[orgo814]

Attached is a really hard picture to understand (sorry.. I don't have an iPhone so that's the best I could do). I'm only concerned with the molecule in the middle which has the double bonds and the Br and Cl substituents at the end. According to the book, the two molecules are the same (with no chirality centers). I get that there are no chirality centers but the Br and Cl switch in the second molecule. Since stereoisomers , by definition, differ in the positions of atoms in space, I don't understand how these are not stereoisomers. The Br and Cl switch positions so one is sticking out towards me and the other is pointing back. Someone please explain to me and be specific. Again, I'm sorry about the bad picture quality.

[orgo814]

FYI- those are double bonds surrounding the middle carbon. It's hard to see because of the picture quality.

[discodermolide]

These are allenes and it looks to me like you have Br at both ends. Lets go with Br and Cl. In that case they are enantiomers due to the twisted nature of the system. gave a look at Orgopete's page:
http://orgo.curvedarrow.com/punbb/viewtopic.php?id=316

[orgo814]

The book says they are the same molecule (not stereoisomers)

[discodermolide]

If you have Br at both ends and it is really difficult to see if this is the case from your picture then they are the same.

[orgo814]

Yes, Br is at both ends. However, it switches with the hydrogen and since it differs with the position of the atoms in space, I'm confused as to why this is not a stereoisomer

[discodermolide]

Its symmetrical, look at the link I gave you

[synthon]

If you rotate 180 degrees about the horizontal axis, each Br of the first drawing assumes the position of the Br in the second drawing.  The allene nature of the structure will add a degree of chirality, but these two drawings are still the same molecule represented in two different ways.

[fledarmus]

Build models! Build models! BUILD MODELS!

Build your two structures and see how they overlap. There is never any reason for wondering, arguing, or debating whether two drawn structures are identical. Stick some marshmallows on the end of toothpicks and set them next to each other.

[orgo814]

I know that exchanging two groups at a chiral center changes it from one enantiomer to another- since this is a trigonal stereogenic center, I'm guessing it doesn't apply to here? So just exchanging the two groups is a simple 180 rotation that keeps it the same? Keep in mind i'm only in orgo 1 so anything about allenes have chiral characteristics isn't applicable to this problem

[Babcock_Hall]

If you built two models, then rotated one as synthon suggested, you should find that it is identical to the other model.