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A Biology textbook question stated that there are three isomers of C5H12, the structural formula of the first two isomers pentane and 2-methylbutane were pictured, the reader was then asked to draw the third isomer of C5H12.

Probably a silly question, nonetheless, why are the following structural formulas I came up with not isomers of C5H12? If someone could elucidate my trouble, I and perhaps others, would be very appreciative. 1:

    H    
    |  
H — C — H  
    |   H   H   H  
    |   |   |   |  
H — C — C — C — C — H  
    |   |   |   |
    H   H   H   H
  1. H atoms removed for simplicity:
C
|
C — C — C
|
C
  1. three lines represent one single bond, so the hydrogen atoms can fit in the structural formula:
C — C — C
        |
        |
        |
    C — C

4:

C — C
|
|
|
C — C
    |
    C

5:

C — C — C 
        |
        C
        |
        C

6.:

C — C — C
|       |
C       C

Edit: I know a similar question has been asked on this website, that bond rotation has to do with my confusion and I think I have just answered my own question 🤦, so I am guessing all structures are pentane, except structure 2 which might be 2-methylbutane. Is this because single bonds allow the atoms they join to freely rotate about the bond axis? My trouble is visualising these structural formulas in 3D.

Finally that leaves us with 2,2-dimethylpropane:

    C
    |
C — C — C
    |
    C
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  • $\begingroup$ Indeed, most of what you have drawn are conformers. They are essentially the same molecules but in different shapes. With time you will be able to visualise the molecules which are often written in 2D representations as actual 3D configurations. You can afford this along with a molecular building kit. $\endgroup$
    – Martin - マーチン ♦
    Dec 28, 2022 at 13:13
  • $\begingroup$ Write these compounds according to the rules for skeletal formula. Better yet look up some 3D representations of molecules. $\endgroup$
    – Mithoron
    Dec 28, 2022 at 16:21
  • $\begingroup$ There is n-pentane, there is 1 way how to side-attach C to n-butane and there is 1 way how to side-attach 2 C to n-propane. $\endgroup$
    – Poutnik
    Dec 28, 2022 at 17:48

1 Answer 1

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According to IUPAC (in physical organic chemistry as well as sterochemistry), an isomer is one of several species (molecular entities) that have the same atomic composition (molecular formula) but different two-dimensional representations in which atoms are shown joined by lines representing single or multiple bonds, without any indication or implication concerning the spatial direction of bonds.

So ignoring the spatial direction of bonds, your examples 1, 3, 4, 5, and 6 are all the same; they are all simply pentane.

Only your example 2 is a different isomer; it shows 2-methylbutane.

The third isomer 2,2-dimethylpropane is missing.

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