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The representations of molecules are used in Chimie to describe the Molécule S (or the Ion S) and their structures. These charts make it possible to describe the molecular connections, the number and the type of Atome S which compose a molecule (or an ion), its form in space or simply to summarily describe the molecule (or the ion) in a simple and fast way. The majority of these representations are especially used in Organic chemistry or Biochimie.

Formulate, representation and projection

Various terms are used to indicate the charts of Molécule S: one thus speaks about empirical formula , of representation of Cram or projection of Fischer .

  • the formulas are used to describe the number and the type of Atome S in the molecule (empirical formula), to show how they are dependant between them (formula of Lewis, developed…). The formulas are especially used to represent simply and summarily the molecules and thus are often used in the chemical equations.

  • the representation of Cram makes it possible to directly describe the three-dimensional structure of a molecule, by a diagram which makes it possible to visualize the molecule such as it exists in space.
  • projections of molecules do not represent them directly : the molecules are projected and flattened on two dimensions (a sheet) in various manners according to projection employed. They make it possible to represent parts of molecules indirectly such that they exist in space by observing strict rules of projection.

Concept of Chemical bond

The purpose of This paragraph is not to explain in detail what is a chemical bond, nor to list all the existing chemical bonds. More information on this subject here

In this article we will especially speak about the covalent bonds and the ionic connections. In reality, a chemical bond is very seldom purely covalent (it is the case of a connection between two identical atoms such as for example in the molecule of Dihydrogène) or purely ionic. The real connection is a mixture of these two characters.

Covalent bond

When two atoms are bound by covalence it shared there one or more doublets of electrons, i.e. two (or more) electrons will belong at the same time to the two atoms. There exist two types of connections of covalence:
  • pure covalence: it shared there doublet of electrons, each atom initially brings an electron at the unmarried state
  • dative covalence: it shared there doublet of electrons, the same atom will bring the doublet.

Ionic connection

The ionic connections are due to the differences of electronegativity (capacity to attract the electrons) atoms present in the molecule.

To include/understand what it is an ionic bond is necessary to consider two atoms dependant between them. If one is more electronegative then it will attract the electrons of the other. The more electronegative atom will be then a little negatively charged, the other a little positively. This difference in load will create an electrostatic force which will attract the two atoms (a positive load and a negative charge attract themselves). If this difference in electronegativity is very important then one speaks about ionic connection. The majority of the connections bringing into play two different elements are partly ionic, the ionic character being generally all the more important as the difference in electronegativity is important.

Representations nonspecific to the organic chemistry

These representations are applicable to all the molecules and all the existing Ion S. They are rather simple of use.

Empirical formula

The empirical formula informs only about the chemical composition of the molecules (or the ions), i.e. about the number and the type of atoms which compose them, and about the electric Charge of the compounds if they are ions. It does not inform about the space fitting of the atoms, nor about the type of the chemical bonds.

To write an empirical formula one indicates the chemical element using his symbol (cf periodic Tableau of the elements), and the quantity of this element by a Chiffre in Indice with the right-hand side of the element concerned. The electric charge of the compound, if it has one of them, is indicated in exhibitor at the end of the formula. The number of elementary charges is indicated by a figure followed by a + if the compound positively (if it is charged misses one or more electron S), or of a - if the compound is negatively charged (if it has an excess of electrons).

Examples

All these examples will not be included in each section .
  • inorganic Chemistry
    • the Water:
    • the sulphuric Acid :
    • the ion Oxonium: (overall this compound is positively charged, its load is of +e, with E the elementary charge)
    • the ion tétraaminecuivre (II): (the Parenthèse S mean that there are four ammonia molecules related to the central copper atom)

  • Organic chemistry

    • the Glucose:
    • the ethanol: or (this second version of the empirical formula, which does not respect exactly the Règle S of above mention writing, is used to propose the function alcohol)
    • the Benzène:
    • the Acetone:

Formulate of Lewis

The formula of Lewis was created by Gilbert Newton Lewis at the beginning of the XX {{E}} century. It makes it possible to represent the connections assembling the Atome S between them (covalent bonds and ionic), but also the electron S of valence not taking part in the connections. The model of Lewis makes it possible to represent the structure of a molecule, but does not allow to show the shape of the molecule in space.

The representation of Lewis is based on simple rules. Of this fact it does not make it possible to describe all the molecules, in particular the complex of metals (as rust).

Formulate of Lewis of the atom

the method described below functions overall for the elements of the three first periods (line of the table) of the periodic Tableau of the elements

To establish the formula of Lewis of a Atome it is necessary to establish its electronic Configuration. For that there exists a simple method:

  • To define the number of electron S of the layer of valence of the element. For that one counts the place of the element on the basis of the left of the table. The Hydrogène thus has an electron in its layer of valence, the Carbone has four of them, the Azote five and the Chlore Sept.
  • Définir the number of lone electrons and doublets not-binders of the element. It is enough for that to know that:
    • if an element has with more the four electrons in its layer of valence, then they all are unmarried; thus hydrogen has a lone electron, carbon has four,
    • when there is more than four electrons of valence, all the electrons being added with the four single people form a doublet not-binder; chlorine has three doublets not-binders and a lone electron and the nitrogen a doublet not-binder and three lone electrons.

Once the electronic configuration of the atom established, one represents his formula of Lewis. The element is represented by its symbol. Around this symbol one places the lone electrons, represented by a point, and the doublets not-binders, represented by a feature.

Formulate of Lewis of the molecule

For the molecules, the formula of Lewis is based on simple empirical rules (which it is not always possible to respect), in particular the rule of the byte or the duet.

