STEREOCHEMISTRY

What is Stereochemistry?

Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules.

Stereochemistry is also known as 3D chemistry because the prefix "stereo-" means "three-dimensionality".The study of stereochemical problems spans the entire range of organic, inorganic, biological, physical and supramolecular chemistries. Stereochemistry includes methods for determining and describing these relationships; the effect on the physical or biological properties these relationships impart upon the molecules in question, and the manner in which these relationships influence the reactivity of the molecules in question (dynamic stereochemistry).

Introduction

Molecules can be drawn a variety of ways on paper.

These flat drawings are actually representations of three-dimensional molecules. This three-dimensional character helps to define the properties of a molecule. Changing this three-dimensional character - even in only one place in the molecule - can drastically alter the properties and applications of the molecule.

The first step in learning about stereochemistry is mastering the terminology involved. The following pages will give terms, definitions, and structural examples on the following topics that are important in stereochemistry.

• Isomers
• Constitutional Isomers
• Stereoisomers
• Conformational Isomers
• Configurational Isomers
1. Geometric Isomers
2. Optical Isomers
• Chirality
• Achirality
• Prochirality
• Enantiomers
• Diastereomers
• Rotation of Light

·         Definitions: Isomers 

Isomers are compounds that have the same molecular formula. To determine whether two molecules are isomers, just count how many atoms of each type are in both molecules. If both molecules have the same count for all of the different atoms, the molecules will be isomers.

For example, consider the following molecules.

The structure on the left has 7 carbons, 13 hydrogens, one bromine, and one oxygen, or a molecular formula of C₇H₁₃BrO. The molecule on the right has the same number of each atom and the same molecular formula. Therefore, these two molecules are isomers

Definitions: Constitutional Isomers

Constitutional isomers are compounds that have the same molecular formula and different connectivity. To determine whether two molecules are constitutional isomers, just count the number of each atom in both molecules and see how the atoms are arranged. If both molecules have the same count for all of the different atoms, and the atoms are arranged in different ways (their connectivity is different), the molecules will be constitutional isomers.

(Recall that connectivity means how the atoms are attached to one another. For example, an ether has a connectivity of C-O-C, and an alcohol has a connectivity of C-O-H.)

For example, consider the following molecules.

In the previous section, it was determined that these compounds were constitutional isomers. Notice that, by definition, constitutional isomers cannot be stereoisomers and vice versa. (Recall that constitutional isomers must have different connectivities, while stereoisomers must have the same connectivity.) Therefore, these molecules are not stereoisomers.
There are two major types of stereoisomers, conformational isomers and configurational isomers.

Definitions: Conformational Isomers

Conformational isomers are stereoisomers that can be converted into one another by rotation around a single bond.  (Note: Conformational isomers are normally best seen using Newman Projections, so this structural representation will be used in this section of the tutorial.

For example, eclipsed, gauche, and anti butane are all conformational isomers of one another. (Recall that eclipsed means that identical groups are all directly in-line with one another, gauche means that identical groups are 60 degrees from one another, and anti means that identical groups are 180 degrees from one another.)

These molecules can be interconverted by rotating around the central carbon single bond. For example, eclipsed butane can be made into gauche butane by rotating 60 degrees and into anti butane by rotating 180 degrees. Similarly, gauche butane can be made into anti butane by rotating 120 degrees

Definitions: Configurational Isomers

Configurational isomers are stereoisomers that can cannot be converted into one another by rotation around a single bond. The two main types of configurational isomers are geometric isomers and optical isomers.

Geometric isomers are molecules that are locked into their spatial positions with respect to one another due to a double bond or a ring structure.

For example, consider the following two molecules.

In the ring on the left, the methyl groups are on the same side of the ring (cis), and in the molecule on the right, the methyl groups are on opposite sides of the ring (trans). These are geometric isomers because the ring structure will not allow these molecules to interconvert

Optical isomers are molecules that differ three-dimensionally by the placement of substituents around one or more atoms in a molecule. Optical isomers were given their name because they were first able to be distinguished by how they rotated plane-polarized light. These molecules are not necessarily locked into their positions, but cannot be converted into one another, even by a rotation around a single bond.

For example, consider the following two molecules.

In the molecule on the left, the chlorine is oriented upward, and in the molecule on the right, the chlorine is oriented downward. (These molecules are presented in Wedge-Dash Notation, which will be covered in more detail in a later section in the tutorial. To learn about this notation now, click here. To navigate back to this page from the Wedge-Dash information page, use the Back button in the browser, not within the tutorial.)

