Isomer definition

Molecules that have the same molecular formula but different structures are called isomers. It is classified as structural isomers and stereoisomers.

Structural isomers

Structural isomers differ in the way the atoms are joined and are in turn classified into positional and functional chain isomers.


Stereoisomers have all the identical bonds and are differentiated by the spatial arrangement of the groups. They are classified as cis-trans or geometric isomers, enantiomers and diastereoisomers.

Chiral or asymmetric center

An atom bonded to four different substituents is called a chiral or asymmetric center. A molecule that has a chiral center has a non-superimposable mirror image of it, called an enantiomer.

Optical activity

The enantiomers have almost all identical physical properties, with the exception of optical activity. One of the enantiomers rotates polarized light to the right (right-handed) and the other rotates polarized light to the left (left-handed).

Stereochemistry in reactions

Radical halogenation reactions on molecules in which chiral centers can be formed produce mixtures of enantiomers in equal amounts or of diastereoisomers in different proportions.

Separation of enantiomers

Enantiomers have almost all the same physical properties, differ in rotation polarized light, but have the same melting and boiling points and identical solubility. Therefore, we cannot apply the traditional methods of separation and we must resort to special techniques. The separation via diastereoisomers, consists of transforming the mixture of enantiomers into a mixture of diastereoisomers by adding a chiral reagent, the diastereoisomers are easily separated by physical methods.

Stereochemistry is the study of organic compounds in space. To understand the properties of organic compounds it is necessary to consider all three spatial dimensions. Pareja de enantiómeros The bases of stereochemistry were laid by Jacobus van't Hoff and Le Bel, in 1874. They independently proposed that the four substituents of a carbon are directed towards the vertices of a tetrahedron, with the carbon in the center of it. .

Isomers are molecules that have the same molecular formula but different structures. It is classified as structural isomers and stereoisomers. Structural isomers differ in the way their atoms are bonded and are classified into chain, position, and function isomers. As an example, let's draw the structural isomers of formula C 2 H 6 O .

isomers 01

[1] Ethanol

[2] Dimethyl ether

cis 2 butene

cis-trans or geometric isomerism is due to restricted rotation around a carbon-carbon bond. This restriction may be due to the presence of double bonds or cycles. Thus, 2-butene can exist in the form of two isomers, called cis and trans. The isomer with the hydrogens on the same side is called cis, and the one with the opposite sides is called trans.

geometric isomers 01

[1] cis-2-Butene

[2] trans-2-Butene

mirror image bromochloroiodomethane

The word chiral was introduced by William Thomson (Lord Kelvin) in 1894 to designate objects that are not superimposable with their mirror image. Applied to organic chemistry, we can say that a molecule is chiral when it and its mirror image are not superimposable.

The maximum number of stereoisomers that a molecule presents can be calculated with the formula (2n), where n represents the number of asymmetric carbons. Thus, a molecule with 2 chiral centers has 4 stereoisomers.

Example 1. Draw the possible stereoisomers of 2-Bromo-3-chlorobutane.

molecules with more than one chiral center

A nomenclature is necessary that distinguishes the stereoisomers of a molecule. Thus, in the case of 2-Chlorobutane the notation must distinguish one enantiomer from the other. Cahn, Ingold and Prelog developed some rules that allow us to distinguish some stereoisomers from others, which I describe below.

Molecules that have a plane of symmetry or a center of inversion are superimposable with their mirror image. They are said to be achiral molecules.

Optical activity is the ability of a chiral substance to rotate the plane of polarized light. It is measured using a device called a polarimeter.

scheme polarimeter

[1] Light source

[2] Unpolarized light

[3] Linear polarizer

[4] Linearly polarized light

[5] Sample cuvette

[6] Rotation in polarized light

[7] Analyzer

projection fischer 00

Projecting consists of drawing a molecule in two dimensions (plane). In the Fischer projection, the molecule is drawn in the shape of a cross with the substituents that go to the bottom of the plane in the vertical and the groups that come out towards us in the horizontal, the point of intersection of both lines represents the projected carbon.


To give R/S notation in Fischer projections the same rules are followed as for a molecule drawn in space.
1. Priorities by atomic number are given to the substituents that start from the asymmetric carbon.
2. It rotates starting with the priority group (a) towards (b) and (c). If the group (d) is in the vertical, the rotation in the direction of the needles gives the notation R and in the opposite direction to the needles S. When the group (d) is in the horizontal it is the opposite.
fischer rs 01
[1] With the group "d" in the vertical
[2] With group "d" on the horizontal

To convert Newman projections to Fischer projections, the spatial shape of the molecule is drawn, arranging it in an eclipsed conformation to make the Fischer projection.


caballete01.png In the sawhorse projection (also called in perspective) the line of observation makes an angle of 45º with the carbon-carbon bond. The carbon closest to the observer is below and to the right. While the farthest one is on the top left.

We are going to see how chemical reactions can introduce chirality in molecules, obtaining products in the form of racemic mixtures or mixtures of diastereoisomers.

Butane halogenates in the presence of bromine and light, at carbon 2, to form a mixture of enantiomers. The radical formed presents enantiotopic faces, which are halogenated with equal probability, giving rise to a racemic mixture (enantiomers in equal proportion).

stereochemistry of reactions 01

The mechanism of this reaction consists of three stages: initiation, propagation and termination. Propagation is the step that determines the stereochemistry of the final product.

Butane halogenation

Stage 1. Initiation

initiation stage

Stage 2. Propagation

propagation stage

Stereoselective reaction

A reaction that leads predominantly to one stereoisomer is stereoselective. Radical halogenations of diastereotopic hydrogens generate diastereoisomers in different amounts, therefore they are said to be stereoselective reactions.
Stereospecific reaction
A reaction that leads exclusively to a particular stereoisomer is said to be stereospecific. In the subject of substitutions and eliminations we will see that SN2 is a reaction that gives only one of the possible stereoisomers, therefore it is a stereospecific reaction.
The halogenation of enantiotopic hydrogens leads to a mixture of enantiomers in equal amounts, therefore it lacks any selectivity.

Difficulties in separating racemates

Enantiomers have almost all the same physical properties, melting points, boiling points, solubility. They only differ in the rotation of polarized light, the dextrorotatory rotates to the right and the levorotatory to the left. Therefore the separation of enantiomers cannot be carried out by conventional physical methods (distillation, crystallization.....). The solution to the problem is based on the difference between the physical properties of the diastereoisomers, which do have different melting, boiling and solubility points that allow them to be separated.
Separation via diastereoisomers
We are going to look for a reaction that converts the racemic mixture into a mixture of diastereoisomers, by binding each enantiomer to a chiral reagent. This mixture is separated by fractional crystallization, distillation or chromatography of the diastereoisomers. Finally, the bond that unites each enantiomer with the chiral reagent is broken and both are separated, obtaining the pure enantiomers.