Stereochemistry . It is the branch of chemistry that is responsible for studying the arrangement that atoms belonging to a molecule adopt in space and how this affects the properties and reactivity of these molecules. The name comes from two terms, first the stereo term that comes from the Greek stereos, which means ‘solid’, and a well-known word, chemistry.
Summary
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- 1 Basics of stereochemistry
- 2 Chirality
- 1 Structural isomers:
- 2 Stereoisomers:
- 3 Enantiomers:
- 4 Diasteroisomers:
- 3 Symmetry in organic chemistry
- 1 Chiral or stereogenic center
- 2 Optical activity
- 4 Importance of stereochemistry
- 5 Sources
Basics of stereochemistry
The foundations of stereochemistry were exposed by Jacobus H. Van’t Hoff and Joseph Achille Le Bel , in the year 1874 . Independently they proposed that the four substituents of a carbon are directed towards the vertices of a tetrahedron, with the carbon in the center of it. To understand the properties of organic compounds, it is necessary to consider the three spatial dimensions.
Previously, Louis Pasteur in 1849 working with tartaric acid salts was the first chemist to observe and describe stereochemistry, who, obtained from the production of wine, observed that crystals of these formed and some of them rotated the plane of polarized light clockwise and counterclockwise ; however, they both possessed the same physical and chemical properties.
Chirality
An object is said to be chiral if it is not superimposable with its mirror image. The term “chiral” was introduced by Lord Kelvin , who stated:
“I call chiral and I say that every geometric figure, or every group of points, has chirality, if its image in a flat mirror, ideally made, cannot be made to coincide with itself.”
Let’s take our left hand as an example, the mirror image of it is our right hand. If we try to superimpose them quickly we will come to the conclusion that it is not possible, since the only way to make the fingers coincide is by rotating one of the two hands 180º but since the back of the hand is different from the palm, it is impossible a hand with the rotated version of the other.
An important part of stereochemistry is dedicated to the study of chiral molecules, among them are known:
Structural isomers:
They are molecules with the same molecular formula but whose atoms are linked in a different order.
Stereoisomers:
They are isomeric substances that have the same composition and connectivity between their atoms but differ in how they are located in space. This different arrangement can cause two stereoisomers to have different properties or react differently.
Enantiomers:
They are non-superimposable mirror image stereoisomers (each is the mirror image of the other, but cannot be superimposed in space), optical isomers, considered non-superimposable mirror images. They have identical physical and chemical properties, except for the direction of rotation of polarized light.
Diasteroisomers:
The opposite of enantiomers: stereoisomers that do not have a mirror image between them. Two compounds are diastereoisomeric if, being stereoisomers, they are not enantiomers, that is, they are not mirror images of each other. The a couple of diastereoisomers can differ in their physical properties and have different reactivity (even in the presence of achiral reagents).
Symmetry in organic chemistry
A symmetry element is a line , a plane or point to which, after applying a symmetry operation (such as a rotation or reflection), it leaves us an object indistinguishable from the original. Let’s consider five symmetry operations:
- Rotation about an axis of symmetry
- Identity
- Reflection with respect to a plane of symmetry
- Reflection with respect to a point of symmetry
- Improper rotation about an improper axis of rotation
Rotation about an axis of symmetry (Cn)
The existence of a Cn axis of rotation in a molecule implies that by rotating the molecule 360 / n degrees the atoms will be in the same position as before rotating it. For example, we can rotate the cis-dichloroethene 180º along an axis perpendicular to the plane of the double bond that passes through its midpoint and leave the molecule unchanged. The “n” will be n = 360/180 = 2, so we say that it is an axis of symmetry of order 2. When there is more than one axis in a molecule, we say that the one with the highest order (major “n”) is the main one . If there are several axes of the same order, we will take as main the one that crosses the largest number of atoms.
Identity (E)
The identity operation consists of rotating the object 360º so that it remains unchanged and would correspond to a C1 axis of rotation. We can see that any object has this element of symmetry. We will say that those molecules to which the only operation that we can apply is identity are asymmetric.
