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Chiral Molecules: Enantiomers, Diastereomers, and Meso Compounds, Study Guides, Projects, Research of Stereochemistry

The Southern Baptist Theological SeminaryStereochemistry

The concepts of chiral molecules, enantiomers, diastereomers, and meso compounds. It covers the relationships between chiral centers and chiral molecules, the assignment of absolute configurations, and the differences between achiral and chiral molecules containing multiple chiral centers. It also includes examples and summaries.

Typology: Study Guides, Projects, Research

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SUPPLEMENTARY NOTES FOR STEREOCHEMISTRY
SOME IMPORTANT CONCEPTS IN STEREOCHEMISTRY
1. RELATIONSHIP BETWEEN SYMMETRY AND CHIRALITY
Asymmetric objects are chiral
Symmetric objects are achiral
2. RELATIONSHIP BETWEEN OBJECTS AND THEIR MIRROR IMAGES
Symmetric objects are superposable with their mirror images. They are one and the same.
Asymmetric objects are nonsuperposable with their mirror images. They are different objects.
In the case of molecules, chiral molecules and their mirror images are different molecules.
Chiral molecules and their mirror images are a kind of stereoisomers called enantiomers.
3. DEFINITIONS
Stereoisomers - Compounds that have the same molecular formula and the same connectivity,
but different arrangement of the atoms in 3-dimensional space.
Stereoisomers cannot be converted into each other without breaking bonds.
Enantiomers - Nonsuperposable mirror images, or chiral molecules which are mirror images.
Chiral, or asymmetric carbon - A tetrahedral carbon atom bearing four different substituents.
Chirality centers, or stereocenters - Asymmetrically substituted atoms in a molecular structure.
The most common type encountered in this course will be the chiral carbon described above.
Diastereomers - Stereoisomers which are not enantiomers (or mirror images).
Meso compounds, or meso forms - Symmetric, or achiral molecules that contain stereocenters.
Meso compounds and their mirror images are not stereoisomers, since they are identical.
Optical activity - The ability of chiral substances to rotate the plane of polarized light by a specific
angle.
Dextrorotatory - Ability of chiral substances to rotate the plane of polarized light to the right.
Levorotatory - Ability of chiral substances to rotate the plane of polarized light to the left.
Specific rotation - The measured angle of rotation of polarized light by a pure chiral sample under
specified standard conditions (refer to textbook for a description of these).
Racemic mixture, racemic modification, or racemate - A mixture consisting of equal amounts
of enantiomers. A racemic mixture exhibits no optical activity because the activities of the
individual enantiomers are equal and opposite in value, therby canceling each other out.
Optical purity - The difference in percent between two enantiomers present in a mixture in unequal
amounts. For example, if a mixture contains 75% of one enantiomer and 25% of the other,
the optical purity is 75-25 = 50%.
Absolute configuration - A description of the precise 3-dimensional topography of the molecule.
Relative configuration - A description of the 3-dimensional topography of the molecule relative
to an arbitrary standard. Absolute and relative configurations may or may not coincide.
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SUPPLEMENTARY NOTES FOR STEREOCHEMISTRY

SOME IMPORTANT CONCEPTS IN STEREOCHEMISTRY

1. RELATIONSHIP BETWEEN SYMMETRY AND CHIRALITY

Asymmetric objects are chiral

Symmetric objects are achiral

2. RELATIONSHIP BETWEEN OBJECTS AND THEIR MIRROR IMAGES

Symmetric objects are superposable with their mirror images. They are one and the same.

Asymmetric objects are nonsuperposable with their mirror images. They are different objects.

In the case of molecules, chiral molecules and their mirror images are different molecules.

Chiral molecules and their mirror images are a kind of stereoisomers called enantiomers.

3. DEFINITIONS

Stereoisomers - Compounds that have the same molecular formula and the same connectivity,

but different arrangement of the atoms in 3-dimensional space.

Stereoisomers cannot be converted into each other without breaking bonds.

Enantiomers - Nonsuperposable mirror images, or chiral molecules which are mirror images.

Chiral, or asymmetric carbon - A tetrahedral carbon atom bearing four different substituents.

Chirality centers, or stereocenters - Asymmetrically substituted atoms in a molecular structure.

