What do isomers mean

The name isomer is from Iso (ίσος, isos Greek = equal) and meros (μέρος, Greek = part) derived. Isomers are chemical compounds with the same molecular formula but different chemical structure and sometimes with different chemical, physical and biological properties. Isomerases are the cell's enzymes that convert isomers into one another.

Isomeric types in organic chemistry

There are a number of very different forms of isomerism - they range from essentially completely different compounds to very subtle differences. All forms can be divided into two groups, the Constitutional isomers and the Stereoisomers.

Constitutional isomerism or structural isomerism


Constitutional isomers have the same molecular formula, but a different structure (constitution). The isomers are therefore generally different substances with different chemical (including reactivity) and physical properties (including melting and boiling point, solubility). One can distinguish four cases:

  • Functional isomers have different functional groups, e.g. ethanol (CH3-CH2-OH) and dimethyl ether CH3-O-CH3.
  • Skeletal isomers have differently branched carbon backbones, such as butanol and 2-methyl-1-propanol.
  • In the case of positional isomers (also called positional isomers), the same functional group is in different positions, which is an example 1,2-Propanediol and 1,3-Propanediol. One speaks of tautomerism when the two isomers can merge into one another in a reversible chemical reaction, as parts of the molecule change their place.
  • Bond isomers (also valence isomers) differ in the number and / or position of σ and π bonds (such as for C3H4 Cyclopropene, propadiene and propyne).


Stereoisomers basically have the same structure (constitution) - and thus also the same empirical formula - but differ in terms of the spatial arrangement (Configuration) of the atoms.

  • Are an extremely important special case Enantiomers, Stereoisomers that relate to one another like image and mirror image and have no plane of symmetry. Enantiomers therefore differ in allCenters of chirality (atoms which, because of four different substituents among these, allow two different orders). Important examples are sugars and amino acids.

Diassereoisomers are divided as follows:

  • As Epimers denotes pairs of diastereoisomers of a molecule with several stereocenters that differ only at one of these centers (e.g. isoleucine and alloisoleucine (see graphic on the right) or glucose and galactose). Epimers are therefore always also diastereomers, but not vice versa.
  • A meso form has several centers of chirality, but with a rigid structure it also has a plane of symmetry and can therefore be made to coincide with the mirror image. Generally valid is the formulation that a meso form has equivalent centers of chirality with opposite configuration. Contrary to what appears at first glance, two meso forms are therefore not enantiomers and are optically inactive.
  • The term is used in sugar chemistry Anomer as a special case of an epimer whose difference lies in the first carbon (relevant when the α- or β-form of a sugar such as glucose is formed).

What all these groups have in common is that an isomer can only be converted into another form by breaking a bond. This does not apply to the last group of isomers, which also fall under the generic term stereoisomers: Conformational isomers are stereoisomers that can be converted into one another by simply rotating single bonds. Thermal energy at room temperature is usually sufficient for this. An example is the ecliptic and the staggered ethane (clearly visible in the Newman projection). The two groups of ethane can in principle be at any angle to one another, the energy difference being less than the thermal energy, so that in a solution the isomeric forms continuously merge and cannot normally be isolated.

Calculation of the maximum number of stereoisomers of a molecule: 2n at n centers of chirality. If meso forms are present, the number of isomers is reduced by one for each meso form.

E.g: Cyclohexane with 2 substituents ---> has 6 centers of chirality, i.e. a maximum of 26 = 32 stereoisomers.

More terms

Optical activity often occurs in connection with stereoisomers, i.e. a chiral molecule rotates the direction of polarization of linearly polarized light.

Despite the linguistic similarity, the term mesomerism does not belong in this subject area.


The following table gives a brief overview of the most important forms of isomerism:

category variant Similarities differences Different chem./phys. properties Transfer only by loosening ties
Constitutional isomers Molecular formula structure Yes Yes
Stereoisomers Diastereomers Molecular formula
spatial arrangement Yes Yes
Enantiomers spatial arrangement, but like image and mirror image optically active, differences in chiral reaction partners (enzymes) Yes
Conformational isomers spatial position Yes No

Isomeric types in complex chemistry

In complex chemistry, too, there are a number of very different forms of isomerism, but all forms can be divided into two large groups, the constitutional isomers and the stereoisomers.

Configuration isomerism (stereoisomerism)

Analogous to the stereoisomerism in organic molecules, a distinction is made between complexes that have the same composition but a different spatial structure. Depending on the geometry of the complex, different forms of stereoisomerism can occur.

  • Completely analogous to a chiral center on carbon in organic chemistry, tetrahedral complexes with four different ligands can exist as an image and a mirror image.
  • In tetrahedral and octahedral complexes a cis / trans isomerism can exist.
  • If there are three identical ligands in an octahedral complex, a distinction can be made between fac (iale) and mer (idional) isomers.

Constitutional isomerism

  • Bond isomerism (salt isomerism) always occurs when a ligand can be connected to the central atom via several different atoms.

Example: nitro complex or nitrite complex

O / X5Co-N or X5Co-O-N-O \ O

with any ligand X (e.g. NH3, H2O ...) and Co (balt) as the central atom

  • Coordination isomerism: If both anions and cations are present as complexes in a complex, the ligands of central atom 1 can be exchanged for central atom 2.

Importance in medicine

In medicine, stereoisomerism is important for many drugs. Medically effective preparations are often in two spatially different arrangements. In the chemical production of some preparations, both are Isomers produced in equal proportions, although only one of the variants is effective in the human body. The other variant is ineffective in the best case, but harmful in the worst case. An asymmetric synthesis strategy must therefore be used for such preparations, or they are produced with the help of genetic engineering or obtained from natural sources.

A prominent and tragic example is the active ingredient thalidomide (Contergan), which exists in two possible enantiomers. Both are in equilibrium, i. H. even if only one enantiomer is given, it is partially converted into the other in the body. It has been suggested that one works as a sleep aid but the other is teratogenic. However, this has not really been proven yet.


  • Eliel, Ernest Ludwig; Lüttringhaus, Arthur; Cruse, Rudolf: Stereochemistry of carbon compounds. Weinheim: Wiley-VCH, 1982 - ISBN 3527250646
  • Eliel, Ernest Ludwig; Wilen, Samuel H.; Hopf, Henning; Mulzer, Johann: Organic stereochemistry. Weinheim: Wiley-VCH, 1998 - ISBN 3527293493

See also

Categories: Organic Chemistry | Inorganic chemistry