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Bromine has the highest molecular weight of the atoms, so this gets the highest priority.
A carbon-carbon double bond is regarded as if the carbon is bonded to two carbon atoms, so in our example below the triple bond gets the highest priority and the single bonded carbon gets the lowest priority, making this compound the S enantiomer: Sometimes, the lowest priority group is not pointing away from you. If you are good at picturing molecules in 3D, you can rotate it to put the highest priority group in back. If you're having trouble rotating the molecule, swap two groups on the molecule so that you put the lowest priority group in back. Assign R or S, keeping in mind that this is the opposite configuration of that carbon.
This strategy is illustrated below: Assigning stereochemistry in a cyclic molecule, or one that contains a ring, can be challenging, but the rules are the same.
Learn the rules for assigning the absolute configuration at a chiral carbon atom in an organic molecule, as well as how to assign R and S stereochemistry.
You'll also see examples that will help you approach problems involving stereochemistry.
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Anyone can earn credit-by-exam regardless of age or education level.If the molecule has just one chiral carbon, these two configurations are called enantiomers.The two possible molecules are mirror images of each other, but they are non-superimposable.When I get to the ring, I like to 'walk' my fingers around one carbon at a time until I find a difference. Notice the methyl that makes the group labeled 1 higher priority. Note that there are two chiral carbon atoms in this molecule.The second is on the ring indicated by the green arrow.When we draw our arrows, we can see that the compound on the left is S and the one on the right is R.When the groups contain more than one atom, we may need to look beyond the first atom to assign the absolute configuration. The first group is CH2 in two chains, so we need to look beyond that position.You learned how to determine if the absolute stereochemistry is R or S, and how to deal with situations where the lowest priority group is not pointing away from you.Finally, you saw how to assign absolute configuration in cyclic molecules. We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities.Carbon, when neutral, is bonded to four groups in a tetrahedral arrangement.What this means is that each group points as far away from the other in three-dimensional space as it can.