The letter R (from rectus, right) or S (fromsinister, left) is used to designate theconfiguration at a chiral center. The four atoms or groups attachedto the chiral center are arranged in a priority order according to atomic number: the higher the atomic number, thehigher the priority. If two atomshave the same atomic number, we move to the next atoms out from thechiral center, or even further, until we observe a difference inatomic number. We then view the molecule from the side opposite the group with the lowest priority. If theremaining three groups in order from highest to lowest priorityform a clockwise array, the configuration is R;if they form a counterclockwise array, the configuration isS.
Procedure: Construct a model of 2-chlorobutane.
29.Which carbon in the chain is a chiralcenter?
a.1
b.2
c.3
d.4
There are four groupsattached to this chiral center
.
30.Which group has the highestpriority?
a.Cl
b.Methyl
c.Ethyl
d.H
31.Which group has the lowest priority?
a.Cl
b.Methyl
c.Ethyl
d.H
32.What is the priority order of the other twogroups?
a.Ethyl > Methyl
b.Methyl> Ethyl
Set the model on thedesktop so that it can be viewed from the side opposite thehydrogen. Put the chlorine atom at the top.
33.When viewing the model, the three remaininggroups in priority- order sequence form a
a.Clockwise array and the model has a Rconfiguration.
b.Counterclockwise array and the model has a Rconfiguration.
c.Clockwise array and the model has a Sconfiguration.
d.Counterclockwise array and the model has a Sconfiguration.
Interchange any two groups attached to the chiral center.
34.What configuration does the model havenow?
a.The same as before.
b.The opposite configuration.
Note that to changeconfiguration we must disconnect and remakebonds, whereas we can change conformation byrotating groups around single bonds.
