[Violagirl]

Hi so I have a lab quiz tomorrow and wanted to know, if you're given a product and need to decide which C's are R/S configuration, I believe you first need to determine whether or not it's chiral and look at it's attached atoms and number them in terms of "importance" with regard to mass number. Then R configurations have attached atoms in a clockwise direction and S configurations have attached atoms in a counterclockwise direction. Is that correct? Also if you have a racemic mixture of products from a Diels-Alder reaction and if a substituent is in the endo position (R) and is in the exo position in the minor product, does that mean it switches to S configuration?

[discodermolide]

Hi so I have a lab quiz tomorrow and wanted to know, if you're given a product and need to decide which C's are R/S configuration, I believe you first need to determine whether or not it's chiral and look at it's attached atoms and number them in terms of "importance" with regard to mass number. Then R configurations have attached atoms in a clockwise direction and S configurations have attached atoms in a counterclockwise direction. Is that correct? Also if you have a racemic mixture of products from a Diels-Alder reaction and if a substituent is in the endo position (R) and is in the exo position in the minor product, does that mean it switches to S configuration?

Look up the Cahn-Ingold-Prelog system of assigning (R) and (S) configuration.
Note: you have to orient the molecule such that the group with the lowest priority is looking away from you, then you start.

[Violagirl]

Thanks a ton! I did do that and have been practicing problems but still a couple of uncertainties. I'm thinking most of the problems I'll come across will relate to boat structures. If you have a carbon group attached to a phenyl substituent, a CH2 group, a double bonded C to another C, and a carbonyl group, would the phenyl or carbonyl group be the higher priority?

[discodermolide]

Thanks a ton! I did do that and have been practicing problems but still a couple of uncertainties. I'm thinking most of the problems I'll come across will relate to boat structures. If you have a carbon group attached to a phenyl substituent, a CH2 group, a double bonded C to another C, and a carbonyl group, would the phenyl or carbonyl group be the higher priority?

I'd say the carbonyl has the higher priority.

[fledarmus]

To assign priority, look at each atom one at a time -

For the example you gave, start with the chiral carbon. The first atom attached to the chiral carbon along each of the four substituents (phenyl, methylene, ethylene, carbonyl) is another carbon - at this point they all have the same priority.

Now look at the next atom along each substituent. The carbon along the phenyl group has three bonds to other carbons, the methylene group has two bonds to hydrogen and one to carbon, the ethylene group has two bonds to carbon and one to hydrogen, and the carbonyl group has two bonds to oxygen (and since you didn't say aldehyde, I'm guessing one bond to carbon). Since the carbonyl has bonds to atoms of a higher atomic number, it would be the highest priority - next would be the phenyl (with three bonds to carbon), then the ethylene (with two) and then the methylene (with one).

The real trick is to go atom by atom looking at atomic number and block out the rest of the structure in your mind, so the sheer size of the substituent won't trick you into ignoring which has the highest atomic number first.