I found over the years that just giving the rules is
overwhelming for naming organic compounds. If you just want the rules click
I am going to take you through all the isomers of methane
Are saturated (all single bonds) hydrocarbons (hydrogen and carbon only).
The rule for naming is they all end with "-ane".
The general formula is CnH2n+2, n is the
number of carbons is used to determine the number of hydrogen atoms. Example
n=5, so H=(2(5) +2)=12
Homologous Series- Did you notice that
as you go down from CH4 to C2H6 (and so on) the
next member is different by 1 carbon and 2 hydrogen?
Now molecular formulas have limitations. You never really know how the
molecule is constructed. So let's look at the structural formulas and name each.
The first 3 alkanes have no isomers (they can only be drawn 1 way). Makes for
a good multiple choice question.
Ignore all the hydrogen's. We only worry about carbon atoms.
Isomers of Butane C4H10
the longest continuous chain of carbon atoms, and end it with -ane.
This is butane, but we call it
n stands for normal
Remaining side chains will be given the ending -yl. They
are placed before the longest chain when naming.
The longest chain is 3 carbons, so "propane". There is one carbon
left. We call this "methyl".
Isomers of Pentane C5H12
Multiple side chains will use prefixes 2 is di-, 3 is tri-, 4 is tetra- and so
Longest chain is 3, so propane
2 separate, one carbon side
chains is dimethyl
Isomers of Hexane C6H14
WAIT....these are different structures with the same names...
When necessary use the lowest number to give the location
of each side chain. You may have to number the longest chain from the right side
to find the lowest number. (NOTE numbers and letters are separated by a
Did you notice we had to number from the right? If we named from the left it
would be 3,3-dimethyl butane.
2+2 vs. 3+3 always use the lowest numbers.
Isomers of Heptane C7H16
Again, each side chain gets a number. Since we have 3, one carbon side
chains, it is called "trimethyl". You better have 3#'s when you have the tri
This has 1(2-carbon) side chain. It is NOT dimethyl. Dimethyl
is 2(1-carbon) side chains. This is an "ethyl" group.
Notice I did not use a number. If was moved one carbon to the
The longest chain becomes hexane.
Isomers of Octane C8H18
(no number needed)
(number it from the right
Rule #5- When there are
2 different side chains name them in alphabetical order using the carbon prefix
(ethyl does not need a #, it can only go on