The benzene ring

The atomic number of carbon is six, which means that the carbon atom has six protons and six electrons. Electrons are present in orbitals around the atom's nucleus. An orbital is defined as the space around the nucleus where an electron with certain energy is most likely to be found. There are typically several different orbitals around a nucleus, representing different energy levels and different spatial distributions.

The electron configuration of carbon is 1s2 2s2 2p2, meaning that the 1s, 2s and 2p orbitals each contain two electrons. Covalent bonds between atoms result from the sharing of one or more pairs of electrons. This is the result of overlapping atomic orbitals that form molecular orbitals. Carbon is able to make four covalent bonds, even though based on its electron configuration only two unpaired electrons - those in the 2p-orbitals - are available. The ability to make four instead of two covalent bonds results from hybridization, whereby one of the 2s electrons is promoted to the 2p orbital (electron configuration: 1s2 2s1 2p3), followed by the formation of four equivalent sp3 orbitals with slightly higher energy than the 2s orbital. The promotion of one of the 2s-electrons to the 2p-orbital can also result in the formation of three sp2 and one 2p orbitals (sp2 hybridization), or in two sp orbitals and two 2p orbitals (sp hybridization). The angle between these different orbitals is always such that the distance between the orbitals is maximal. Thus, the four sp3 orbitals make an angle of 109.5°. This can be visualized as a regular tetrahedron, with the C-atom in the center and the orbitals pointing into the corners. The three sp2 orbitals make an angle of 120° with each other, and lie in a plane. The two sp orbitals make an angle of 180° with each other.

If the bond formed between two adjacent atoms is symmetric along the axis between the two nuclei, we refer to the bond as a o-bond. In contrast, the bond that is formed between two electrons in the p-orbitals is called a n-bond. These molecular orbitals have a very different shape. The single bond between two carbon atoms (-C-C-) is formed between electrons in the sp3-orbitals (o-bond) of two adjacent carbon atoms (Figure 1a). A double bond between two C-atoms (-C=C-) consists of a o-bond formed between electrons in the sp2 orbitals of two adjacent C-atoms, and a n-bond formed between two 2p-orbitals (Figure 2-1).

Figure 2-1. Formation of a o and a n-bond between two sp2 hybridized carbon atoms. The p-orbital has two lobes, one above and one below the C-atom.

A single bond has a length of 1.54 A (1A = 1Angstrom = 10-10 m). A double bond is 1.34 A. In conjugated molecules there is an alternation between single and double bonds, such that the n-electrons can be shared by all C-atoms that are part of the conjugated system. These electrons are referred to as delocalized electrons, because they are less confined to the axis of the bond. A conjugated molecule is considered aromatic if it contains a cyclic n-system with (4n+2) n-electrons (n = 1, 2, 3, ...). Hence, benzene is the simplest aromatic compound (n = 1). The n-electrons of benzene are present in a molecular orbital that lies above and below the plane formed by the C-atoms (Figure 2-1). The bond between the C-atoms is 1.39 A, which lies in between the length of the single and double bond.

Figure 2-1. Formation of a o and a n-bond between two sp2 hybridized carbon atoms. The p-orbital has two lobes, one above and one below the C-atom.

Figure 2-2. Spatial representation (ball-and-stick model) of benzene, with C-atoms in grey and H-atoms in white. The dotted lines between the C-atoms represent the delocalized electrons. The image on the right shows the surface area of the highest occupied molecular orbital (HOMO). Note how the n-electrons are above and below the benzene ring.

Figure 2-2. Spatial representation (ball-and-stick model) of benzene, with C-atoms in grey and H-atoms in white. The dotted lines between the C-atoms represent the delocalized electrons. The image on the right shows the surface area of the highest occupied molecular orbital (HOMO). Note how the n-electrons are above and below the benzene ring.

The benzene ring is usually depicted as one of two mesomeric structures (2.1). The double arrow indicates that the true structure of the molecule lies somewhere in between the two drawn structures. It is therefore more accurate to use structure 2.2, since the six C-C bonds of the ring are identical, with the n-electrons delocalized over the entire ring. The configuration shown in 2.2 is, however, less convenient for drawing reaction mechanisms.

The delocalization of the n-electrons is energetically favorable, and this affects the reactivity of aromatic compounds: There is a tendency towards restoring aromaticity. This is why aromatic compounds, in contrast to regular alkenes (linear chains of carbon atoms containing at least one double bond), do not easily undergo addition reactions, whereby a double bond is replaced by two single bonds. Aromatic compounds show a preference for substitution reactions, which means that atoms are replaced.

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