Isomerism is a chemical phenomenon characterized in that molecules with the same number of atoms have different chemical or physical properties. Thus, it occurs in isomers, which in turn are molecules whose chemical formulas are the same, but their structures or spatial arrangements differ substantially from each other..
We have constitutional or structural isomerism, and spatial isomerism, which concentrates a lot of attention on stereochemistry. The flowchart in the image below shows that isomers are mainly divided into the two types of isomerism just mentioned: constitutional and stereoisomers..
All isomers, for a certain compound, have the same chemical formula. But the structures that these atoms can build and their proportions is unique for each molecule or isomer. Also, although the structure itself is the same, the atoms or groups can occupy different spatial positions..
The flow chart shows some examples of isomers that will be discussed in the next sections..
Constitutional or structural isomers are those that differ in the ordering of the molecular backbone. That is, their atoms are not linked in the same way, so the structure or functional groups in question can become very different..
For example, for the same chemical formula C7H10Or there may be multiple molecules that meet that number of atoms; all will have a single oxygen atom, but the structural arrangement of the other atoms will be unique for each isomer.
And hence the chemical and physical properties of all constitutional isomers for formula C7H10Or they will also differ greatly from each other.
On the other hand, constitutional isomers can be very simple, varying only in the relative position of a single atom or group. In the next section you will see an example of this and the above said.
Stereoisomers are those that have the same structural ordering, but differ in the positions and spatial orientations of their atoms or groups..
For their existence to be possible, molecules must contain stereogenic centers, the most famous in organic chemistry being asymmetric or chiral carbon atoms; that is, with four different substituents.
In addition to asymmetric carbons, the considered molecule must lack elements of symmetry. Otherwise, it is impossible for the spatial ordering to become stereoisomerism..
Enantiomers are stereoisomers consisting of non-overlapping mirror images. For example, the right and left hands are not overlapping, as are shoes or gloves. The next section will address the classic example of enantiomerism: bromochloro domethane: CBrClI.
Diastereomers, on the other hand, are also stereoisomers but they are not mirror images. That is to say, that placed one in front of the other they are not the reflection of themselves. Of the best known diastereomers we have the cis-trans isomerism, also called geometric isomerism, especially when applied to alkenes.
Conformers, although within the spectrum of stereoisomerism, are not actually isomers. They are spatial configurations that a molecule, at a certain moment or energy state, acquires through the rotations of several of its bonds. We will see in the next section the example of the conformers of the n-hexane.
Finally we have rotamers, which resemble conformers in the sense that they depend on the rotation of covalent bonds..
However, a single bond between two carbon atoms is taken as a reference, and the relative positions, whether eclipsed or alternated, of the substituents for both carbons are compared. Later we will see the example of propylene rotamers.
Above we have two molecules whose chemical formulas are C6H13I. In the first, we have 2-iodohexane, while in the second, 3-iodohexane. They are constitutional isomers of the positional type, since the skeleton is the same, with the difference that the position of the iodine atom varies from one carbon to another..
Above we have another example of constitutional isomers for the chemical formula C3H8O. Molecules I and II are 1-propanol and 2-propanol, respectively. They are both alcohols. Instead, molecule III is about ethyl-methyl-ether.
Therefore, we speak of another functional group. Note that the three molecules have the same number of atoms, but their structures are different..
Above we have the trans-2,3-dichloro-2-butene isomer, and below we have the cis-2,3-dichloro-2-butene isomer. In the first, the chlorine atoms are in opposite positions with respect to the double bond, while in the second they lie on the same side of the double bond..
The backbone for both molecules is the same, but they differ in the spatial positions of the chlorine atoms; therefore, they are stereoisomers, and since they are not mirror images, they become diastereomers.
This time, instead of a double bond, we have a cyclohexane ring for the compound 3-fluoro-2-methylcyclohexane. In the left isomer, the CH groups3 and F are below the plane of the ring; while in the one on the right, the CH3 is up and the F is down.
Therefore, the one on the left would correspond to the cis isomer, and the one on the right would be the trans isomer. They are therefore diastereomers, because again they are not mirror images.
We have above three conformers of the n-hexane, which result from the rotations and "bends" of the carbon chain in different orientations. They all correspond to the same molecule of n-hexane with different levels of folding, which will depend on the environment and the internal energy of each molecule.
In 3-bromo-2-pentanol we have another example of a pair of diastereomers. Note that once again, in the isomer above we have OH and Br pointing in the opposite directions of the skeleton plane; while in the lower isomer, both groups point towards the reader's direction.
And once again, we have another pair of diastereomers, with the difference that now the difference lies in the spatial position of a single group: the ethyl, -CHtwoCH3.
This type of isomers has a very specific name: epimers, which are diastereomers whose variation is observed only in the spatial configuration of a single asymmetric carbon; in this case, the one bound to the -CHtwoCH3.
Note that the two bromochloroiodomethane molecules, CBrClI, are enantiomers because one is the non-superimposable reflection of the other. No matter how hard you try, its four atoms will never match. They are said to possess opposite spatial configurations.
In the example of the 2-bromo-4-ethylcyclohexane pair of molecules, the enantiomerism is even more evident. If we try to flip the molecule on the right, we will see that the -CH grouptwoCH3 it will not be located below the ring but above.
Similarly, the CH3 It will not point up the ring but below it. The spatial configurations are reversed, they are opposite. This is another of the most important characteristics of enatiomerism..
In propylene rotamers, to finish, we rotate the single bond between C1 and Ctwo: H3C-CH = CHtwo. The group = CHtwo is in an eclipsed position with one of the Hs of the CH group3 front on the rotameter on the left. Meanwhile, the = CHtwo and H are in alternate positions on the right rotamer.
Depending on the eclipse or not of the groups, we will have steric hindrance, which will destabilize the rotamer. Therefore, the rotamer on the right is more stable than the one on the left..
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