The chemical hybridization is the "mix" of the atomic orbitals, whose concept was introduced by chemist Linus Pauling in 1931 to cover imperfections of the theory of the valence bond (TEV). What imperfections? These are: molecular geometries and equivalent bond lengths in molecules like methane (CH 4).
According to TEV, in methane the C atomic orbitals form four σ bonds with four H atoms. The 2p orbitals, with formas shapes (bottom image) of C are perpendicular to each other, so the H should be a few from others at a 90º angle.
Additionally, the 2s (spherical) orbital of C binds to the 1s orbital of H at an angle of 135º with respect to the other three H. However, experimentally it has been found that the angles in CH 4 are 109.5º and that Furthermore, the lengths of the C – H bonds are equivalent.
To explain this, a combination of the original atomic orbitals must be considered to form four degenerate hybrid orbitals (of equal energy). Here chemical hybridization comes into play. What are hybrid orbitals like? It depends on the atomic orbitals that generate them. Also, they exhibit a mixture of their electronic characteristics.
Sp hybridization
For the case of CH 4, the hybridization of C is sp 3. From this approach, molecular geometry is explained with four sp 3 orbitals separated at 109.5º and pointing towards the vertices of a tetrahedron.
In the image above, you can see how the sp 3 orbitals (green) establish a tetrahedral electronic environment around the atom (A, which is C for CH 4).
Why 109.5º and not other angles, in order to "draw" a different geometry? The reason is because this angle minimizes the electronic repulsions of the four atoms that bind to A.
Thus, the CH 4 molecule can be represented as a tetrahedron (tetrahedral molecular geometry).
If, instead of H, C formed bonds with other groups of atoms, what would then be their hybridization? As long as the carbon forms four σ bonds (C – A), their hybridization will be sp 3.
It can consequently be assumed that in other organic compounds like CH 3 OH, CCl 4, C (CH 3) 4, C 6 H 12 (cyclohexane), etc., the carbon has sp 3 hybridization.
This is essential for sketching organic structures, where single bonded carbons represent points of divergence; that is, the structure does not remain in a single plane.
Interpretation
What is the simplest interpretation for these hybrid orbitals without addressing the mathematical aspects (the wave functions)? The sp 3 orbitals imply that they were originated by four orbitals: one s and three p.
Because the combination of these atomic orbitals is assumed to be ideal, the resulting four sp 3 orbitals are identical and occupy different orientations in space (such as in the p x, p, and p z orbitals).
The above is applicable for the rest of the possible hybridizations: the number of hybrid orbitals that is formed is the same as that of the atomic orbitals that are combined. For example, sp 3 d 2 hybrid orbitals are formed from six atomic orbitals: one s, three p and two d.
Bond angle deviations
According to the Valencia Shell Electronic Pair Theory of Repulsion (RPECV), a pair of free electrons occupies more volume than a bonded atom. This causes the links to move apart, reducing the electronic voltage and deviating the angles from 109.5º:
For example, in the water molecule the H atoms are bonded to the sp 3 orbitals (in green), and also the unshared pairs of electrons ":" occupy these orbitals.
The repulsions of these pairs of electrons are usually represented as “two globes with eyes”, which, due to their volume, repel the two σ O – H bonds.
Thus, in water the bond angles are actually 105º, instead of the 109.5º expected for tetrahedral geometry.
What geometry then does H 2 O have? It has an angular geometry. Why? Because although the electronic geometry is tetrahedral, two pairs of non-shared electrons distort it to an angular molecular geometry.
Sp hybridization
When an atom combines two p and one s orbitals, it generates three sp 2 hybrid orbitals; however, one p orbital remains unchanged (because there are three of them), which is represented as an orange bar in the upper image.
Here, all three sp 2 orbitals are colored green to highlight their difference from the orange bar: the "pure" p orbital.
An atom with sp 2 hybridization can be visualized as a flat trigonal floor (the triangle drawn with the sp 2 orbitals colored green), with its vertices separated by 120º angles and perpendicular to a bar.
And what role does the pure p orbital play? That of forming a double bond (=). The sp 2 orbitals allow the formation of three σ bonds, while the pure p orbital one π bond (a double or triple bond involves one or two π bonds).
For example, to draw the carbonyl group and the structure of the formaldehyde molecule (H 2 C = O), proceed as follows:
The sp 2 orbitals of both C and O form a σ bond, while their pure orbitals form a π bond (the orange rectangle).
It can be seen how the rest of the electronic groups (H atoms and the pairs of electrons not shared) are located in the other sp 2 orbitals, separated by 120º.
Sp hybridization
In the upper image an A atom with sp hybridization is illustrated. Here, one s orbital and one p orbital combine to form two degenerate sp orbitals. However, now two pure p orbitals remain unchanged, which allow A to form two double bonds or one triple bond (≡).
In other words: if in a structure a C complies with the above (= C = or C≡C), then its hybridization is sp. For other less illustrative atoms - such as transition metals - the description of electronic and molecular geometries is complicated because the d and through f orbitals are also considered.
The hybrid orbitals are separated at an angle of 180º. For this reason the bonded atoms are arranged in a linear molecular geometry (BAB). Finally, in the image below the structure of the cyanide anion can be seen:
References
- Sven. (June 3, 2006). Sp-Orbitals.. Retrieved on May 24, 2018, from: commons.wikimedia.org
- Richard C. Banks. (May 2002). Bonding and Hybridization. Retrieved on May 24, 2018, from: chemistry.boisestate.edu
- James. (2018). A Hybridization Shortcut. Retrieved on May 24, 2018, from: masterorganicchemistry.com
- Dr. Ian Hunt. Department of Chemistry, University of Calgary. sp3 hybridization. Retrieved on May 24, 2018, from: chem.ucalgary.ca
- Chemical Bonding II: Molecular Geometry and Hybridization of Atomic Orbitals Chapter 10.. Retrieved on May 24, 2018, from: wou.edu
- Quimitube. (2015). Covalent Bonding: Introduction to Atomic Orbital Hybridization. Retrieved on May 24, 2018, from: quimitube.com
- Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition., P. 51). Mc Graw Hill.