| |
>>
View the other Key Equations and Concepts in this
chapter
Stereoisomers
>> Parts of this equation/concept include:
| A. Identifying Chiral Centers |
A chiral center is an sp3 carbon (four single
bonds) that has a different group at each bond. Remember to consider
not just the next atom in the bond, but all the bonds that follow
it when determining whether the group is the same.
>> Example 1
Do the following molecules have chiral centers? If so, what atoms
are chiral centers?
Solution:
- This molecule has one chiral center. It is the carbon on the
right side of the chain, with a methyl, hydroxyl, and hydrogen
group. The fourth group has an isopropyl chain.
- This molecule also has one chiral center, which is the carbon
in the center. The groups connected to this carbon are hydroxyl,
ethyl, methyl, and hydrogen.
- This molecule has two chiral centers, the two carbons in the
middle. The one on the left end is eliminated for its two hydrogen
groups and the one on the right end has two methyl groups.
>> back
to the Top of the Page
| B. Assigning R
and S to Chiral Centers |
The designation R or S depends on the orientation
of the groups around the chiral center. If there is more than one
chiral center, each one will be labeled R or S.
To designate the orientation, each group is assigned a priority.
The group with the highest priority is labeled a, then b,
c, and d. Groups are assigned higher priority based
on the atomic number (Z), where higher atomic numbers get
higher priority. The atom directly bonded to the chiral center is
considered first. If the atom is the same for two groups, the atoms
bonded to that atom are considered, and so on until there is a difference.
(If there is never a difference, it wasn't a chiral carbon.) Once
the groups are labeled, orient the d to the back, then determine
whether a-b-c goes clockwise (R) or
counterclockwise (S).
>> Example 2
Assign R and S values to the molecules in Example 1.
Solution:
-
The atoms bonded to the chiral carbon are H, O, and C. Since
oxygen has the highest atomic number, it is labeled a.
Since hydrogen has the lowest atomic number, it is d.
The two carbons have the same atomic number, so the next level
of bonding must be considered. The carbon of the methyl group
is bonded only to hydrogens. The other carbon is bonded to
three more carbons. Since carbon has the higher atomic number,
the methyl group is labeled c and the more complicated
group is b.
Tips: You will probably need a model the first few
times you determine R and S. If you use one,
you don't need to build the entire molecule. Instead, assign
specific colors to a, b, c, and d,
and build the model as it is shown in the structure. Remember
that the dashed wedges indicate that the group is behind the
plane and solid wedges mean that the group is in front of
the plane. Solid lines indicate that the group is in the plane.
After the groups are assigned, rotate the d to the
back and determine the orientation of a, b,
and c. If you are consistent about the colors you use,
and pick and familiar sequence (e.g., a = red, b
= white, and c = blue), it will be easier to remember.
It may also help to use a mnemonic to remember R and
S (e.g., R is " right.")
In this example, when d is oriented toward the back
the remaining a, b, and c look like  .
Then direction (counterclockwise) can be determined. This
example has an S chiral center.
-
The order of the groups are a = OH (oxygen has a higher
Z), b = ethyl (two carbons), c = methyl
(only one carbon), and d = hydrogen (Z = 1).
This is also counterclockwise and S.
-
This example has two carbons. The groups are prioritized
for the starred chiral carbon shown below
Therefore this is an R enantiomer.
For the other chiral center, the priorities are:
Therefore this is an S enantiomer.
>> View
the other Key Equations and Concepts in this chapter
|
 |
