#### • Class 11 Physics Demo

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# how to find chiral centers

Question:I understand that a chirality center has 4 different things attached, but how do you find the chiral center when it has multiple rings? I cant tell how to find the chiral centers of cholesterol.

Answers:Since cholesterol isn't symmetrical, any carbon that has four different substituents attached is a chirality center. In comparing substituents, you may have to extend the comparison to one or more carbons. Keep going until you find a difference. Don't stop at a CH2 on either side.

Question:I understand how to name them using R and S but need help finding them, I know that a chiral center is a carbon that is attached to 4 different substituent groups, but when i look at a more complicated molecule I get confused.

Answers:Seems like you know the correct rules. Isolate each carbon and examine the 4 possible bonding sites

Question:i just don't get it. If you could give me an example of one as well, that would be great. Thank you in advance

Answers:A compound that has only one chiral (or asymmetric center) will always have stereoisomers. However, a compound that has two asymmetric centers and is symmetric will have three stereoisomers. A pair of enantiomers and a meso compound. The two enantiomers are optically active and the mseo compound, the answer to your question is not, therefore it is achiral. The reason it is achiral is because when you take the mirror image of the compound, it will be the same. Therefore it is superimposable and is not optially active. An example is CH3CHBrCHBrCH3 Here you can see that the two asymmetric centers are the two C's bonded to the Br's. It is hard to explain without pictures but if you draw the dashed-wedge diagram for one of the three CH3CHBrCHBrCH3 configurations and try try to take the mirror image of that, you will have the same configuration. This is the meso compound. An easy way to tell if it is a meso compound is if it has a plane of symmetry. In the case of CH3CHBrCHBrCH3, there is a plane of symmetry between the two CHBr's. But be careful of rings and other exceptions. Good Luck!

Question:And if there is two chiral center, does that mean that it is not chiral? And if there is three chiral centers, does that mean that it is chiral? I am studying for a test, and was wondering if there is a pattern to chirality just by looking at the chiral atoms (carbons). Thanks in advance!

Answers:If a molecule has one chiral center, the entire molecule is chiral. If there are two chiral centers, the entire molecule is STILL chiral. No mater how many chiral centers you have, the molecule will not be identical (i.e., superimposable) to its mirror image.