What is the systematic name of this compound:
My try: 3-propylpentane.
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You need to take the longest chain first, i.e. the basis of the hydrocarbon skeleton is hexane. It has a ethyl group at position 3, so the IUPAC name will be 3-ethylhexane.
First of all, make sure to find the longest chain. In this case, it is 6 starting from top right carbon to bottom right carbon. Then, number the chain. You will find that the C atom bonded to the branch has the number 3.It is not 4 because the branch should have the lowest locant and so the numbering should start from top right C atom and not from the bottom one. So, the name of the compound is 3-ethyl hex-ane.
The most important thing to do is to be able convert the representation like those in your question to line-angle drawing and then number those carbons from 1 to the end in all directions. Try numbering then completely: For instance, you are talking about this compound. Then it helps to see where every thing is and where there are branches.