STEREOCHEMICAL

Keisomeran due to the asymmetric carbon atom, optical keisomeran.

Before there was the theory of valence, chemist / physiologically French Louis Pasteur (1822-1895) have recognized the influence of molecular structure on the properties of individual molecules combined. He managed to separate the racemic tartaric acid (sodium ammonium salt actually) to (+) and (-) under the direction of hemihedral crystal face (1848).
Both compounds have physical properties (eg melting point) and the same chemical, but there are differences in the optical properties in solution of each compound. Both rotate the plane of polarization of light, in other words having an optical activity. Rotation of the second type of compound, which mengkur rotational strength of both compounds, have the same absolute value but opposite sign. Because the molecules are free in solution, this difference can not be explained by differences in crystal structure. Unfortunately at that time, although the existing atomic theory, theory of valence has not been there. With these conditions can not explain the Pasteur discovery.

In the 1860's, the German chemist Johannes Adolf Wislicenus (1835-1902) found that two types of lactic acid is known at that time are both α-hidroksipropanoat acid CH3CH (OH) COOH, instead of acid β-hidroksipropanoat HOCH2CH2COOH. He further suggested that a new concept for menjelaskna stereoisomer should be made for this phenomenon. Konse recently stated that the two compounds that have the same structural formula can be in two dimensu stereoisomer when the arrangement of the atoms in different rooms.
In 1874, van't Hoff and Le Bel independently proposed the theory of tetrahedral carbon atom. According to this theory, the lactic acid that can be depicted in Figure 4.4. One of the lactic acid lactic acid is the mirror image of them. In other words, relations between the two compounds as a relationship right hand and left hand, and therefore called the antipodes or enantiomers. Thanks to the theory of van't Hoff and Le Bel, a new chemistry, stereochemistry, is growing rapidly.

Both isomers or the antipodes, as related to the right and left hand
In the central carbon atom in lactic acid, four different atoms or gigus bound. Carbon atoms are called the asymmetric carbon atom. Generally, the number of stereoisomers will be as many as 2n, n is the number of asymmetric carbon atoms. Tartaric acid has two asymmetric carbon atoms. However, due to the presence of molecular symmetry, the number stereoisomernya less than 2n, and another one is optically inactive stereoisomer (Figure 4.5). All these phenomena can be consistently explained by the theory of tetrahedral carbon atom.

However, because of symmetry, meso-tartaric acid is optically inactive.

Geometrical isomers

Van't Hoff explains fumaric acid and maleic keisomeran because of restrictions on the double bond rotation, a different explanation for the optical keisomeran. Isomers of this type is called the geometrical isomers. In the trans form subtituennya (in the case of fumaric and maleic acid, carboxyl group) are located on different sides of the double bond, while the cis isomer was subtituennya located on the same side.
Of the two isomers are diisoasi, van't Hoff was named isomer which readily releases water into maleic anhydride cis isomer because the cis isomer the two carboxy groups are close to each other. By heating to 300 ° C, acid fuarat turned into maleic anhydride. This is quite logical because the process must involve cis-trans isomerization process of the shipyard which is sufficiently high energies (Figure 4.6) ï ¼ Ž
Because some couples have been known geometrical isomers, geometric isomers theory gives a good dukunagn van't Hoff for the structural theory.

Figure 4.6 maleic acid geometrical isomers (cis form) has two carboxyl groups are close, and easy to release water into anhydride (maleic anhydride).