1.3 Atomic orbitals of carbon atoms; hybridization
1.3 Atomic orbitals of carbon atoms; hybridization
The electron configuration of a carbon atom in its ground state is 1s22s22p2. Reasoning from the electron configuration and the direction of atomic orbital, the bonding electrons are only two 2p electrons that occupy the 2p orbital because 2s electrons are not involved in the bond formation because these form a unshared electron pair. Thus, hydride of carbon would be expected to be CH2, and its bond angle should be 90°, which is the angle between two 2p electrons. (Figure 1.2 )
Figure 1.2 Hypothetical hydride of carbon; CH2
However, studies about methane and its substituted species (e.g. chloroform CH3Cl) reveal that each of those four hydrogen atoms in methane is identical. The fact means that a carbon must use four identical atomic orbitals with four unpaired electrons, but not an s orbital and three p orbitals. It can be explained in term of hybridization that means mixing an s orbital and three p orbitals to make up four equivalent new orbitals, and the resulting orbital is called sp3hybridized orbital.
One may wonder that it is impossible to decide which of those expected structures 11~13 can be the true structure of methane without some direct observations. However, if substituted methanes should have the same structure as methane, it is possible to deduce the structure of methane from the numbers of the isomers of substituted methanes.
Since some investigations reveal that only one compound exists as methylene chloride, the tetrahedron should be acceptable as the structure of methane. Atomic orbitals of an sp3hybridized carbon atom that forms tetrahedral structure is shown in Fig 1.4. Methane is made up by four bonds which is formed by the overlap of each sp3orbitals of a carbon atom with a 1s orbital of a hydrogen atom. Therefore, direction of those atomic orbitals determines the geometry of a molecule.
In addition, chemistry about a molecule that is formed by substituting three hydrogen of methane with each different atoms or groups (hereafter referred to as "ligand") such as a comparatively simple molecule bromochlorofluoromethane CHBrClF provides a more critical evidence for the tetrahedral structure of methane.
Here we have learned the pair of isomers 14 and 15. 14 and 15 are differed in their three-dimensional arrangement of atoms, even though they seem to be similar on the two-dimensional plane. Generally, isomers, though identical in their structural formula, differ in the arrangement of those atoms in space are called stereoisomers. Though there are some classes of stereoisomers, a pair of nonidentical images such as a right hand and a left hand or a real image and its mirror image is called a pair of enantio isomers or simply a pair of enantiomers. Though the terms antipode and mirror image isomer are also used, the term enantiomer will be exclusively employed in this textbook. 14 and 15 are differed in their steric configuration around carbon atom.
Next, we will discuss on the relations between stereoisomers including enantiomers. While it is clear that they are isomers with different structure, is there any difference in their property or chemical reactivity? Stereochemistry is one of a branch of science that involves the study about physical and chemical properties of various compounds especially associated with its three-dimensional structures. Chemistry could not make any progress without stereochemistry. Stereochemistry is an essential part of chemistry. Then, is it necessary to separate the study of stereochemistry from the study of chemistry?
There is a good reason for stereochemistry to be recognized as an important and independent field of chemistry. First of all, physical and chemical properties of stereoisomers are sometimes different so much that it is possible to obtain much information about the relation between molecular structure and properties. Next, a definite and considerably elaborate procedure is necessary to represent information about three-dimensional structure of a molecule on a two-dimensional sheet of paper and then to reproduce original three-dimensional information from it.