[Bioionic]

Started Lewis dot structure on Friday's lecture.  One of the tricky problems was C3H6, but I solved it. I started to look at molecules with more carbons like C4, C5-8.  I got myself confused at this point, as how to know when the carbon becomes a ring.  Is there a rule of thumb for this.  I really do not know how to tell if the carbon chain will stay as a chain or form a ring.

Thanks

[nj_bartel]

You can't tell.  There are ring and straight chain isomers of those.

[Astrokel]

Do you mean cyclic? If you know the general formula, then you should be able to tell if the compound is aliphatic or cyclical.

For example

Aliphatic alkane has a general formula of C_nH_2n+2 whereas in cyclic it is C_nH_2n

Cycle alkene has a general formula of C_nH_2n-2

[Bioionic]

Maybe I am just thinking to far ahead. My prof never covered  rings yet, but I was just curious as we use them a lot in Biology.

Thanks

[Borek]

If you know the general formula, then you should be able to tell if the compound is aliphatic or cyclical.

Not exactly - in your examples you are using additional information about whether they are alkenes or not. Just looking at the formula you can't tell.

See

http://en.wikipedia.org/wiki/Degree_of_unsaturation

[Astrokel]

I didn't know about that, thanks!