the radicals derived from the straight-chained alkanes. To uniquely name an alkane, you need
to do three things: 1) identify the longest chain to obtain the root name of the molecule, 2) identify the
radicals bound to the longest chain, and 3) assign numbers to tell where the radicals are located.
The longest chain has 8 carbon atoms, so the molecule is an octane. It has an ethyl group bound to
carbon #3, and a methyl group bound to carbon #4. We call it 3-ethyl-4-methyloctane. Ethyl comes
before methyl because we put the radicals in alphabetical order. Another example is:
You might think the root would be hexane, but it is not. The root is octane,
because we have to follow the longest chain and the chain curves upward.
There is a methyl group on the chain, so the compound is methyloctane.
But what is the number? Is it 5-methyloctane or 4-methyloctane?
The rule is simple-always use the smaller number, so the name is 4-methyloctane.
Copied with permission from Foundations of Chemistry: Applying POGIL Principles, by David Hanson
Pacific Crest Publishing
What if you have more than one of the same radical? Use the prefixes you learned for covalent
compounds:
There is one more rule that you will need for complicated molecules. Sometimes the radical is
branched. For example, consider the following molecules:
In the second molecule, the three carbon radical is branched, so you have to be more specific. If you
are lucky, you won’t find too many of these complicated ones to name.
Exercises
3. Apply the rules and name the following alkanes.
4. Draw the structures for the following alkanes:
a. 4-methyldecane
b. 2, 3-dimethyl-4-propyloctane
c. 3-ethyl-4,4,5-triethylnonane
Turn in #1-4 for extra credit!!!
This is our little secret! If I find out you shared, your points will be forfeited!
Copied with permission from Foundations of Chemistry: Applying POGIL Principles, by David Hanson
Pacific Crest Publishing