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 ​Before we look at configurations, we need to study chirality and associated ideas.

Chirality and Achirality

image\Ch 19 fig 19-5.png

Figure 19-5

If a molecule does not have a plane of symmetry, it is chiral. A chiral molecule is not superimposable on its mirror image. The structures represented in Figure 19-5 are mirror images. These mirror images are non-superimposable. The molecules depicted in Figure 19-5 are chiral.

image\Ch 19 fig 19-6.png

Figure 19-6

We will now consider Figure 19-6. The mirror images of the molecule depicted are superimposable. Hence it is an achiral molecule. Notice that two of the atoms attached to the central carbon atom are the same, namely the hydrogen atoms.
Achirality can also be recognized by looking at the molecular structure. When we are analyzing the structure of a molecule, we should look and determine whether there is any plane of symmetry in the molecule. Plane of symmetry reflects achirality.
The figure given below shows a stereoisomer of tartaric acid. Notice that this compound has two chiral (stereogenic) centers. But, there is a plane of symmetry and thus the molecule itself is achiral and optically inactive. Such compounds that contain one or more stereogenic centers, but are achiral, are called meso compounds. Hence, having a stereogenic center or chiral carbon does not always lead to chirality of the entire molecule.

image\Ch 19 page 265 g.png

Absolute Configuration

image\Ch 19 Absolute Con.png


In Fischer projection, the horizontal lines represent bonds that are toward you, and the vertical lines represent bonds that are pointing away from you.


Absolute configuration is the arrangement of substituents around the stereogenic center of a chiral molecule. The Fischer projection and the absolute configuration of the amino acid alanine is shown in Figure 19-7.

R-S System of Representation

The R-S system of representation is a convenient and essential way of looking at molecules. The R-S convention is done by prioritizing the substituents that are bonded to the chiral carbon.
The following rules will familiarize you with the R-S naming of compounds.
  1. The orientation of the molecule should be in such a way that the lowest priority group is pointing away from you.
  2. First, prioritize each group that is bonded to the chiral carbon. The priority is based on atomic number. Higher the atomic number of the atom (in the group) that is connected to the chiral carbon, higher the priority of that group. For example, if the four groups connected to the chiral carbon are CH3,H, OH, and Br, then the bromine atom (atomic number 35) has the highest priority. This is followed by the oxygen atom (atomic number 8) of the hydroxyl group, then the carbon atom of the methyl group, followed by the hydrogen atom (atomic number 1). It is important to realize that the priority is determined by the atomic number of the atom that is directly connected to the chiral atom.
  3. If two groups that are attached to a chiral carbon are isotopes (same atomic number, different mass numbers), the heavier isotope takes precedence.
  4. If two groups have the same atom connected to a chiral carbon, then the next atom along the chain determines the priority. If that too fails (if it is the same atom), then go to the next atom to determine which group has higher priority. For example, -CH2F has a lower priority than -CH2I. If the groups contain unsaturations such as double or triple bonds, consider that the atoms on both ends are duplicated depending on the number of bonds.

image\Ch 19 page 267 g1.png

  1. After prioritizing, draw an arrow starting from the first priority group to the third priority group through the second priority group. This is illustrated in the example given below.
  2. If the arrow points in the clockwise direction, the configuration of that chiral carbon is R. If the arrow points in the counterclockwise direction, the configuration of that chiral carbon is S.

image\Ch 19 page 267 g2.png

If the given orientation of a molecule shows the lowest priority group pointing toward the viewer, the orientation should be changed so that the lowest priority group points away form the viewer. In Fischer projection, if the lowest priority group is attached to a vertical bond (the one that points up or down), the molecule should be viewed from another angle so that the lowest priority group points away from the viewer. This can be achieved by doing 2 two-group switches or interchanges. If only 1 two-group interchange is done, the opposite configuration results. See the example given below.


 Find the absolute configuration of the molecule shown below in terms of R-S notation.


image\Ch 19 Example 19-1.png



First, we have to think about the order of priority of the substituents. The order is as follows:


OH > CH2CH3> CH3> H

We can simplify the structure by looking at the three substituents that determine the configuration. Let's redraw them as shown below:

image\Ch 19 Example 19-1 sol.png

Since the direction of priority is clockwise, the configuration is R.

image\Ch 19 page 269 g1.png



Draw the L configuration of the compound shown in Example 19-1.


For L configuration, the direction of priority should be counterclockwise. So the structure can be best represented as follows:

image\Ch 19 Ex 19-2.png

image\Ch 19 Ex 19-2 sol.png


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