The most common type encountered in this course will be the chiral carbon described above.

Diastereomers - Stereoisomers which are not enantiomers (or mirror images).

Meso compounds, or meso forms - Symmetric, or achiral molecules that contain stereocenters.

Meso compounds and their mirror images are not stereoisomers, since they are identical.

Optical activity - The ability of chiral substances to rotate the plane of polarized light by a specific

angle.

Dextrorotatory - Ability of chiral substances to rotate the plane of polarized light to the right.

Levorotatory - Ability of chiral substances to rotate the plane of polarized light to the left.

Specific rotation - The measured angle of rotation of polarized light by a pure chiral sample under

specified standard conditions (refer to textbook for a description of these).

Racemic mixture, racemic modification, or racemate - A mixture consisting of equal amounts

of enantiomers. A racemic mixture exhibits no optical activity because the activities of the

individual enantiomers are equal and opposite in value, therby canceling each other out.

Optical purity - The difference in percent between two enantiomers present in a mixture in unequal

amounts. For example, if a mixture contains 75% of one enantiomer and 25% of the other,

the optical purity is 75-25 = 50%.

Absolute configuration - A description of the precise 3-dimensional topography of the molecule.

Relative configuration - A description of the 3-dimensional topography of the molecule relative

to an arbitrary standard. Absolute and relative configurations may or may not coincide.

4. RELATIONSHIPS BETWEEN CHIRAL CENTERS AND CHIRAL MOLECULES - The term chiral center refers

to an atom in the molecular structure. The term chiral molecule refers to the entire molecule.

The presence of one chiral center renders the entire molecule chiral. The presence of two or more chiral

centers may or may not result in the molecule being chiral. In the examples given below the chiral centers

are indicated with an asterisk. The vertical broken line represents a plane of symmetry.

5. RELATIONSHIPS BETWEEN CONFORMATIONS AND CHIRALITY - The primary criterion to determine

molecular chirality is the absence of any symmetry elements. However, some achiral molecules have chiral

conformations. For example the chair conformations of 1,2-disubstituted cyclohexanes are chiral, yet the

molecule as a whole is considered achiral. On the whole, we can apply the following criteria.

a) If the contributing conformations average out to an achiral conformation, then the molecule is considered

achiral. Such molecules do not show optical activity. In the case of 1,2-disubstituted cyclohexanes the two

most stable conformations are chiral. If we could freeze and isolate one of them, it would exhibit optical

activity. But because they are mirror images in equilibrium, their optical activities cancel out and the sample

is optically inactive. A similar example is illustrated by the conformations of (2R,3S)-1,2-dichlorobutane,

which again is achiral, even though some of its conformations are chiral.

OH O

Ibuprofen. One chiral center

renders the molecule chiral

H 3 C CH 3 H 3 C CH 3

cis -1,2-dimethylcyclohexane

is an achiral molecule

trans -1,2-dimethylcyclohexane

is a chiral molecule

Cl

Cl

Cl

Cl

Chiral conformations in equilibrium.

The molecule is achiral

(2R,3S) -2,3-dichlorobutane

H 3 C H

Cl

Cl

H 3 C H

Cl

H 3 C H

H CH 3

Cl

Cl Cl

H 3 C CH 3

H H

Cl

H 3 C

H

H

CH 3

Cl

Cl

CH 3

H

H

H 3 C

Cl

Cl

H CH 3

H 3 C H

Cl

4. If there are atoms containing double or triple bonds, count them twice or thrice respectively. This holds

for each of the atoms involved in the double or triple bonding.

C

H 2 C

C H 3

H O

H

C H 3

1

2

(^4 3) C

H 2 C

C H

H O

H

C H 3

1

2

3

4

C H 3

C H 3 C

H 2 C

C H 3

H O

H

O H

1

2

4 3

C

H 2 C

C H

H O

H

C H 2

1

2

3

4

C H 3

C H 3

O H

C

H 2 C

C H

H 2 C

H

C H 2

1

2

3

4

C H 3

C H 3

OH

H 3 C

C

H C

CH 2

H 3 C

H

C H 2

1 2

3

4

CH 3

C C

becomes C C

C C

C C

becomes C C

C

C

C

C

becomes C

C H

CH 2

H 3 C

H

C H 2

CH 3

H 3 C

C H 3

5. Although not obvious from the above examples, when duplicating the atoms involved in double or triple

bonding they are also being crossed over at the same time. This only becomes apparent when the atoms

involved in multiple bonding are not of the same kind, as in the examples shown on the following page.

