Optical
isomerism
(Chirality)
Kevin Augustin Tinapay
Mark Burlaos
Jhon Ivan Nava
Donita Jane Artigas
Pretest
1. Which of the following groups has the highest priority according to the Cahn-Ingold-Prelog
sequence rules?
a) CH3
b) CH2Cl
c) CH2OH
d) CHO
2. Which is the correct assignment of chirality at C2 and C3 of the
following molecule?
a) 2S,3S
b) 2R,3R
c) 2S,3R
d) 2R,3S
3. Which of the following groups has the lowest priority according to the Cahn-Ingold-Prelog
sequence rules?
a) C≡CH
b) CH=CH2
c) CH(OH)CH3
d) CH2CH2OH
4. Which of the following has the (R)
configuration?
a)
b)
c) d)
5. Which is the correct assignment of chirality at C2 and C4 of the
following molecule?
a) 2S,4S
b) 2R,4R
c) 2S,4R
d) 2R,4S
Introduction
Chirality
Molecules that form nonsuperimposable mirror images, and thus exist as enantiomers, are said to
be chiral molecules. For a molecule to be chiral, it cannot contain a plane of symmetry. A plane of
symmetry is a plane that bisects an object (a molecule, in this case) in such a way that the two
halves are identical mirror images.
Chirality is a geometric property of some molecules and ions. A chiral molecule/ion is non-
superposable on its mirror image. The presence of an asymmetric carbon center is one of several
structural features that induce chirality in organic and inorganic molecules. The term chirality is
derived from the Ancient Greek word for hand.
Content
The mirror images of a chiral molecule or ion are called enantiomers or optical isomers. Individual
enantiomers are often designated as either right-handed or left-handed. Chirality is an essential
consideration when discussing the stereochemistry in organic and inorganic chemistry. The
concept is of great practical importance because most biomolecules and pharmaceuticals are
chiral.
Chiral molecules and ions are described by various ways of designating their absolute
configuration, which codify either the entity's geometry or its ability to rotate plane-polarized
light, a common technique in studying chirality.
Chirality means a molecule that is mirrored
won't be superimposable.
Explanation:
A chiral molecule can usually be found if there
is no plane of symmetry, an example in every
day life of this is your hands. (They are mirror images but one can't be put onto the other such that
they would appear the same).
Picture of chiral hands:
To apply this to molecules one must first find a specific atom whether it be a Carbon or other,
where there are four different substituents bonded to the atom.
Four different substituents bonded to a chirality center:
Sometimes it appears that the atom bonded directly to the atom of interest, as a chiral center, is the
same as another one bonded to the atom of interest, so its necessary to look a bit further to see if
substituent is indeed different from the other.
Most commonly, chiral molecules have point chirality, centering around a single atom, usually
carbon, which has four different substituents. The two enantiomers of such compounds are said to
have different absolute configurations at this center. This center is thus stereogenic (i.e., a
grouping within a molecular entity that may be considered a focus of stereoisomerism), and is
exemplified by the α-carbon of amino acids.
The special nature of carbon, its ability to form four bonds to different substituents, means that a
mirror image of the carbon with four different bonds will not be the same as the original
compound, no matter how you try to rotate it.
Understanding this is vital because the goal of organic
chemistry is understanding how to use tools to
synthesize a compound with the desired chirality,
because a different arrangement may have no effect, or
even an undesired one.
A carbon atom is chiral if it has four different items bonded to it at the same time. Most often this
refers to a carbon with three heteroatoms and a hydrogen, or two heteroatoms plus a bond to
another carbon plus a bond to a hydrogen atom. It can also refer to a nitrogen atom bonded to four
different types of molecules, if the nitrogen atom is utilizing its lone pair as a nucleophile. If the
nitrogen has only three bonds it is not chiral, because the lone pair of electrons can flip from one
side of the atom to the other spontaneously.
Any atom in an organic molecule that is bonded to four different types of atoms or chains of atoms
can be considered "chiral".
Chirality of Molecules
Molecules that are not helical like DNA can also be chiral, as long as they cannot be superimposed
with their mirror image. There are two notations for their different configurations: The older D/L
system is often used for biomolecules such as sugars and amino acids and assigns a relative
configuration to the entire molecule, and the R/S system assigns a so-called "absolute
configuration" to a single chiral center. A chiral center is most often a carbon atom with four
different substituents.
D and L come from the Latin dexter and laevus for right and left, while R and S come from rectus
and sinister for right-handed and left-handed. Since the assignments are based on different rules, a
D configuration must not always coincide with an R configuration for a single molecule, and the
same applies to L and S.
An optical isomer can be named by the direction in which it rotates the plane of polarized light. If
an isomer rotates the plane clockwise as seen by a viewer towards whom the light is traveling, that
isomer is labeled (+). Its counterpart is labeled (-). The (+) and (-) isomers have also been termed
d- and l-, respectively (for dextrorotatory and levorotatory). This labeling is easy to confuse with
D- and L- and is therefore not encouraged by IUPAC.
The fact that an enantiomer can rotate polarised light clockwise (d- or +- enantiomer) does not
relate with the relative configuration (D- or L-) of it.
The R/S system is another way to name an optical isomer by its configuration, without involving a
reference molecule such as glyceraldehyde. It labels each chiral center R or S according to a
system by which its ligands are each assigned a priority, according to the Cahn Ingold Prelog
priority rules, based on atomic number.
This system labels each chiral center in a molecule (and also has an extension to chiral molecules
not involving chiral centers). It thus has greater generality than the D/L system, and can label, for
example, an (R,R) isomer versus an (R,S) — diastereomers.
