Drawing Simple Alkanes

(semi-structural formulae)

Previous: Naming Simple Alkanes

If I were to draw a methane molecule it might look like this: 

CH4.

Ethane and propane could be drawn thus:

CH3-CH3    and    CH3-CH2-CH3

The next molecule in the series has the molecular formula C4H10 , so might look like this:

CH3-CH2-CH2-CH3

 

or this:

CH3-CH-CH3
 
|
  
 
CH3
  

 

The first is, indeed, butane. The second is called methyl propane. That is, it is propane because its longest chain of carbon atoms is three carbons in a row (prop-), but it has a side chain that is one carbon long (meth-). The two molecules have the same molecular formula, C4H10, but different structural formulae, as shown. Therefore they are isomers.

 

The next molecule in the series has the molecular formula C5H12.
     
     
CH3-CH2-CH2-CH2-CH3
     
     
  CH3  
  |  
CH3-CH2-CH-CH3
     
     
 
CH3
  
 
|
  
CH3-CH-CH3
 
|
  
 
CH3
  

All three of these compounds fit the molecular formula. They are, left to right, pentane, methyl butane and dimethyl propane. Again, they are all isomers of each other. If you have a 3D model of dimethyl propane you can see that it makes no difference, indeed, no sense, to worry about which carbons are the ends of the three carbon chain, and which carbons are the methyl side chains.

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