Hybridization (chemistry)

Example of methane

The theory of the orbital hybrids was developed by Linus Pauling to explain the geometry of molecules like the Méthane (CH₄). The difficulty of comprehension of the latter has due the reasons suivantes.
It is about a molecule made up of an atom of Carbone related to four atoms of Hydrogène. The electronic Configuration of carbon in its basic state is 1s ² 2s ² 2p_x ¹ 2p_y ¹ , or

$C \ quad
\frac{\uparrow\downarrow}{1s}\;
\frac{\uparrow\downarrow}{2s}\;
\ frac {\ uparrow \,} {2p_x} \;
\ frac {\ uparrow \,} {2p_y} \;
\ frac {\, \,} {2p_z}$

(orbital atomic the 1s is lower in energy than orbital the 2s, the latter being itself lower in energy than orbital the 2p).

First of all, it would seem that the carbon atom should form only two covalent bonds because of existence of two orbital 2p to half filled. However, CH₂ or Méthylène does not exist as such but only like a grouping inside molecules. This reasoning does not make it possible to explain the existence of methane.

In addition a reasoning implying a excited state would not make it possible to explain the properties of methane. If an electron of orbital the 2s is excited and occupies orbital the 2p_z, carbon can then be related to four hydrogen atoms (4 orbital of carbon is with half filled). However, the form of orbital the 2s and 2p being very different, covering with the orbital ones of the various hydrogen atoms will be different, and the four CH connections will not have same energy what is contrary with the experimental results.

Hybridization

A manner of answering the problem of the existence and geometry of this type of molecule are the hybridization of orbital atomic. Historically, this concept appeared to explain the chemical bonds in very simple systems (like methane). It proved then that this theory is more largely applicable and it forms today part of the bases of the comprehension of the Organic chemistry. It is less easily applicable to the branches of chemistry for lequelles are concerned heavy atoms. The theory of hybridization for the chemistry of the metals transition more complicated and is led to results definitely less precise.

The orbital ones (orbital atomic, orbital molecular hybrids or orbital) constitute a model representing the way in which the electron S behave around the atomic nuclei. In the case of hybridization, the model is based on orbital atomic hydrogen. The orbital hybrids are mixtures of these orbital atomic where they are recovered in various proportions. The orbital atomic ones used as base are those of hydrogen because it is about the one of the only cases for which it is possible to solve exactly the equation of Schrödinger. The orbital ones obtained then are slightly deformed in the heavier atoms like the Carbone, the Azote and the Oxygène.

The first stage in the construction of the orbital hybrids is the excitation of one (or several) electron (S). To simplify, the continuation of the text deals more particularly with the example of the methane molecule. The Proton constituting the core of a hydrogen atom attracts one of the electrons of valence of carbon. This one occupies a excited state then, with an electron 2s occupying orbital a 2p. Consequently, the influence of the core of the carbon atom on the electrons of valence increases because of increase in the effective Charge (load actually felt by the electrons: it is equal to the load of the core decreased by the écrantage caused by the other electrons). Combination of these forces (attraction by the hydrogen core and modification of attraction by the carbon core) led to new mathematical functions, orbital hybrids. In the case of the carbon atom related to four hydrogen atoms, it creates for itself four new orbital: orbital the 2s mixes with orbital the 2p to form four orbital hybrids sp³:

$C^ {*} \ quad
\frac{\uparrow\downarrow}{1s}\;
\ frac {\ uparrow \,} {2s} \;
\ frac {\ uparrow \,} {2p_x}
\ frac {\ uparrow \,} {2p_y}
\ frac {\ uparrow \,} {2p_z}$ becomes $C^ {*} \ quad
\frac{\uparrow\downarrow}{1s}\;
\ frac {\ uparrow \,} {sp^3} \;
\ frac {\ uparrow \,} {sp^3}
\ frac {\ uparrow \,} {sp^3}
\ frac {\ uparrow \,} {sp^3}$

In CH₄, these four orbital sp³ are recovered with the four orbital 1s of the four hydrogen atoms what leads to the formation of four connections sigma. These four connections have the same length and same energy, which is in conformity with the experimental results.

The structure of the other organic molecules is explained in a similar way. For example in the ethylene, the orbital atomic ones of carbon mix to form orbital hybrids sp², and covering between the atoms of carbon and with those of hydrogen led to the formation of four CH connections of the type sigma and to a double connection DC (connection of the sigma type - axial covering superimposed by a connection of the type pi - side covering.

The proportion of character p is not restricted with whole values, and there exist for example orbital hybrids sp^2.5. In this case, the geometry is deformed compared to the orbital hybrids " idéales". According to the rêgle of Bent, the character p of a connection is all the more large as it is directed towards an electronegative element more .

Form molecules

Allied theory VSEPR, the hybridization of orbital makes it possible to explain the geometry of the molecules qualitatively:

These examples are valid when the central atom does not carry in more one pair of electrons. If such is the case, this one is deducted with X_i but the angles of connections are different. It is the case for example for the molecule of Eau H₂O: the geometry is quite tetrahedral (by taking of account the electronic doublets), but the tetrahedron is deformed (angle HOH of 104,5 degrees).

References

L. Pauling, J. amndt Chem. Plowshare 53 (1931), 1367
L. Pauling, Natural The off the Chemical Jump , Cornell University Close (ISBN 0801403332)
Clayden, Greeves, Warren, Wothers. Organic Chemistry. Oxford University Close (2001), ISBN 0198503466.

See too

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