C

H C

CH 2

CH

C H 2

1 2

3 4

C O

becomes C O

O C

becomes C

C H

H 3 C C H 2

C H 3

C N

becomes C N

N C

OH

CH 3

C CH 3

H

O CH 2

OH

CH

CH 3

CH 3

C

O

H O

H 3 C

H

If the lowest priority group is positioned on the plane of the paper, we can momentarily exchange it with

whatever group happens to be positioned in the back, then assign configuration, then reverse it.

C

C H 3

H HO

Br 1 2

3

C

C H 3

Br HO

H

momentarily exchange

the H and the Br atoms

Configuration is now ( R )

Reverse to obtain the

correct configuration

( S )

C

C H 3

H HO

Br

ASSIGNING ABSOLUTE CONFIGURATIONS IN CYCLIC MOLECULES

Cyclic molecules are frequently represented on paper in such a way that the ring atoms are all lying on the

plane of the paper, and substituents are either coming out of the paper towards the front or towards the back.

It is therefore easy to assign configuration to any chiral centers forming part of the ring, since the lowest

priority substituent will be either pointing to the front or to the back. However, always make sure there is

in fact a chiral center present. The fact that a 3-D representation is given does not necessarily mean there

is a chiral center in the molecule.

Br H

No chiral centers present anywhere

Br H CH 3

1

2 3

A chiral center is present with the (R) configuration

CH 3

O

H

H H

H

CH 3

H 3 C H

H

H

1

(^32)

Although it may look cumbersome, sometimes it helps to spell out the structure in more detail to see the order of priorities clearly

ASSIGNING ABSOLUTE CONFIGURATIONS IN FISCHER FORMULAS

The key points to keep in mind regarding Fischer projection formulas are:

1. Horizontal lines represent bonds to the chiral carbon that are coming out of the plane of the paper towards

the front, whereas vertical lines represent bonds going behind the plane of the paper towards the back. Thus,

Fischer formulas are easily translated into “bow tie” formulas, which are 3-D formulas.

COOH

CH 3

HO H

Fischer formula

"Bow tie" formula

HO (^) C H

COOH

CH 3

2. The lowest priority group bonded to the chiral carbon must always be shown as a horizontal bond.

The process of assigning ( R ) or ( S ) configuration to the chiral carbon is the same as outlined before, but since

the lowest priority group is pointing towards the front, the configuration obtained directly from a Fischer

formula is the opposite of the actual one.

COOH

CH 3

HO H

1

2

3

The order of priorities follows a clockwise direction in the Fischer

formula. Therefore the actual configuration of this molecule is ( S ).

Once we know the actual configuration, we can represent the molecule in any of several possible ways

using 3-D formulas. Thus the formulas shown below all represent the same molecule as given above in

Fischer projection form. That is to say, all have the ( S ) configuration at the central carbon.

COOH

H 3 C OH

H

COOH

H CH^3

HO

COOH

H 3 C OH

H

COOH

HO CH 3

H

CH 3

HO^ COOH

H

Identifying planes of symmetry in Fischer formulas is relatively easy, since they are planar representations.

The following illustrations show examples of both chiral and achiral molecules.

CHO

H OH

H OH

CHO

achiral

CHO

H OH

HO H

CHO

chiral

CHO

H OH

H OH

CH 2 CH 3

achiral

The process of assigning absolute configurations to the chiral centers in molecules containing two or more

of them is basically an extension of the process followed for molecules containing only one. However, it

helps to isolate the chiral centers and deal with one at a time to avoid confusion.

The following example illustrates this point. In this example we have numbered the carbon atoms in the

main chain according to IUPAC rules that will be studied later. We have also marked the chiral carbons with

asterisks.

Once the chiral centers have been identified, we focus on one at a time (shown as a red dot). First, we isolate

carbon-2, then we assign priorities to the groups bonded to it, and assign configuration. In this case the

configuration of carbon-2 turns out to be ( R ). A similar process for carbon-3 also leads to ( R ) configuration.

