Optical rotation vs Stereochemistry

[pm1]

Are they related?

I keep falling in a trap where they give me the stereochemistry and I think from that I can deduce the optical rotation. Is that not true?

For example, in a TPR passage they give you a product from a Sn1 reaction. The reactant had S stereochemistry and the product, R stereochemistry.
Then, the question tells us that the reactant optical rotation is -83.6 and asks for the optical rotation of the product. I chose +83.6, thinking that it was just inverted, but that's wrong. Can I establish a relationship between optical rotation with stereochemistry? or that's completely wrong?

Thank you!

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[milski]

You can but you have to consider the stereochemistry correctly. If it was an Sn1 reaction, it would have gone through a carbocation - any stereochemistry at that carbon atom would have been lost and you would get a racemic mixture of both S- and R- products. That's provided that the C atom which had the leave group was a chiral center in the reactant. Any other chiral centers will stay the same as they were before the reaction.

 

[chiddler]

enantiomers have opposite optical rotation. mesos have 0 optical rotation. racemic mixtures also have 0 NET optical rotation. optically active compounds are those with no axis of symmetry (meaning chiral compounds)

 

[pm1]

enantiomers have opposite optical rotation. mesos have 0 optical rotation. racemic mixtures also have 0 NET optical rotation. optically active compounds are those with no axis of symmetry (meaning chiral compounds)

okay.. but can we actually tell what is the optical rotation based on stereochemistry?

TPR says there is no relationship between optical rotation and a molecule's absolute stereochemistry.

 

[chiddler]

okay.. but can we actually tell what is the optical rotation based on stereochemistry?

TPR says there is no relationship between optical rotation and a molecule's absolute stereochemistry.

no we can't. optical rotation is based on the specific molecule you're looking at. so some molecule that is R might be -58 degrees. Another similar molecule that is R might be +13 degrees. There is no relationship if they are different molecules.

if they are the same molecules with different stereochemistry, then there is room for prediction. not all stereoisomers, of course (diastereomers will not tell you much), but only some (like i wrote, enantiomers.)

 

[pm1]

no we can't. optical rotation is based on the specific molecule you're looking at. so some molecule that is R might be -58 degrees. Another similar molecule that is R might be +13 degrees. There is no relationship if they are different molecules.

if they are the same molecules with different stereochemistry, then there is room for prediction. not all stereoisomers, of course (diastereomers will not tell you much), but only some (like i wrote, enantiomers.)

okay! Thank you!!