From A guide to naming organic compounds. This is Section 2:
Structural Isomerism

To start off this section I would like to show you a molecule:

            H   H   H   H
            |   |   |   |
        H - C - C - C - C - H
            |   |   |   |
            H   H H-C-H H
                    |
                  H-C-H
                    |
                  H-C-H
                    |
                    H
What would we call this molecule? When you think about it, it's really just a hexane with a methane hanging off it. Why not a butane with a propane attached? Because, as I said in Section 1, you have to find the longest carbon chain.

To find the length of the main chain here you need to find the longest one. So lets start counting from the bottom one and then, when you get to the join, start counting left. We don't go right because that would not make it the longest one. (remember that a chain can bend on paper, it'll still be a chain)

The longest chain there has six carbon atoms and would therefore be known as Hexane, but what about that Methane that is hanging off it? We can't simply call the compound methyl-Hexane because that doens't give us enough information to draw it again, we have to specify enough information to tell someone how to draw the compound as we see it again, not a variant of the compound. In order to do that we have to explain where that Methane is coming from. The name of the above example is 3-methylhexane; the three denotes the location of the methane.

It is worthy to note that the above example can also be drawn as it is below:

               H   H   H   H   H   H
               |   |   |   |   |   |
           H - C - C - C - C - C - C - H
               |   |   |   |   |   |
               H   H H-C-H H   H   H
                       |
                       H
Where did I get the three from in the above name? Well, what you need to do is start counting from the end of the carbon chain that will result in the lowest number. You see, 4-methylhexane would give us enough information, but technically it's better to say 3-methylhexane.

Another molecule:

                         H
                         |
                       H-C-H
                         |
                       H-C-H
                         |
             H   H   H H-C-H H   H   H   H
             |   |   |   |   |   |   |   |
         H - C - C - C - C - C - C - C - C - H
             |   |   |   |   |   |   |   |
             H   H H-C-H H   H   H   H   H
                     |        
                 H H-C-H        
                 |   |        
             H - C - C-H      
                 |   |
                 H   H
The name of the above compound is 5-ethyl,6-propyldecane. It's a bit of a mouthful, but it's simple to come up with.

If I replace all of the carbons in the longest chains with numbers then we'll see how I came up with the name:

                         H
                         |
                       H-C-H
                         |
                       H-C-H
                         |
             H   H   H H-C-H H   H   H   H
             |   |   |   |   |   |   |   |
         H - C - C - 5 - 6 - 7 - 8 - 9 - 10- H
             |   |   |   |   |   |   |   |
             H   H H-4-H H   H   H   H   H
                     |        
                 H H-3-H        
                 |   |        
             H - 1 - 2-H      
                 |   |
                 H   H
The longest chain there is 10 atoms long as you can see, so the base name will be decane. The reason why I numbered the decane the way I did was so that the other carbon chains that come off of the base-decane have the lowest numbers possible. If I had numbered it the other way then it would be called 5-propyl,6-ethyldecane. It's still got the information, but is technically incorrect. Also, don't make the mistake that I made when I first wrote this node, when you write out the name of the molecule the alkyl groups are put in alphabetical order (5-ethyl,6-propyldecane, not 6-propyl,5-ethyldecane)

I imagine that you can already see how complex this system could make naming many molecules, but it's certainly alot better than the previous 'system'. Essentially this will only get confusing if you try to do everything at once. If you see a molecule and you have to name it, take it step by step. Ask yourself "Which of these is the longest chain?" and then 'If I number the carbons, which will result in the name having the lowest numbers?". Don't try to do everything all at once.

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