[SIUEWIHG8435JDFGJ]

Hi,

We're learning about symmetry in organic chemistry right now and I'm having a hard time understanding rotational symmetry. I've looked at the lecture videos again and again, looked up additional videos on YouTube, and can't quite seem to grasp the concept.

For rotational symmetry, does that mean that if I manage to find a way to rotate the molecule, as long as I didn't go a full 360, and it looks the same, I have a symmetrical molecule? After rotating it, I believe I can change my viewpoint to see if it looks the same? Please correct me if I'm wrong.

One of the practice problems we had was:


The answer key said that molecule A (the top one) had rotational symmetry while B (the bottom one) didn't have such symmetry. I've drawn in the yellow line of where I believe the rotational axis is.

For A, if we rotate it 180 degrees and look at it from the other side, wouldn't it look exactly the same?

The same goes for B, so why is A symmetrical but B not?

Update: I was thinking over this problem while in the shower and it seems that I can't change my viewpoint, right? Because doing so would mean I've gone a full 360, so of course things would look the same. Is that true for all rotations then? That I can't change my viewpoint.

Thanks.

[Dan]

Draw molecule A on a piece of paper, then take a pencil and hold it perfectly vertical with the point of the pencil in the centre of the six-membered ring. That is the C₂ symmetry axis.

[fledarmus]

For A, if we rotate it 180 degrees and look at it from the other side, wouldn't it look exactly the same?

Update: I was thinking over this problem while in the shower and it seems that I can't change my viewpoint, right? Because doing so would mean I've gone a full 360, so of course things would look the same. Is that true for all rotations then? That I can't change my viewpoint.

Thanks.

Yes - as you've noticed, if you rotate both the object and your point of view, you're simply looking at the same side of the same object. Like looking at the moon, all you see is one face.

Do what Dan says. The axis of symmetry in your molecule A is perpendicular to your paper.