Isomerism

The term isomerism comes from the Greek ίσος ( isos = identical) and μερος ( meros = left) isos . Isomerism was noticed the first time in 1827, when Friedrich Woehler prepared the Cyanic acid (H-C=N=O) and noted that, although its composition elementary is the same one as that of the fulminic Acid (H-N=C=O, prepared by Justus von Liebig the previous year), the chemical properties of these substances are radically different. This discovery was in contrast with the theories of the time within the framework of which it was thought that the properties of a substance were entirely determined by its empirical formula.

In Organic chemistry, one speaks about isomerism when two Molécule S have same the rough Formule but have semi-developed formulas or developed formulas different. These molecules, called isomer , have physical properties, chemical and biological different.

One distinguishes various isomerisms, mainly isomerisms of constitution and configuration (stereoisomery). There are also isomerisms of conformation.

Isomerism of constitution

The isomerism of constitution corresponds to isomerisms indicating of the different sequences of Atome S. Isomers of constitution have for only common point their empirical formula; they are not consisted of same the chemical functions.

Isomerism of chain

The isomerism of chain indicates the isomers which differ by their carbonaceous Chaîne (skeleton). these isomers are characterized by their identical chemical properties and physical properties different.

Example: C4H10

butane méthylpropane CH3-CH2-CH2-CH3 CH3-CH-CH3 | CH3

Isomerism of position

The isomerism of position qualifies the isomers whose functional grouping is placed on carbons different from the carbonaceous chain, which wants to say that it is the function which moves inside the skeleton. These isomers are characterized by their physical properties different and chemical slightly different.

Example: C3H7OH

propan-1-ol propan-2-ol CH2-CH2-CH3 CH3-CH-CH3 | | OH OH

Isomerism of function

The isomerism of function characterizes the isomers from which the functional groups are different, therefore of physical properties and chemical different.

Example: C2H6O

ethanol oxidizes dimethyl CH3-CH2-OH CH3-O-CH3

Stereoisomery

The Stéréoisomérie indicates isomers of Configuration, i.e. the molecules of identical constitution but from which the space organization of the atoms is different. Isomers of configuration in two great groups are classified: the enantiomers and the diastereoisomers.

Enantiomery

Also called isomer optics , the enantiomer S is two molecules which are the image one of the other by a mirror and are not superposable: they present a Chiralité indeed. They are symmetrical, they are thus not superposable with their images in the mirror. The most concrete example is that with the hands.

Diastéréoisomérie

The diastereoisomers (which one writes also diastereoisomers ) are the stéréoisomères which are not enantiomers.

Diastereoisomery of configurations Z and E

When, on each of the two sides of a Double connection, one finds two groups different, one distinguishes two configurations: Z and E . Free rotation around the double connection is not possible, because of presence of a Liaison π: it results a plane molecule from it having two possible configurations.

By using the set of priorities according to the convention CIP (Cahn, Ingold, Prelog) one defines two isomerisms as follows:

  • the configuration Z characterizes the diastereoisomers whose priority groups are same side of the plan formed by the double connection (and perpendicular to the plan of the molecule). The abbreviation Z comes from German zusammen who means “together”.
  • the configuration E characterizes the diastereoisomers whose priority groups are opposite side one of the other by the plan formed by the double connection. The term E comes from German entgegen who means “opposite”.

example: 3-aminobut-2-ènoïque acid, opposite. The priorities are COOH > H and NH2 > CH3. There are thus in the first case the priority groups on the same side of the plan: it is thus the representation of acid (Z) - 3-aminobut-2-ènoïque. Contrary, in the second representation, the priority groups are opposite: the molecule represented is thus the acid (E) - 3-aminobut-2-ènoïque.

In a general way, the configurations Z are rarer because the priority groupings (often bulkiest) are destabilized by their steric Encombrement. But certain configurations can be stabilized, in particular by Chélation.

NB: One uses less the terms cis and trans which rather qualifies relative positions of groupings without taking account of their priority. For example, on the illustration of the configuration E , one will say that the H is in trans CH3 , and that the H is in cis NH2 .

Other types

There is diastereoisometry between two isomer molecules having even enchâinement connection, which comprise two or several centers of Chiralité (of the atoms carrying 4 different substituents) and which are not enantiomers. Example: the forms (R) - (S) and (R) - (R) of the tartaric acid are diastéréomères and have physical properties different.

Epimery

Two épimères differ between them only by the absolute configuration from one asymmetrical Carbone, for an ose one does not speak any more epimery if that touches asymmetrical carbon determining the series D or L of this last.

Anomery

It is a particular case of épimèrie for carbon 1 of the oses. It makes it possible in particular to describe convention α and β. If the function hydroxyl of carbon 1 is below the plan (representation of Haworth), the ose is known as α (alpha) whereas if the hydroxyl of carbon 1 is above the plan, the ose is known as β (béta).

Example: α-glucose: on the image, carbon 1 is on the right and the function hydroxyl is neither in top nor in bottom (it would be necessary to choose to have α or β).

This nomenclature is very important to describe the chemical bonds contracted in disaccharides and the polysaccharides.

Example: the Saccharose (α-D-glucopyrannosyl (1->2) β-D-fructofurannoside) is a dissacharide formed dependant d'α-glucose and d'β-fructose in α1-2.

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