The rule of the byte implies that each atom, of the second and third period, must approach the electronic configuration of the rare gases (of great stability) by having eight electrons in its layer of valence. The rule of the duet applies only to the hydrogen atom, this one must have two electrons in its layer of valence.

To establish the model of Lewis of a molecule it is initially necessary to establish the formula of Lewis of each one of her atoms. Then these atoms are connected so that each one of them complies with the rule of the byte or the duet. For that one initially shares the lone electrons of each atom. Then if that is not enough, connections of dative covalence are used or one divides certain nonflexible doublets to obtain two lone electrons. (See the examples)

Examples

  • Here, the lone electrons are simply shared, only one possibility.
  • This ion positively, it is charged thus misses/> an electron, from where the empty rectangle on the oxygen which represents a missing electron. To form this ion, one of the two nonflexible doublets of oxygen was broken, results two lone electrons, one from it forms a doublet with the third hydrogen, the other is absent what corresponds to the positive load. A molecule whose formula of Lewis comprises a lone electron is called " radical". This configuration is very unstable, the radicals are thus very reactive.
  • example of made up organique.
  • the nonflexible doublets of sulfur have being broken to form this compound.

Formulate developed plane

The developed formula planes makes it possible to represent in very simple manner and rapid the structure of a molecule, as well as the chemical bonds. But it does not make it possible to represent the shape of the molecule in space.

The plane developed formula is to some extent a simplified formula of Lewis. Indeed, the representation is almost identical, but one does not show the doublets not-binders to simplify and reduce the writing. Generally the connections are represented with 90°, but are sometimes represented under different angles to approach the real structure of the molecule in space (for example 120° around a double connection carbone=carbone).

Examples

  • Formula developed of the molecule of ethanol

Representation of Cram

The representation of Cram makes it possible to show the form in the space of a molecule, and its structure. On the other hand it does not represent the chemical bonds (not difference in multiple connection and simple connection).

To illustrate the shape of the molecule the various directions which its chemical bonds can take are codified in this manner:

  • a connection in the plan (of the sheet) is represented by a simple feature
  • a connection which is directed towards the reader is represented by a full Triangle, pointed towards the plan
  • a connection which moves away from the reader is represented by a hatched triangle pointed towards the plan

One represents in a more precise way the orientation of the chemical bonds while varying the angles between the connections to stick to more close with reality. This representation in particular makes it possible to visualize the atoms of asymmetrical Carbone S and to determine if a molecule is chiral.

Example

Representation of Cram of the molecule of ethanol.

Representations specific to the organic chemistry

These representations are specific to the Organic chemistry because they use rules specific to the connections carbon-carbon or carbon-hydrogen .

Formulate semi-developed (planes)

The semi-developed formula, as its name indicates it, is a condensed form of the developed Formule. One does not represent any more the Carbon-Hydrogen connections which are condensed in the form of: (with N the number of hydrogen atoms related to the carbon atom). The connections carbon-carbon are represented and simple connection and multiple connections are distinguished.

Examples

  • , the molecule of ethanol.
  • , the molecule of acetone.

Topological formula

The topological formula is a manner simplified and fast to represent the structure of a organic molecule.

One does not represent any more the atoms of carbon nor the hydrogen atoms, but the connections carbon-carbon, these connections are represented by an oblique feature. In a topological formula the carbon atoms are thus located at the intersection of two segments. One makes the distinction between multiple connections and simple connections: a simple connection will be represented by a feature, a double linking by two parallel segments.

One represents the connection between heteroatoms (elements other than carbon and hydrogen), or the functional groups, and carbons to which they are bound by a segment; one places the functional group, or heteroatom, at the end of this segment. The hydrogen atoms not being represented, a segment whose end is not related to any functional group corresponds in fact to.

Examples

  • topological formula of the molecule of ethanol
  • topological formula of the molecule of Glucose

Projection of Newman

The projection of Newman is very useful to study different the Conformation S (one passes from a conformère to another by rotation around a connection simple carbon-carbon) from a Composé organics. This projection is generally used only with tetravalent carbons (related to four other atoms).

To represent a molecule in a projection of Newman it is necessary to look at the molecule in the axis of a connection simple carbon-carbon, one does not represent these two carbons in a projection of Newman, but the connections.

The connections of carbon in the foreground are represented by three segments on the basis of the same point, the Angle S between each one of these segments are of 120° as in reality. Carbon in background is represented by a circle on which one places the three remaining connections. (see the example)

This representation makes it possible to easily visualize the effects of interactions steric between groupings carried by two adjacent carbons. It also makes it possible to easily determine conformation Z or E of the Alcène resulting from a reaction of elimination.

Example


  • Schéma descriptive of the passage of the representation of Cram to the representation of Newman for the ethanol molecule.

Projection of Fischer

The projection of Fischer is especially used to represent sugars. She was invented by Hermann Emil Fischer.

The carbon atoms are not represented, they are located at the intersection of the horizontal segments and the vertical feature. The carbonaceous chain is represented by the vertical line so that the connections represented on the vertical moves away from the reader. The connections represented on the horizontal one leave the plan of the sheet towards the reader (see the example). Carbon more oxidized is at the beginning of chain (in top here C=O).

This representation makes it possible to differentiate easily the chiral enantiomer S L or D.

Example



  • Schéma of the passage of the representation of Cram to the projection of Fischer for the molecule of D-Glucose

Projection of Haworth

The projection of Haworth makes it possible to represent the cyclic structure of the Monosaccharide S with a simple three-dimensional prospect.

In this projection, one represents neither the atoms of carbon nor those of hydrogen but on the other hand, the connections carbon - carbon are represented by a feature. Moreover, the connections closest to the reader are in fat.

Example



Projection of Haworth of the α-D-glucopyranose

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