The R/S system has no fixed relation to the (+)/(-) system. An R isomer can be either
dextrorotatory or levorotatory, depending on its exact ligands.
The R/S system also has no fixed relation to the D/L system. For example, one of glyceraldehyde's
ligands is a hydroxy group, -OH. If a thiol group, -SH, were swapped in for it, the D/L labeling
would, by its definition, not be affected by the substitution. But this substitution would invert the
molecule's R/S labeling, due to the fact that sulfur's atomic number is higher than carbon's,
whereas oxygen's is lower. [Note: This seems incorrect. Oxygen has a higher atomic number than
carbon. Sulfur has a higher atomic number than oxygen. The reason the assignment priorities
change in this example is because the CH2SH group gets a higher priority than the CHO, whereas
in glyceraldehyde the CHO takes priority over the CH2OH.]
For this reason, the D/L system remains in common use in certain areas, such as amino acid and
carbohydrate chemistry. It is convenient to have all of the common amino acids of higher
organisms labeled the same way. In D/L, they are all L. In R/S, they are not, conversely, all S —
most are, but cysteine, for example, is R, again because of sulfur's higher atomic number.
In 1813 Jean Baptiste Biot noticed that plane-polarized light was rotated either to the right or the
left when it passed through single crystals of quartz or aqueous solutions of tartaric acid or sugar.
Because they interact with light, substances that can rotate plane-polarized light are said to be
optically active. Those that rotate the plane clockwise (to the right) are said to be dextrorotatory
(from the Latin dexter, "right"). Those that rotate the plane counterclockwise (to the left) are called
levorotatory (from the Latin laevus, "left").
Examples
Chiral molecules rotate polarized light. Their enantiomers (aka optical isomers) rotate light in
equal magnitude and opposite direction. If we know how much a molecule rotates light (specific
rotation), we can actually determine the concentration of each enantiomer based on the observed
rotation of the light as it passes through the solution.
If two enantiomers are known to be in solution and polarized light is not rotated as it passes
through, the solution is racemic—that is, it has equal concentrations of each enantiomer.
3-chloro-2,3,4,5-tetrahydroxypentanal
From top down, the chiral centers are R, S, and R. Including stereochemistry, the name of this
molecule is (2R,3S,4R)-3-chloro-2,3,4,5-tetrahydroxypentanal.
Fischer projection R and S
assign our priorities based on atomic mass and trace around the top three priorities. If the lowest-
priority group is on the vertical like in the molecule on the left, the stereochemistry is as it looks;
if the lowest-priority group is on the horizontal like in the molecule on the right, stereochemistry
is flipped! Let’s see what it looks like before trying on our own.
The chiral center on top is an S for the molecule on the left and an R for the molecule on the right.
1-aminoethanal
The molecule on the left has the hydrogen in the back, so its easy to solve. The one on the right,
though, has the oxygen in the back. What we need to do from here is a) swap the priority values
{1 and 4 here}, b) trace a circle around 1, 2, and 3, and c) take the opposite result from the trace.
For example: if, after the swap, the trace is counterclockwise the actual configuration is an R.
S-1-aminoethanol and R-1-aminoethanol
After swapping priorities, the circle traced is counterclockwise. It looks like an S, but it’s
actually an R because we swapped priorities.
Conclusion
Chirality is a geometric property of some molecules and ions.
A chiral molecule/ion is non-superposable on its mirror image.
The term chirality is derived from the Ancient Greek word for hand.
The presence of an asymmetric carbon center is one of several structural features that
induce chirality in organic and inorganic molecules.
Chirality means a molecule that is mirrored won't be superimposable.
Any atom in an organic molecule that is bonded to four different types of atoms or chains
of atoms can be considered "chiral".
Post-test
1.Chiral is a term that is used to describe a characteristic of an object. What is the meaning of the
term?
a. The characteristic of having a nonsuperposable mirror image.
b. The characteristic of being different about one carbon.
c. The characteristic of one being a straight-chain compound and the other cyclic.
d. The characteristic of one being a straight-chain compound and the other being branched.
2.The molecule that is chiral is __?__.
a. 1-chloroethane
b. 1,1-dicholorethane
c. 1,1,2-trichloroethane
d. 1,2-dichloro-1-iodoethane
3.If a molecule and its mirror image cannot be superimposed, the two isomers have the special
name of
a. racemic mixture
b. chiral pairs
c. achiral pairs
d. enantiomers
4. Optical activity of isomers is
a. only a characteristic of racemic mixtures.
b. is the ability to change the frequency of the light.
c. the ability to rotate the plane of polarized light.
d. the ability to change the color of a light source.
5. A sample is placed in a polarimeter and is found to rotate the plane of the light clockwise. The
compound is
a. at equilibrium
b. dextrorotatory
c. levorotatory
d. none of these
References
Rechts oder links: In der Natur und anderswo (in German),
Henri Brunner,
Wiley-VCH, Weinheim, 1999.
ISBN: 9783527299744
Händigkeit - leben in einer chiralen Welt (in German),
Anne J. Rüger, Joshua Kramer, Stefan Seifermann, Mark Busch, Thierry Muller, Stefan Bräse,
Chem. Unserer Zeit 2012, 46, 294–301.
DOI: 10.1002/ciuz.201200579
https://www.clutchprep.com/organic-chemistry/chirality
https://en.m.wikibooks.org/wiki/Organic_Chemistry/Chirality
http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/chirality.html
