This question was given by my teacher in class:
Draw the structure of 2,3-diethylheptane.
So I drew the following:
But now if I name the compound according to the longest chain, it comes out to be 3-methyl-4-ethyloctane! How can I properly draw these structures?
What I usually do when I start numbering is look for branching and start numbering at the end with the closest branch. Also, keep in mind that with branching you'll want to consider numbering from one of the branches (if it'll yield a long chain.)
I'm using the structure you made as an example:
Here I numbered from the side with the most branching. But then I numbered it in two different ways. One way was starting with the straight chain which gave me a 7-carbon chain. Then I renumbered it (the image on the right) starting from one of the branches. This gave me an 8-carbon chain. (You could also number starting from the other branch, but that gives you 7 as well.)
Choose the longest chain. Since stuff that looks like a side chain can actually be part of the main chain, seriously keep this in mind during exams. It's totally okay to number the same alkane in different ways and play around with it to find which is the longest chain.
4-ethyl-3-methyloctane
Now, if you're having trouble drawing them:
I would draw just a zig-zag pattern (like I drew above) containing the number of carbons you need. You need a heptane, so start out with a 7 carbon chain.
This will just be your bare, skeleton structure. Then number the carbons, then add the ethyls to carbons 2 and 3.
HOWEVER, the question is flawed. You actually can't have an ethyl on carbon number 2 in this case, because it just makes the chain longer.
First, according to the name that you were provided, 2,3-diethylheptane, you did draw the correct structure. The problem actually lies with the name: it does not follow the standard IUPAC nomenclature. While it is possible to draw the structure, as you have, it's not the correct name for it.
Any name derived from an organic alkane structure should start with the longest chain from start to end. From your structure the "2-ethyl" branch is actually part of the longest chain, which then includes 6 other carbons leading to the furthest end. Now you have to deal with a methyl group and an ethyl group on that octane chain: in this case, both lie on the same side, so there is no competition for which has priority in determining the numbering. So from the end that they're both closest, the methyl is on the 3-carbon; the ethyl is on the 4-carbon.
It is possible to get correct structures from incorrect names; the system is degenerate in that sense. However there is (ideally) only 1 correct IUPAC name for any organic structure.
