[AzMa1]

First off, why are there (according to a problem in my book) only two ethylheptanes, 3-ethylheptane and 4-ethylheptane? Why can't 2-ethylheptane exist?

Also, would 5-ethyl-2-methylhexane be an isomer for nonane? The structure is drawn in my chemistry book but according to this page http://www.3rd1000.com/chem301/nonane.htm , that isn't an isomer.

Likewise, would something like 2,3-diethylpentane exist? If it doesn't why not? I think I'm missing something crucial here and I can't find it in my book.

Thank you for any help.

[enahs]

Draw the structures out.

First off, why are there (according to a problem in my book) only two ethylheptanes, 3-ethylheptane and 4-ethylheptane? Why can't 2-ethylheptane exist?

The parent name is the longest carbon chain. When you add the ethyl-group, the carbon back-bone can change. As a result, when you put the ethyl group on the second carbon in heptane, the longest carbon chain becomes 8 carbons long, and thus no-longer heptane.

Also, would 5-ethyl-2-methylhexane be an isomer for nonane?

Again, draw the structure. What is the longest carbon chain? That is not a valid name. Once you draw the structure and name it properly, you will see it is on your list.

Likewise, would something like 2,3-diethylpentane exist? If it doesn't why not? I think I'm missing something crucial here and I can't find it in my book.

Again, draw that structure. The longest carbon back-bone changes, the name changes. It again is on your list.