Systematic and preferred names of acetamide

My book says to use carboxamide when it is a carboxylic acid derivative (acyl). [Ethanamide](https://en.wikipedia.org/wiki/Acetamide) $\ce{CH3CONH2}$ is a carboxylic acid derivative, so shouldn’t it be named **ethan-1-carboxamide** rather than **ethanamide**?


[![enter image description here](https://i.stack.imgur.com/BFjBB.png)](https://i.stack.imgur.com/BFjBB.png)


I am confused with this nomenclature and use of amide, carboxamide and imide. When should I use **carboxamide** and **ethanamide**?




The first thing to do when constructing a systematic name is to find the **parent structure** - be it a cycle, or in this case an acyclic chain.


You choose the parent structure with the most skeletal carbon atoms - that is the **longest chain**. Here the longest chain in 2 carbon atoms so we start with a parent chain of **ethane**.


Ethane been substituted with -$\ce{NH2}$ and $\ce{O}$ which describes an amide group. So combining the amide suffix and the ethane parent, we get **ethanamide**.


#### NB: carboxyamide group


A carboxyamide group includes the carbon atom, so if you *incorrectly*declared the $\ce{CH3}$ group as the parent *methane*, you could say it is substituted with a carboxyamide group and would be called *methanecarboxamide*.


An example of the correct use of *carboxyamide* is *cyclohexanecarboxyamide* (pictured below). Here the cyclohexane ring system is the parent and the can be described as carboyamide substituted.


[![cyclohexanecarboxyamide](https://i.stack.imgur.com/DiK0G.gif)](https://i.stack.imgur.com/DiK0G.gif)


The minutiae of systematic naming is described in the IUPAC Blue Book. Here the relevant rule is [P-44.3.2](https://iupac.qmul.ac.uk/BlueBook/P4.html#440302)


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