[SniperDogs]

I'm talking about R and S configuration.  Imagine that you have a benzene.  One of the carbons is chiral.  It is attached to a hydrogen, which is pointing towards you.  That carbon is also attached to a propyl group.  The carbon on the right of the chiral carbon is double bonded to the next carbon on the ring.  There are no other double bonds or groups.  How do you know if the propyl group is higher or lower priority than the group with a double bond. 

The name of the compound if 3-isopropylcyclohexene (the hydrogen on the chiral carbon is facing towards you).  Any help will be appreciated.

[dunno260]

You assign priority the same way you do in an acyclic system.  For your compound you start at carbon 3.  Hydrogen is obviously the lowest priority group.  The next lowest is going to be carbon 4 because its a CH2-C.  The next will be the carbon of the isopropyl group (CH bonded to two carbons) then carbon 2 of the ring has highest priority (CH double bond C).  As you have written it you have your steering wheel going to the right, however the lowest priority group is pointing towards you so you reverse it to get S.

http://www.cem.msu.edu/~reusch/VirtualText/sterism3.htm has an example of a more difficult cyclic compound that should help you be able to assign priority in carvone which is a little more difficult than your example.

[SniperDogs]

I understood what you said - but wouldn't the steering wheel would be going to the left?

C2 is highest priority (on the right), then the isopropyl group (on top), and then C4 (on the left).  That's S configuration.  But since H is pointed towards you, it would be R.  I'm really confused!!

(btw... u're right... that's the right answer in the book)

[dunno260]

Either you flipped the structure so that the hydrogen is back in your description or you are doing your substituent number incorrectly.  I have attached a picture of the structure as you described it.