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Basic Naming Alkanes Rules
For you to really understand this, you need an example. So, throughout
this guide, I am referring to this molecule when I am explaining.
*Note: All the hydrogen is all understood to exist.
1. Find the longest chain. This chain may just be a single straight line or
it may turn. Count the number of carbons in this chain. This will be the
last piece of your name.
2. Are there double/triple bonds? If there are, then make sure that you
have the appropriate suffix (ene for double bonds, yne for triple bonds).
In this example, there are eight carbons in the longest chain, and there is a
double bond, so the last piece of the name will be "octene".
If you have multiple bonds, then you must also add a number in
front of this last name.
Count your carbons. On the carbon that has the multiple bond, put
its number in front of the last name. Since the double bond in the
octene is on the third carbon, then the last piece of the name will
now be 3octene.
3. Now look at substituent groups. Find the group that has the lowest
name alphabetically. Here, we have one ethyl group and two methyls.
Since ethyl comes before methyl in the alphabet, we will look at it first.
How many of these molecules are attached? Here, we have only one
ethyl, so we will write "ethyl". There are two methyls, so attach a
prefix and write "dimethyl".
What are the positions of the groups? The ethyl is attached to
carbon 5, so we will write "5ethyl". The methyl groups are attached
to carbons 2 and 6, so we write "5ethyl 2,6dimethyl".
4. If you have any cis or trans, they go at the very beginning of the
5. Now put the pieces together. The piece describing the substituent
groups goes first. The piece describing the longest chain goes last. So, the
name of this molecule is "5ethyl 2,6dimethyl 3octene".