Qvault stereocenters

[FeralisExtremum]

Wanted to double check this one. The solution says there are 2 stereocenters (as marked with asterisks), but shouldn't the correct answer be 3, due to one of the double bonds?

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[nickh]

For stereocenters, don't you need all 4 to be different? With a double bond, you will only left with 3 connections, which can't be a stereocenter. There could be a stereocenter on the other tertiary C by the ring, if the ring wasn't symmetrical. So 2 stereocenters in total.

 

[FeralisExtremum]

A stereocenter means that by swapping the position of two groups, you would get a stereoisomer. Double bonds can be stereocenters if they allow for cis/trans configurations. The double bond on the far left can't be cis/trans because one of its carbons is attached to identical groups. But the double bond to the right does have cis/trans, so it qualifies as a stereocenter.

 

[dawg289]

You said "A stereocenter means that by swapping the position of two groups, you would get a stereoisomer."

If we flip the C=C bond to make it from trans to cis, the two products are not mirror images, which is what stereoisomers are. These cis and trans molecules are diastereomers, which are not stereoisomers. Any carbon in an alkene bond cannot be a stereocenter.

 

[FeralisExtremum]

You said "A stereocenter means that by swapping the position of two groups, you would get a stereoisomer."

If we flip the C=C bond to make it from trans to cis, the two products are not mirror images, which is what stereoisomers are. These cis and trans molecules are diastereomers, which are not stereoisomers. Any carbon in an alkene bond cannot be a stereocenter.

1. Stereoisomers are not limited to mirror images - those are enantiomers, a specific type of stereoisomer.

2. Diastereomers are stereoisomers

3. It's not the carbon in an alkene bond that is considered the stereocenter - it is the double bound itself, when cis/trans is possible

I think you may be mixing up the term chiral vs stereocenter. Keep in mind "stereoisomer" is a general term that means you have the same atoms present and the same bond connectivity, but a different 3d arrangement. It encompasses many different sub-types of isomers:

 

[DAT Destroyer]

Wanted to double check this one. The solution says there are 2 stereocenters (as marked with asterisks), but shouldn't the correct answer be 3, due to one of the double bonds?

Be careful on a question like this it is unlikely to land on the DAT. The correct answer is 3 stereocenters since the double bond carbon is indeed counted and is classified as a stereocenter, in addition to the 2 chiral carbons. It is always best to reference this material from a standard Organic Chemistry textbook. Only count the double bond that leads to cis trans isomers when looking at sterocenters. Tricky indeed.