AP Biology Question

So I know that enantiomers are like mirror images of each other, but not identical (superimposable) upon one another. On Wikipedia, I read that it was like our hands, similar but not identical (you can't rotate one hand into another. Makes sense. But I'm not sure how this would help me identify the enantiomers. All the molecules look identical.

The answer is (D).

What is different about D? enter image description here

  • $\begingroup$ Curious as to why this post was downvoted without explanation. I believe I provided proper context and any attempts, because this problem is just identifying a property. $\endgroup$ – Saketh Malyala Oct 22 '17 at 23:36
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    $\begingroup$ Most likely it was downvoted because it's a homework like question with very little effort shown towards solving it. You haven't demonstrated drawing any mirror images or superimposing them. $\endgroup$ – Zhe Oct 23 '17 at 0:00
  • $\begingroup$ i've done literally all i can. i can't post my drawings here because i don't have a camera. all the molecules are literally identical no matter how hard i try to differentiate them from one another. @Zhe do you know any way in which D is different? $\endgroup$ – Saketh Malyala Oct 23 '17 at 1:28
  • $\begingroup$ In your defence I’ll admit that these are exceedingly terrible representations because the way the central tetrahedron is drawn makes everything look like the same old methane. But if you look closely notice that not every corner contains a hydrogen. Imagine if the corners were coloured differently for each of the groups (methyl, $\ce{COOH}$ or $\ce{NH2}$) and then try again. $\endgroup$ – Jan Oct 23 '17 at 6:33

The key point is that for four of the five pairs, the structure on the left can be rotated to make a structure exactly like the one on the right. For example, the left structure in (A), if rotated 120° about the C-CO$_2$H bond, is equivalent to the structure on the right.

For two "enantiomeric" structures, it is not possible to do this. It might be helpful to draw every possible rotation of the structures in (D), to see that it is impossible to produce the structure on the right by rotating the one on the left.

  • $\begingroup$ i never noticed the clockwise/counerclcokwise aspect ahhh THANKS $\endgroup$ – Saketh Malyala Oct 23 '17 at 5:11
  1. If you see duplicated groups attached to the C (e.g. 2 H's or 2 CH3's), then you can rule them out. For molecule to be chiral it should have a chiral center (C connected to 4 different groups). This helps you easily identify that the only possible answer is D.

  2. Then you could check if molecules are mirror images of each other. If not, you need to rotate them to be so. If it's impossible - they are not enantiomers.

  3. And finally when you have mirror images you can try to rotate one of the molecule to superimpose them. It still may be possible if molecule has symmetry.


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