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Alkanes: Introduction and Chemical Bond

Alkanes: Hybridization and Chemical Bonding

The ability of carbon to form four bonds was first proposed by Kekulé in 1858 ("Carbon is four-based, i.e. 1 carbon atom = Î = 12 is equivalent to 4 hydrogen atoms"). Kekulé recognized that one carbon atom was able to bind to four other "one-based" atoms. The steric structure, i.e. a model in which the bonds point towards the four corners of a tetrahedron, was postulated by J. H. Van't Hoff and J. A. Le Bel in 1874. An explanation for this empirical hypothesis, which was based on a number of experimental results, could not be given until many years later using quantum mechanics and its application to atomic models and chemical bonding. In its simplest form, the application of quantum mechanics allows for two different approaches to the explanation of chemical bonding, VB (valence bond method) and MO (molecular orbital theory). At this point, we will consider only VB theory which initially is more straightforward and descriptive.

The simplest alkane, methane (CH4), will serve as a model compound. The central C atom has in its valence shell one s orbital and three p orbitals (px , py and pz ). If a methane molecule were to be hypothetically generated by adding four hydrogen atoms to the carbon atom, it would show three identical C-H bonds using the p orbitals and one separate bond using the s orbital. However, there is experimental evidence showing that all four C-H bonds are identical. Moreover, such a hypothetical methane molecule would not have tetrahedral geometry.

Wrong MO model
Tetrahedral model

Applying VB theory, the four identical C-H single bonds and the tetrahedral geometry can be explained by hybridization of the four valence orbitals (s, px , py and pz ) to form four sp3-hybrid orbitals. These have identical shape and energy and their steric orientation spans a regular tetrahedron. The bond angle in a tetrahedron is 109.5°. The four hybrid orbitals are used to form four bonds to the H atoms.

Construction of methane model
Building blocks
C atom with 4 sp3 hybrid orbitals
4 H atoms with one s orbital each
Notation with stereochemical representation of bonds

The construction of models of higher alkanes can be conceived in a similar manner. Ethane can be built up by using two sp3-hybridized carbon atoms and six hydrogen atoms.

The use of models is very helpful in describing the principle of molecular construction in organic chemistry and the variety of structures resulting therefrom.

Construction of ethane model
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