Reflection with respect to a plane of symmetry (σ)
When in the molecule there is a plane of symmetry for each atom on one side of it, we can find another of the same type and in the same position and distance from the plane, as if we reflected one side of the plane in a mirror. When there are axes of rotation and planes of symmetry in the molecule, we can analyze the relationship between them. Thus, we will say that a plane of symmetry is vertical and we will designate it σv when it contains the main axis of rotation. If a plane is perpendicular to the main axis of rotation, we will say that it is horizontal (σh). If the plane is parallel to the main axis and bisects the angle formed by two axes of rotation C2, we will say that it is diagonal (σd).
The left and right hands are not superimposable; in fact, the word chiral is derived from the Greek “cheir” which means hand.
A chiral object does not have planes of symmetry or points (centers) of symmetry. A plane of symmetry passes through the object and divides it in two such that the image on one side of the plane is the mirror image (reflection, as seen in the mirror ) of the other. That is, there is a point-to-point correspondence between the two sides of the plane; For each point located on one side of the plane there will be another equal, at the same distance, on the other side of the plane. A point of symmetry (or center of inversion) is located on the object in such a way that any point can be reflected through this center and leave the object intact.
Every object has a mirror image, the molecules also and therefore can be classified as chiral or achiral (non-chiral).
Chiral or stereogenic center
There is an easy way to predict whether or not a molecule is chiral, and that is to find the chiral centers. A chiral (or stereogenic) center is obtained when one central atom and four other different atoms or groups of atoms come together, adopting a tetrahedral molecular geometry. For example, a carbon atom attached to other atoms (or groups of atoms) that are different from each other. The central atom does not have to be of carbon necessarily, existing chiral centers on atoms of sulfur , silicon , etc.
Whenever a molecule has a single stereogenic center, it will be chiral but if it has two or more centers, this is not true, with achiral isomers (see meso forms). It is important to note that there may be molecules that even without possessing stereogenic centers can be chiral, so the absence of a chiral center does not necessarily indicate that the compound is achiral (to ensure this, it is necessary to verify the absence of planes and centers of symmetry). As an example of these molecules are compounds that have the rotation around sigma CC bonds impeded (for example, due to steric effects or to possess double bonds), an example being the allenes ( accumulated dienes ).
Optical activity
The optical activity of a chemical compound is called the property that certain molecules have of being able to rotate the plane of a polarized light beam. Chirality is a necessary but not sufficient condition for the existence of optical activity.
Only those molecules that are chiral, that is, asymmetric, possess optical activity. Due to the different location of atoms in space, chiral molecules interact with polarized light to different degrees, so that two pairs of enantiomers rotate the plane of polarized light in the same angle but in a different direction (one to the other). right: clockwise, and the other to the left: left-handed) if they are in the same concentration in the solutionIt will not rotate the plane of the polarized light. Stereoisomers that are related to each other, other than enantiomerism, can present specific rotation values without any relation between them, (The specific rotation of each substance by its concentration is added, giving towards the rotation of the problem solution).
Importance of stereochemistry
– This branch of chemistry is very important in the area of polymers. For example, natural rubber consists of repeating units of cis-polyisoprene, almost 100%, while synthetic rubber consists of units of trans-polyisoprene or a mixture of both. The resilience of the two is different and the physical properties of natural rubber are still far superior to the physical properties of synthetic.
– Other important cases include polystyrene and polypropylene , whose physical properties are increased when their tacticity is correct.
– In medicine , the most representative case about the importance of stereochemistry is the so-called thalidomide disaster, a drug synthesized in 1957 in Germany , prescribed for pregnant women in the treatment of morning sickness. However, it was shown that the drug could cause deformations in babies, after which the drug was thoroughly studied and it was concluded that one isomer was safe while the other had teratogenic effects, causing severe genetic damage to the embryo growing.
– The human body produces a racemic mixture of both isomers, even if only one of them is introduced.
– Provides knowledge for general chemistry, whether inorganic, organic, biological, physicochemical or polymer chemistry.
