First Rules for Naming Alkanes

Previous: Drawing Simple Alkanes

To consolidate, so far we have applied three rules for naming simple alkanes.

Rule 1) Find the longest chain of carbons. (It does NOT have to be straight.) This goes at the end of the name.

Rule 2) Identify any side branches. These go just before the longest chain name.

Rule 3) If a side chain occurs more than once, indicate how many time it occurs. This goes just before the branch name. di - two, tri = three, tetra = four. After that the prefixes are the same as for chain-length.

 

On to C6H14, where we find a new complication.

The usual candidates are:

hexane
CH3-CH2-CH2-CH2-CH2-CH3
 
methyl pentane
CH3-CH2-CH2-CH-CH3
 
|
 
 
CH3
 
CH3-CH2-CH-CH2-CH3
 
|
 
 
CH3
 
 
and dimethyl butane
 
CH3
 
 
|
 
CH3-CH2-C-CH3
 
|
 
 
CH3
 
 
CH3
   
 
|
   
CH3-CH-CH-CH3
    |
    CH3
 

Alas! We find two methylpentanes and two dimethylbutanes! How can we tell them apart? Indeed, are they really different?

The answer to the second question is yes. Were you to assemble models of each of the methylpentanes you would find no way to make them identical without breaking bonds. In real life (ie, chemically speaking) they have different melting points, different boiling points and produce different products in reactions. So they really are different chemical compounds.

How can we tell them apart? Name them. Add another rule to our list:

Rule 4) When necessary, indicate which carbon the side-branch comes from by counting from the end of the main chain that gives the lowest possible number. The left methyl pentane is 2-methyl pentane and the right one is 3-methyl pentane. Note that the two molecules below are both 2-methyl pentane.

  

5 CH3-4CH2-3CH2-2CH-1CH3     1CH3-2CH-3CH2-4CH2-5CH3

  |                          |

     CH3                      CH3

As a matter of passing, have you already realised that the two dimethyl butanes are 2,2-dimethyl butane and 2,3-dimethyl butane? There is no such compound name as 3,3-dimethyl butane. (Itís 2,2-dimethyl butane named from the wrong end.)

 

Let us look at a problem raised when we get to C7H16. By now you should be able to build and/or draw heptane and two different methylhexanes.

Remember. 

There are exactly four different dimethylpentanes. Can you draw/build them all? Can you find exactly one trimethylbutane?

This is ethylpentane.

CH3-CH2-CH-CH2-CH3

|

   CH2

|

   CH3

Whatís this?

CH3-CH2-CH2-CH-CH3

           |

              CH2

           |

              CH3

Why would you be wrong if you said 2-ethylpentane? (HINT: Go back to rule 1.)

Next:More Naming Rules

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