Why are these structural formulas not isomers of C5H12? [closed]








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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  

```

2. H atoms removed for simplicity:



```
C
|
C — C — C
|
C

```

3. 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

```



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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