Is it correct to put a methyl group on each end of a skeletal formula? [closed]








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On my organic chemistry exam there was a skeletal formula that had, supposedly one methyl group at each end of the skeletal formula and one somewhere in the middle.


[![Skeletal formula from my exam question](https://i.stack.imgur.com/jkOIn.png)](https://i.stack.imgur.com/jkOIn.png)


But is it correct if you draw a formula like that and then put those methyl groups on each end? I feel like it's very misleading because I thought that those were just the ends of the main chain, so *not* methyl groups. I lost a lot of marks on that question because of this confusion and I just really want to know if it's even correct to write it that way.




Here, all three terminal carbons are *also* methyl groups.
Carbon labels may be omitted or displayed, e.g. for the straight angles or implicit hydrogen atoms.
Both structures **1** and **2** below are *identical* and represent 3‐methylhexane:


[![3‐methylhexane](https://i.stack.imgur.com/giKtE.png)](https://i.stack.imgur.com/giKtE.png)


From *IUPAC Recommendations 2008* [[1](https://doi.org/10.1351/pac200880020277), p. 290]:



> 
> ### GR-1.4 Terminal single bonds
> 
> 
> As discussed in GR-2.1.2, unlabeled atoms are assumed to be carbon atoms, and so terminal single bonds are assumed to represent methyl groups. Unlabeled bonds should not be used to represent unspecified or variable attachment points (see GR-9), as such diagrams are extremely prone to misinterpretation.
> 
> 
> [![GR-1.4 Terminal single bonds](https://i.stack.imgur.com/pjCF5.png)](https://i.stack.imgur.com/pjCF5.png)
> 
> 
> 


Further [[1](https://doi.org/10.1351/pac200880020277), pp. 300–302]:



> 
> ### GR-2.1.2 Labeling of carbon atoms
> 
> 
> […]
> 
> 
> It is acceptable to add labels for terminal carbon atoms connected to unlabeled carbon atoms, but only when it is possible to do so without overlapping other portions of the diagram.
> 
> 
> [![terminal C atoms and unlabeled chain](https://i.stack.imgur.com/TtJnq.png)](https://i.stack.imgur.com/TtJnq.png)
> 
> 
> When ethane, ethene, ethyne, and related molecules are drawn with only one explicit bond, both terminal carbon atoms must be labeled explicitly to prevent the molecule from being interpreted as a stray line or set of lines.
> 
> 
> [![explicit labeling of short chains](https://i.stack.imgur.com/V9CpI.png)](https://i.stack.imgur.com/V9CpI.png)
> 
> 
> 


### References


1. Brecher, J. Graphical Representation Standards for Chemical Structure Diagrams (IUPAC Recommendations 2008). *Pure and Applied Chemistry* **2009**, 80 (2), 277–410. DOI: [10.1351/pac200880020277](https://doi.org/10.1351/pac200880020277). (Free Access)


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