[ellieahlquist]

We're learning how to name compounds in my ochem class.
I'm having issues naming a few of them, and was wondering if anybody could point me in the right direction.

For this one, I numbered the straight-chain carbons from 1 to 9 (right to left). There is an ethyl group on the third carbon, a propyl group on the 5th carbon, and a methyl group on the 7th carbon. I named it: 3-ethyl-7-methyl-5-propylnonane, but my online assignment said it was incorrect.


Similarly, this is cyclopentane with two substituents. One is a methyl group on the first carbon, and the other is an ethyl group on the second carbon. So I named it 2-ethyl-1-methyl-cyclopentane.

I'm uncertain where I went wrong in either of these problems, so help would be appreciated.

[Archer]

Start with the lowest number.

Your numbers should be in ascending order.

[ellieahlquist]

Start with the lowest number.

Your numbers should be in ascending order.

My professor said according to the IUPAC they should be in alphabetical order?

edit: I just tried it in case my professor was wrong, and the answer was incorrect.

[TwistedConf]

I named it: 3-ethyl-7-methyl-5-propylnonane, but my online assignment said it was incorrect.

I agree with you on this one.

So I named it 2-ethyl-1-methyl-cyclopentane.

The numbers are 1 and 2 no matter which substituent you start with, so the earlier alphabet substituent gets the lower number (ie. 1-ethyl). This is actually the same kind of thing as you have going on in the first acyclic example. It's also the trans stereoisomer, but you might not be there yet.

Also, you are correct that names are constructed in alphabetical, not numerical, order.

[ellieahlquist]

I named it: 3-ethyl-7-methyl-5-propylnonane, but my online assignment said it was incorrect.

I agree with you on this one.

So I named it 2-ethyl-1-methyl-cyclopentane.

Thank you, you are right. I emailed my TA and we haven't done anything with "trans" or "cis" yet. So much for being proactive on my assignment
The numbers are 1 and 2 no matter which substituent you start with, so the earlier alphabet substituent gets the lower number (ie. 1-ethyl). This is actually the same kind of thing as you have going on in the first acyclic example. It's also the trans stereoisomer, but you might not be there yet.

Also, you are correct that names are constructed in alphabetical, not numerical, order.

[Archer]

My apologies for giving out mis-information, I glanced at the molecule, saw that you had been told it was wrong and thought that you had not numbered them correctly.
 
I should have drawn it out because I had assumed it was a different molecule.

Also if I had carefully considered my answer with the thought of providing you with the rules of nomenclature rather than just how to answer your question I wouldn't have made such a mistake! So again I sincerely apologise.