Confusion about meso compound

[orangepopsicle]

so I was taking a DAT Qvault test and the question was, which of the following are optically inactive?

The answer was this below which confused me:

"In order to be optically active, a molecule has to lack an internal plane of symmetry. This is the ONLY requirement. Absence or presence of a chiral center is not a guarantee (meso compounds have chiral centers yet are optically inactive, while certain compounds are optically active without having a chiral center). If you trace the two paths around the ring starting at the stereo bond, you will see that they are the same. The other molecules are all asymmetrical around the cyclohexane ring.

"


It seems from the answer worded above they are saying that the molecule above has some sort of plane of symmetry, which suggests it is a meso compound, how can the molecule above me a meso compound? I don't see any plane of symmetry other than the 2 double bonded oxygens but on the bottom half of the ring there is no way to call that symmetrical. So confused, please help!

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[FeralisExtremum]

This molecule has a vertical plane of symmetry - it just doesn't look that way on paper. Notice that there is a wedge connecting the bottom group to the rest of the ring - that indicates it is coming out of the page, and the double bond to the CH2 you see is sticking straight up vertically (out of the page) and not off to the side as it appears. In reality that bottom group is flat and perpendicular to the ring, but that can't be accurately represented in 2D, so the wedge is there to indicate it's actual 3d orientation.

Even without the wedge to guide you, you should try to get familiar with the orientation of molecules in 3d vs their 2d representation.

 

[orangepopsicle]

So a molecule can be considered meso even if only a part of it is symmetrical and one of its substituents aren't? I thought that the plane of symmetry had to apply to the entirety of the molecule, like for the molecule above to be meso the double bond would have to be removed and would have to be a single bond and the CH2 would have to be a CH3 wouldn't it?

 

[orangepopsicle]

nvm i figured it out!