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View Full Version : Topscore EXAM #1 Q. 98 DoctaWalnuts 06-10-2009, 06:54 PM How is http://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/Cyclooctatetraene.png/100px-Cyclooctatetraene.png (http://en.wikipedia.org/wiki/File:Cyclooctatetraene.png) Nonplanar? Each carbon is SP2? Topscore says this is non-aromatic (not anti-aromatic) because although it has 8 electrons, they are non-planar? Thank you! Sublimation 06-10-2009, 07:24 PM How is http://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/Cyclooctatetraene.png/100px-Cyclooctatetraene.png (http://en.wikipedia.org/wiki/File:Cyclooctatetraene.png) Nonplanar? Each carbon is SP2? Topscore says this is non-aromatic (not anti-aromatic) because although it has 8 electrons, they are non-planar? Thank you! I know but the most stable conformation of this molecule is a "tub". DoctaWalnuts 06-10-2009, 07:37 PM So, it's not planar because of the tub? What other molecules would have this? Maybe C16H16? I guess multiples of C8H8? http://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/COT-all-cis.png/150px-COT-all-cis.png (http://en.wikipedia.org/wiki/File:COT-all-cis.png) GiTsticker 06-10-2009, 07:40 PM I thought the most stable conformation would be a lawn-chair. DoctaWalnuts 06-10-2009, 07:53 PM I thought the most stable conformation would be a lawn-chair. How do we know? GiTsticker 06-10-2009, 08:00 PM Well it would make sense because that's what cyclohexane does right? The lowest energy conformations occur when it is shaped like a chair. The slightly higher transition state is the boat (analogous to the tub here) which flips over to another chair. I could be wrong, but it seems like there would be less steric hindrance that way with the carbons orientated trans of each other. DrCheese 06-10-2009, 08:47 PM Yea I always wondered about that. What other cyclic rings are also non planar? joonkimdds 06-10-2009, 10:19 PM how do we know what's planar and what's not? DoctaWalnuts 06-10-2009, 10:27 PM how do we know what's planar and what's not? Planar usually means sp2...that's the easiest way to memorize it IMO Aceofspades 06-10-2009, 11:14 PM how do we know what's planar and what's not? The rings from 3 carbons to 7 carbons are planar. At ring sizes of 8 to 12 carbons you have some twisting out of plane. After that, ring sizes 13+ tend to be planar again. For the DAT, all you have to know is that rings are planar except cyclooctane (8C). They probably won't ask about rings bigger than that. |