5.3 Molecules with Multiple Chiral Centers | Enantiomers, Diastereomers, and Meso Compounds | OChem

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Thank you for the explanation of "Meso" 👍 Enjoyable learning time 👍

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Thank you for your kind words, and if my fast paced passion for chemistry gets too fast, slow down the playback speed under the settings. Happy Studying!

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[@komalchahal9671]

@15:12 isn’t the molecule on lower left side is R and S, when configurations are not matched?

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[@jaiagrawal9963]

I have a doubt at 15:25 those two compounds are enantiomer even though they have a center of symmetry. Why?

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[@ChadsPrep]

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[@MisterJ34]

can you explain at 23:33 when you did R&S how you found that, its tripping me up a little, please and thank you

[@ChadsPrep]

No problem - so at this point we are looking at the left carbon with Br attached we assign priority based on atomic number so Br = 1, the top carbon is number 2 (because it is closer to the next Br atom than the bottom carbon, bottom carbon = 3 and the H = 4 if you trace with your finger or connect with a pen going from 1 -> 2 - > 3 you are moving clockwise or make in a 'right' (R) turn. So its R, the other carbon makes a left turn = S. Hope that helps!

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check this video if you need a refresher on assigning R and S kzbin.info/www/bejne/pGrQiImDZbibis0

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[@suhaylakamarudheen704]

Hi Chad! Could you please explain why you flipped the molecule at 5:20 (the blue drawing)? Did you flip it, so we can see that they both have wedges (hence their enantiomers)? But the ones on the bottom are diasteriomers as they’re not superimposable images to the molecule? (Ie. they have one wedge and one dash) Thank you so much Chad. I’ve been watching your videos since I was in my first year of undergrad!!! My favourite channel on KZbin!

[@ilovecows303]

why are the R AND R for bromine and S and S still enantiomers?

[@jibraankhan9667]

Is blue structure of (2R,4S)-2-bromo-4-chloropentane identical to the structure with two hash bonds @ 5:40?

[@bethanyharrison3310]

Question for you sir: Imagine if we’d been looking at the mirror image (through the plane of the board) of the very last example (which was (2R,3S)-2-chloro-3-methylhexane just as a reminder). This would render the stereochemistry 2S and 3R. So my question is: do we now list these numerically or alphabetically in the name’s starting parenthetical? (I’m suspecting alphabetical since IUPAC loves the alphabet)

[@shim525]

Sir, I have a question for meso compounds. Cis-1,4-dichlorocyclohexane is a meso or not? It has two stereocenters in the molecule with a plane of symmetry and its mirror image is superimposable, but it does not have any chiral centers in the molecule. In this case, is it a meso or not?

[@reydeleon4091]

For your question, it would just be achiral and optically inactive. For a meso compound, there must be both the presence of chiral centers and a plane of symmetry. In this case, there are no chiral centers to begin with so it is just achiral. Hopefully this answer helps.

[@Kenzo_254]

When you talk about calculating number of stereisomers do you mean number of enantiomers? Because you have got me mixed up.

[@MrYahya0101]

You said diastereomers are those that are not mirror images. Would that include the same molecule with a rotation around one of its bonds for example I think in cis trans diastereomers, there is only rotation and no inversion of configuration? What about the same molecule (no rotation, no inversion). Would that be its own diastereomer? Thank you.

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[@MrYahya0101]

Hey Chad, I assume when rotating the molecule around to see if it has a plane of symmetry, one shouldn't rotate in a way that changes the configuration right? Like, rotating around a chiral carbon may change the configuration and thus the R:S ratio of 1:1? Or does rotating around a bond typically not affect the configuration?

[@ChadsPrep]

Great question Youssef and your intuition at the end was correct! If you're just simply rotating around a bond then it won't change the configuration. Nice!

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[@MrYahya0101]

If you have 3 chiral centers could that still be a meso compound?

[@ChadsPrep]

Yes! It has to have at least 2 and the most common examples you're likely to see have 2 chiral centers but the key is that it has to have AT LEAST 2. Hope this helps!

[@MrYahya0101]

​@@ChadsPrep But if it has 3 then R+S wouldn't be in same ratio, so it would be optically active no? I also wanted to ask you about the cyclohexane you drew. You drew cyclohexane and said that it has plane of symmetry so it has superimposable mirror image. When you draw the mirror image do we consider the configuration of stereocenters to be reversed, eventhough they are not chiral centers? For non-superimposable mirror images, you can just say that the chiral centers were reversed, but superimposable mirror images have no chiral centers, how would you identify them? Thanks.

[@ChadsPrep]

You're exactly right Youssef, meso compounds are only possible with an even number of chiral centers. But they can exist with an odd number of stereocenters. Consider 1,2,4-cyclopentanetriol. Carbon atoms 1 and 2 will be chiral centers. Carbon atom 4 will be a stereocenter but not a chiral center (you can't assign it R/S). If carbons 1 and 2 have opposite configurations (one R and one S) it will be a meso compound. My bad but hope this helps!

[@ChadsPrep]

When you draw a mirror image the configurations of stereocenters won't necessarily be reversed. I haven't seen this come up before but if I consider the cyclohexane example and other such similar examples the stereocenters aren't reversed in the mirror image. I really want to say that will always be true, but the use of the word ALWAYS is dangerous. But I'm leaning that direction. Hope this helps!

[@MrYahya0101]

@@ChadsPrep Thanks Chad! So some molecules will have plane of symmetry and still not be meso compounds (cyclohexane), and some will not have a plane of symmetry (like the one at 18:30) and still be meso compounds? I mean doesn't that just mean we shouldn't use "plane of symmetry" to identify meso compounds?

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