The letter R from rectus right or S from sinister left is us

The letter R (from rectus, right) or S (from sinister, left) is used to designate the configuration at a chiral center. The four atoms or groups attached to the chiral center are arranged in a priority order according to atomic number: the higher the atomic number, the higher the priority. If two atoms have the same atomic number, we move to the next atoms out from the chiral center, or even further, until we observe a difference in atomic number. We then view the molecule from the side opposite the group with the lowest priority. If the remaining three groups in order from highest to lowest priority form a clockwise array, the configuration is R; if they form a counterclockwise array, the configuration is S.

Procedure: Construct a model of 2-chlorobutane.

29. Which carbon in the chain is a chiral center?

a. 1

b. 2

c. 3

d. 4

There are four groups attached to this chiral center

.

30. Which group has the highest priority?

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 two groups?

a. Ethyl > Methyl

b. Methyl> Ethyl

Set the model on the desktop so that it can be viewed from the side opposite the hydrogen. Put the chlorine atom at the top.

33. When viewing the model, the three remaining groups in priority- order sequence form a

a. Clockwise array and the model has a R configuration.

b. Counterclockwise array and the model has a R configuration.

c. Clockwise array and the model has a S configuration.

d. Counterclockwise array and the model has a S configuration.

Interchange any two groups attached to the chiral center.

34. What configuration does the model have now?

a. The same as before.

b. The opposite configuration.

Note that to change configuration we must disconnect and remake bonds, whereas we can change conformation by rotating groups around single bonds.

Solution

a

a

d

a

a

b