Structural formula

The term structure formula, in chemistry is a collective term for chemical modes of representation that provide information about how atoms connected in a molecule and are arranged in space. Structural formulas show the atomic bonds and - partially - the chemical structure. They are particularly used in the organic chemistry. As structural formulas of increasing complexity electrons formulas Valenzstrichformeln, wedge line formulas and skeletal formulas can be viewed. A special position also take projection formulas (eg the Fischer projection and the Newman projection) a.

A Brief History structural formulas

After preliminary work by Charles Frédéric Gerhardt, Hermann Kolbe, Edward Frankland August von Kekulé could specify the number of possible bonds to other atoms in 1857 for many chemical elements. The number of bonds an atom to other atoms is known since 1860 ( as suggested by Emil Erlenmeyer ) value. Kekulé formulated the quadrivalency for the carbon atom. In 1865 Kekulé realized the chemical structure of aromatic compounds such as benzene and naphthalene.

Alexander Butlerov used at the meeting of natural scientists in Speyer in 1861, the name Chemical structure.

Alexander Crum Brown used in 1864 for the first time a structural formula ( for acetic acid) in which the bonds between the atoms was marked by dashes.

Since 1864 also structural formulas were used in addition to the molecular formulas of the organic substances.

Construction

In organic chemistry, the so-called skeletal formula is often used not written in the C and H atoms but implies (assuming ) are. The representation of the carbon skeleton via the drawing of the bonds between the carbon atoms. For each carbon atom of a corner is drawn. Since carbon atoms usually form four atomic bonds, the number of added hydrogen atoms is calculated by the number of bonds of the carbon atom of 4 is subtracted. In aromatic rings, the number of delocalized electrons also is taken into consideration. In the benzene ring are for example 6 delocalized electrons. Consequently, on each of the six carbon atoms of the ring, only one hydrogen atom is bonded and not two as in cyclohexane.

Example

An example is the representation of hexamethylenetetramine in the skeletal formula is:

As the formula shows, it is common in the skeleton formula, to omit the representation of carbon atoms and hydrogen atoms. Due to the fact that a carbon atom has four bonds, and is a hydrogen atom can form a bond, the number of the hydrogen atoms can be calculated, which are bonded to a carbon atom. At each carbon atom in the example more hydrogen atoms are so bound. The name hexamethylenetetramine results from the number of methyl groups ( 6 = hexa ) and the number of amine groups (4 = Tetra ), see Greek numerals.

Long unclear was the structure of benzene and thus its structural formula: The molecular formula of this aromatic hydrocarbon is C6H6. The construction of the corresponding structural formula was in 1865 by Friedrich August Kekulé von Stradonitz. Unlike in the picture on the stamp on the right of the benzene ring has neither single nor double bonds at fixed positions - all CC bonds in the ring are identical.

Spatial representation

Although structural formulas indicate the exact bonding, however, offer some limited information on the exact spatial structure of a molecule. The data necessary for the reduction to two dimensions compressions lead to bond lengths and angles are distorted.

To illustrate the spatial representation of molecules in a drawing, bonds can be drawn as wedges or dashed lines. A wedge is a bond, which is out of the paper plane, while a dashed line represents a bond that goes into the plane of the paper. This is especially for the representation of the structure of chiral substances of importance.

An accurate spatial representation can be achieved via the space-filling model. Previously kits were used, today announced the three-dimensional representation on a computer is common (see Molecular Modeling ).

Semi- structural formula

To complement and simplify the presentation, half structural formulas are used.

From empirical formulas often does not reveal to what substance it is. However, since it is often not necessary or possible to draw the structural formula of a molecule, half- structural formulas have been introduced.

The molecular formula C3H8O gives only the ratio of occurring in a molecule atoms, but does not provide information on the chemical class of compound. Half structural formula CH3 -CH2-CH2 -OH or CH3 -CH ( OH) - CH3, which includes the same number of kinds of atoms in total, this is illustrated in and indicates that it is an alcohol. In this form corresponds to the semi- structural formula of the constitutional formula.

In addition to the "real" half- tree formulas ( CH3 -CH2-CH2 -OH) has been in daily laboratory between the empirical formula ( C3H8O ) and the half structural formula lying common form C3H7OH naturalized, although this is often not unique. In the example C3H7OH remains open whether this is to 1- propanol ( n- propanol ) or the isomeric 2-propanol ( isopropanol ). Clearly, however, the notation n- C3H7OH for 1 -propanol or iso- C3H7OH or C3H7OH i- 2- propanol. As a unique intermediate forms are found occasionally CH3 - (CH2) 2 -OH or H3C - (CH2) 2 -OH.