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enter image description here

I understand that in the first example, the molecule is chiral as there is no symmetry. For the second and third example, I see no symmetry in the 3d conformation after twisting the right hand side. In the third example, I do see symmetry if we twist the right hand side so that the carbon molecules are in one plane. What am I missing?

Edit: typo corrected. Also I was not clear in that the picture I attached is the answers the professor provided. I see why the first one is right as there is no symmetry. But I did not see why the second and third ones were considered chiral and achiral, as I saw symmetry in the third example and not in the second. From the answers though, I am understanding that we consider if the molecule is super imposable with it's mirror image?

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  • $\begingroup$ Are the answers you have circled correct? $\endgroup$ – YUSUF HASAN Oct 8 '18 at 9:16
  • $\begingroup$ Also, welcome to Chemistry.SE! $\endgroup$ – YUSUF HASAN Oct 8 '18 at 9:17
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    $\begingroup$ I think you have made a typo. You write the molecule is not chiral as there is no symmetry; the molecule is chiral. And in the third example is a meso compound, so you have to find symmetry. I don't really understand your question, it seems to me you have done it right. $\endgroup$ – Martin - マーチン Oct 8 '18 at 9:18
  • $\begingroup$ For E and F, look here: chemistry.stackexchange.com/questions/48995/… $\endgroup$ – user55119 Oct 8 '18 at 19:46
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Although you have arrived at the correct answers and seem to have followed a reasonable process for getting there, it's important to be precise with the use of terms like symmetry and chirality. For a molecule to be chiral, it must be non-superposable with its mirror image. It's important to really embed this definition before talking about symmetry and things like chiral centres. For example, it is possible for molecules to be chiral without any chiral centres, and it's possible for molecules with multiple chiral centres to be achiral.

  • the guaranteed way to determine if a molecule is chiral: draw (or build a model) of the mirror image and determine whether they are superposable

Now, symmetry is a different, but related, topic. A molecule with a point or plane of symmetry will be achiral. However, molecules with a rotational axis of symmetry can still be chiral, eg. many often-used chiral ligands are C2 symmetric.

So, in answer to your question, "What am I missing? - I would just say: the rigor in use of the terminology. Your approach of drawing different conformers of the molecules with two chiral centres and looking for a plane of symmetry (that shows the molecule is a meso compound, and therefore achiral) is sound and led you to the correct answers.

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    $\begingroup$ I would slightly augment Explained's fine answer with a caution that, after you build a model of the molecule in question, be sure to consider all of its easily accessible conformations to see if any of them are superimposable on its mirror image. If so, the molecule is achiral. $\endgroup$ – iad22agp Oct 8 '18 at 13:14

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