Homework Help: Chemistry

Posted by Sam on Wednesday, June 10, 2009 at 9:22pm.

This is a bit weird to me. We're doing R and S configurations and I think of it as a steering wheel - steering to the right is an R configuration and steering to the left is an S configuration.

These questions are drawn out as molecules in our book and I listed them going clockwise.
So..

If a chiral center has an H, OH, C-CH3 and CH3 shouldn't it be an R configuration? Why is it S..(according to the solutions manual)

If a chiral center has a H, OCH3, CO2H, HOCH2, then shouldn't that be R as well.. Why is it S?

In the compound Prostaglandin E1 there are 4 chiral centers. Apparently there are 3 Rs and 1 S. I don't understand the S. Shouldn't that be an R?

Is there anyway anyone can explain the priority thing to me because that's where I'm confused when it comes to bigger molecules. Thanks!

Chemistry - Dr Russ, Thursday, June 11, 2009 at 11:06am
You need to be able to visulaise the molecule or get yourself a set of models.

Identify the chirality centres (most commonly an sp3 C with 4 different groups attached)
Assign the priority to each group (high = 1, low = 4) based on atomic number (or mass if same atomic number) of the atom attached to the chirality center (remember the first point of difference rule)
Position the lowest priority group away from you as if you were looking along the bond, C nearest to you.

For the other 3 groups, determine the direction of high to low priority (1 to 3)
If this is clockwise, then the center is R (Latin: rectus = right)
If this is counter clockwise, then it is S (Latin: sinister = left)

In you first example with the H away from you the order would need to be

OH, C-CH3 and CH3
anticlockwise for S - hard to draw on here
C-CH3
\
H3C-C-OH

For PE1 you need to make a model. Yes, there are 4 chiral centres.

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