CHO

H OH

H OH

CH 2 OH

1 2 3

4

C H OH H C OH CH 2 OH

O H 1

2

3

Carbon-2 (red dot) has the ( R ) configuration

C

H C OH

H OH

C

1

2

3

O H

H

OH

H

Carbon-3 (red dot) also has the ( R ) configuration

The IUPAC notation used to indicate the configurations at carbons 2 and 3 is therefore (2 R , 3 R ).

ASSIGNING CONFIGURATION TO CONFORMATIONALLY MOBILE SYSTEMS

This is probably one of the trickiest situations to deal with, especially when the molecule is shown to us in

conformations such as a cyclohexane chair, or represented by Newman projections. It is a good idea in these

cases to work with models, because one cannot help but to turn the molecule around until the of the lowest

priority groups are positioned where they should be, and the priorities of the groups attached to the chiral

centers can be clearly seen.

In the case of cyclohexane chairs and other rings, it’s a good idea to flatten the ring and position it on the

plane of the paper, with the lowest priority groups pointing towards the back when possible.

Br

Br

H H Br Br

S R cis

Br Br H H Br

R trans Br

R

H

H

Br

R OH

S cis

Br H

OH

Br

S H

trans

S

OH

Br

OH

In the case of Newman projections, it helps to rotate one of the carbons around the C–C bond under

consideration until as many similar groups as possible are aligned (eclipsing each other), then rotate the

structure sideways to obtain a side view, rather than a projection, then assign priorities and configuration.

Br H

CH 3

CH 3

H Br

Br

H CH 3

Br H

CH 3 H

3 C^ CH^3

H Br

Br^ H

R R

Chiral molecules with two or more chiral centers can also have stereoisomers which are not their mirror

images. Such sets of stereoisomers are called diastereomers. In the following example the two molecules

shown are stereoisomers (same connectivity but different spacial arrangement of the atoms) but they are not

mirror images. Their relationship is one of diastereomers.

Cl

Cl Cis- and trans -1,2-dichlorocyclohexane. are examples of diasteromers

Cl

Cl

R S R R

As a corollary, we can state that cis/trans pairs of disubstituted cyclohexanes (or any other rings for this

matter) are always diastereomers. Notice that we have referred to such sets before as geometric isomers.

Geometric isomers are in fact a subcategory of diastereomers.

The following example is an illustration of open chain molecules with a diastereomeric relationship.

Also notice that in the above examples one of the members in each pair is chiral and the other is not.

Diastereomeric sets are frequently made up of molecules where one of the molecules is chiral and the other

is not. Also notice that although they are not mirror images, part of their structures do mirror each other. It

is frequently the case that one half of one molecule mirrors one half of the other one, but the other halves

are identical.

H 3 C CH 3

H Br

Br^ H

R R

H 3 C CH 3

Br Br

H H

S R

(2 R , 3 R )-2,3-dibromobutane meso -2,3-dibromobutane

Cl

Cl

In this pair of cis/trans isomers, the top half of one molecule

mirrors the top half of the other one (the chiral centers have

opposite configurations), while the bottom halves are the

same (the chiral centers have the same configuration).

Cl

Cl

R S R (^) R

If n = number of chiral centers, the maximum possible number of stereoisomers is 2 n

EXAMPLE 1: Possible combinations for 2,3-dibromobutane.

C C

H Br H Br

CH (^3)

CH (^3)

C C

Br H Br H

CH (^3)

CH (^3)

C C

H Br Br H

CH (^3)

CH (^3)

C

C H Br

Br H

CH (^3)

CH (^3)

CH (^3)

CH (^3)

H Br H Br

CH (^3)

CH (^3)

Br H Br H

CH (^3)

CH (^3)

H Br Br H

CH (^3)

CH (^3)

Br H H Br

(S)

(R)

(R) (S)

(S) (S)

(R) (R)

Diastereomers

Meso forms (same) Enantiomers

n 1 2 3 4 5

2 n^ 2 4 8 16 32

SUMMARY OF RELATIONSHIPS BETWEEN MOLECULES WITH TWO OR MORE CHIRAL CENTERS