The chirality It is a geometric property in which an object can have two images: one right and one left, which are not interchangeable; that is, they are spatially different, even though the rest of their properties are identical. An object exhibiting chirality is simply said to be 'chiral'.
The right and left hands are chiral: one is the reflection (mirror image) of the other, but they are not the same, since when placing one on top of the other their thumbs do not coincide.
More than a mirror, to know if an object is chiral, the following question should be asked: does it have “versions” for both the left and right sides?
For example, a left-handed desk and a right-handed one are chiral objects; two vehicles of the same model but with the steering wheel on the left or right; a pair of shoes, as well as feet; spiral staircases in the left direction, and in the right direction, etc..
And in chemistry, molecules are no exception: they can also be chiral. The image shows a pair of molecules with tetrahedral geometry. Even if the one on the left is turned over and the blue and purple spheres are made to touch, the brown and green spheres will “look” out of the plane..
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With molecules it is not so easy to define which is the left or right "version" just by looking at them. For this, organic chemists resort to the Cahn-Ingold-Prelog (R) or (S) configurations, or in the optical property of these chiral substances to rotate polarized light (which is also a chiral element)..
However, it is not difficult to determine whether a molecule or compound is chiral just by looking at its structure. What is the striking feature of the pair of molecules in the upper image??
It has four different substituents, each with its own characteristic color, and also the geometry around the central atom is tetrahedric..
If in a structure there is an atom with four different substituents, it can be stated (in most cases) that the molecule is chiral.
Then it is said that in the structure there is a center of chirality or stereogenic center. Where there is one, there will be a pair of stereoisomers known as enantiomers.
The two molecules in the image are enantiomers. The greater the number of chiral centers that a compound possesses, the greater its spatial diversity..
The central atom is generally a carbon atom in all biomolecules and compounds with pharmacological activity; however it can also be one of phosphorus, nitrogen or a metal.
The center of chirality is perhaps one of the most important elements in determining whether a compound is chiral or not..
However, there are other factors that may go unnoticed, but in 3D models, they reveal a mirror image that cannot be superimposed..
For these structures it is then said that instead of the center they have other elements of chirality. With this in mind, the presence of an asymmetric center with four substituents is no longer enough, but the rest of the structure must also be carefully analyzed; and thus be able to differentiate one stereoisomer from another.
Compounds shown above may appear flat to the naked eye, but they really are not. To the left is the general structure of an allene, where R denotes the four different substituents; and on the right hand side, the general structure of a biphenyl compound.
The extreme where R meet3 and R4 could be visualized as a "fin" perpendicular to the plane where R lies1 and Rtwo.
If an observer analyzes such molecules by positioning the eye in front of the first carbon bound to R1 and Rtwo (for alene), you will see R1 and Rtwo to the left and right sides, and to R4 and R3 up and down.
If R3 and R4 remain fixed, but are changed R1 on the right, and Rtwo on the left, there will then be another "spatial version".
It is here that the observer can then conclude that he found an axis of chirality for allene; the same happens with biphenyl, but with the aromatic rings in the vision.
Note that in the previous example the chirality axis lay in the C = C = C skeleton, for allene, and in the Ar-Ar bond, for biphenyl.
For the compounds above called heptahelcenes (because they have seven rings), what is their axis of chirality? The answer is given in the same image above: the Z axis, that of the propeller.
Therefore, to discern one enantiomer from another, you have to look at these molecules from above (preferably).
In this way, it can be detailed that a heptahelicene rotates clockwise (left side of the image), or counterclockwise (right side of the image).
Suppose you no longer have a helicene, but a molecule with non-coplanar rings; that is, one is located above or below the other (or they are not on the same plane).
Here the chiral character does not rest so much on the ring, but on its substituents; these are the ones that define each of the two enantiomers.
For example, in the ferrocene in the upper image, the rings that "sandwich" the Fe atom do not change; but the spatial orientation of the ring with the nitrogen atom and the group -N (CH3)two.
In the image the group -N (CH3)two points to the left, but in its enantiomer it will point to the right.
For macromolecules or those with unique structures, the picture begins to simplify. Why? Because from their 3D models it is possible to see from a bird's eye if they are chiral or not, as with the objects in the initial examples.
For example, a carbon nanotube can show patterns of turns to the left, and therefore it is chiral if there is an identical one but with turns to the right..
The same occurs with other structures where, despite not having centers of chirality, the spatial arrangement of all their atoms can adopt chiral forms..
We speak then of an inherent chirality, which does not depend on an atom but on the whole set..
A chemically forceful way of differentiating the "left image" from the right one is through a stereoselective reaction; that is, one where it can only occur with one enantiomer, while with the other no.
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