Study Material

Isomerism

What is isomerism?

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. examples of isomerism

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 isomeris

Flow diagram for the different types of isomers in organic chemistry.
Flow diagram for the different types of isomers in organic chemistry.

m 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. examples of isomerism

Some examples of isomers are shown in the flow chart and will be discussed in later sections.

Types of isomerism

Constitutional examples of isomerism

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. examples of isomerism

For example, for the same chemical formula C 7 H 10 O there may be multiple molecules that meet that number of atoms; they will all have a single oxygen atom, but the structural arrangement of the other atoms will be unique for each isomer.

And therefore the chemical and physical properties of all the constitutional isomers for the formula C 7 H 10 O will also differ greatly from each other. examples of isomerism

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 aforementioned. examples of isomerism

Stereoisomers examples of isomerism

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. examples of isomerism

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

Enantiomers are stereoisomers consisting of non-overlapping mirror images. For example, the right and left hands are not overlapping, as are shoes or gloves. In the next section, the classic example of enantiomerism will be addressed: bromochloro domethane: CBrClI. examples of isomerism

Diastereomers examples of isomerism

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 examples of isomerism

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. The example of n- hexane conformers will be seen in the next section .

Rotamers examples of isomerism

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. The example of propylene rotamers will be seen later.

Examples of isomerism

Iodine hexane examples of isomerism

examples of isomerism: Constitutional isomers of iodine hexane.
Constitutional isomers of iodine hexane.

Above we have two molecules whose chemical formulas are C 6 H 13 I. In the first, we have 2-iodohexane, while in the second, we have 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. examples of isomerism

3 H 8 O

examples of isomerism

Above we have another example of constitutional isomers for the chemical formula C 3 H 8 O. 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.

2,3-dichloro-2-butene

Geometric isomers of 2,3-dichloro-2-butene.
Geometric isomers of 2,3-dichloro-2-butene.

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. examples of isomerism

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.

3-fluorine-2-methylcyclohexane

examples of isomerism: Diastereomers of 3-fluorine-2-methylcyclohexane. 
Diastereomers of 3-fluorine-2-methylcyclohexane.

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 3 and F groups lie below the plane of the ring; while in the one on the right, CH 3 is up and 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 thus diastereomers, because again they are not mirror images. examples of isomerism

Conformers of n- hexane

examples of isomerism: Conformers of n-hexane.
Conformers of n-hexane.

Above we have three conformers of n -hexane, which result from the rotations and “bends” of the carbon chain in different orientations. They all correspond to the same n- hexane molecule with different levels of folding, which will depend on the environment and the internal energy of each molecule.

3-bromo-2-pentanol

examples of isomerism: Diastereomers of 3-bromo-2-pentanol.
Diastereomers of 3-bromo-2-pentanol.

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 in the direction of the reader.

1-bromo-3-chloro-5-ethylcyclohexane

Diastereomers of 1-bromo-3-chloro-5-ethylcyclohexane. 

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: ethyl, -CH 2 CH 3 .

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 linked to -CH 2 CH 3 .

Bromochlorooyodomethane

The pair of enantiomers of bromochloro domethane. 
The pair of enantiomers of bromochloro domethane.

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 have opposite spatial configurations. examples of isomerism

2-bromo-4-ethylcyclohexane

2-bromo-4-ethylcyclohexane enantiomers.
2-bromo-4-ethylcyclohexane enantiomers.

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 2 CH 3 group will not be located below the ring but above it.

Similarly, CH 3 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.

Propylene rotamers

Rotamers of propylene. 
Rotamers of propylene.

In propylene rotamers, to finish, we rotate the single bond between C 1 and C 2 : H 3 C-CH = CH 2 . The = CH 2 group is in an eclipsed position with one of the Hs of the front CH 3 group in the rotamer on the left. Meanwhile, = CH 2 and H are in alternate positions on the rotamer on the right. examples of isomerism

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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