Definitions: Chiral

A molecule is chiral if it is not superimposable on its mirror image. Most chiral molecules can be identified by their lack of a plane of symmetry or a center of symmetry. Your hand is a chiral object, as it does not have either of these types of symmetry.

The molecule on the left has a plane of symmetry through the center carbon. This is a mirror plane; in other words, one half of the molecule is a perfect reflection of the other half of the molecule. This molecule is not chiral because of its mirror plane.

Definitions: Achiral

A molecule is achiral if it is superimposable on its mirror image. Most achiral molecules do have a plane of symmetry or a center of symmetry. Achiral molecules that contain a stereocenter are called meso.

The molecules discussed in the previous section are achiral because they possess either a plane of symmetry or a center of symmetry.

Definitions: Prochiral

A molecule is prochiral if the addition of a new group or an exchange of one group on the molecule would create a new stereocenter and, therefore, a chiral molecule. A prochiral atom must be bonded to three different groups before any change is made.

For example, consider the following molecules.

The molecule on the left is prochiral because a new stereocenter can be made by replacing one group on the carbon marked with an asterisk (*) with a new one. The molecule on the right is prochiral because a new stereocenter can be made by adding a new group to the carbon marked with an asterisk.

Definitions: Enantiomers

Now that chirality within a molecule has been discussed, the relationships between two or more chiral molecules can be determined.

Enantiomers are chiral molecules that are mirror images of one another. Furthermore, the molecules are non-superimposable on one another. This means that the molecules cannot be placed on top of one another and give the same molecule. Chiral molecules with one or more stereocenters can be enantiomers. It is sometimes difficult to determine whether or not two molecules are enantiomers. For introductory purposes, simple molecules will be used as examples. More complex examples will be given later.

For example, consider the following molecules.

These molecules are mirror images of one another. Additionally, these molecules are non-superimposable because if one of these molecules, the one on the right, is flipped 180 degrees (so that the chlorines are aligned, as shown below), the stereochemistry is different (one chlorine is wedged and the other is dashed). Therefore, these molecules are enantiomers. (Note: When flipping molecules using the Wedge-Dash notation, groups that are wedged become dashed, and groups that are dashed become wedged.)

Definitions: Diastereomers

Diastereomers are stereoisomers that are not mirror images of one another and are non-superimposable on one another. Stereoisomers with two or more stereocenters can be diastereomers. It is sometimes difficult to determine whether or not two molecules are diastereomers. For introductory purposes, simple molecules will be used as examples. More complex examples will be given later.

For example, consider the following molecules.

These molecules are not mirror images of one another. Additionally, these molecules are non-superimposable because if one of these molecules is flipped 180 degrees (so that the alcohols and methyls are aligned, as shown below), the stereochemistry is different at one carbon (the alcohols) and the same at another carbon (the methyls). Therefore, these molecules are diastereomers.

Definitions: Rotation of Light

Optical isomers are named because they can rotate a plane of polarized light. Light is plane-polarized if all of the light waves are vibrating in the same, parallel, direction. This is shown below with a single wave (line) below.

If plane-polarized light is rotated 45 degrees to the left (counterclockwise), this is known as levorotatory (l, -); this is shown below in the image on the left. If plane-polarized light is rotated 45 degrees to the right (clockwise) this is known as dextrorotatory (d, +); this is shown below in the image on the right. Both d and l rotations are considered by looking in the direction the light is traveling; in other words, the light is moving away from you, not toward you.

Three-Dimensional Representations: Cyclic Structures

Cyclic, or ring, structures can also be drawn in a variety of ways. Rings can have many different sizes, with the smallest ring being cyclopropane (three carbons). The most common ring types in organic chemistry are cyclopentane (five carbons) and cyclohexane (six carbons). Cyclohexane will be discussed in this tutorial, and its three main structural types can be seen above. From left to right, these structures are the Wedge-Dash Notation, the Haworth Projection, and the Chair Conformation.

Three-Dimensional Representations: Straight-Chain Structures

There are four main types of representations for straight-chain molecules, as shown above. From left to right, these structures are the Sawhorse Projection, the Fisher Projection, the Newman Projection, and the Wedge-Dash notation.

http://www.chemeddl.org/resources/stereochem/introduction1.htm