What is a Benzene Ring?
Benzene rings are an organic molecule with a planar geometry. A benzene ring is 6 carbon atoms in a ring with alternating double bonds between them. Therefore there are also 6 hydrogen atoms. It is an aromatic structure.
Structure of Benzene Rings
Benzene rings are the simplest type of aromatic rings; They consist of 6 carbons and 6 hydrogens(C6H6). Because of its classification as an aromatic ring, they also have an unsaturation value of 4. This U value is due to the presence of 3 double bonds and the overall ring structure of the molecule.
Additionally, the molecule is completely flat because of the sp2 hybridization of all the carbons within the ring; the carbons have a bond angle of 120 degrees in between the other carbons. Thus, there exist no conformations for this 6-membered ring—neither chair nor boat conformations.
The molecule also doesn’t possess any stereocenters/stereoisomers; the molecule is achiral and has several internal planes of symmetry, making its NMR rather unique for an aromatic ring.
In addition, its important to note that phenyl groups(benzene derivatives) are named differently from alkenes and cycloalkanes. The nomenclature for benzene derivatives can become incredibly complex.
Resonance of Benzene Ring
As depicted in the 3rd representation of benzene above, this molecule has a resonance structure. Thus, it is resonance stabilized. While most other ring structures have a lowest energy conformation, this molecule is particularly stabilized through its delocalization of its electrons in the 2P orbitals and has no favorable conformation.
The electrons in the P orbital (Pi bonding orbital in this instance) are generally high (enough) in energy and can usually act as a fair HOMO in most reactions. However, because the molecule has an equally favorable resonance structure with electrons delocalized both above and below the ring, in a phenomenon called aromaticity, the molecule isn’t willing to use these delocalized electrons to act as a nucleophile unless there exists an incredibly good electrophile. This is in contrast to regular alkene chemistry where it is common to see Pi bonds (C=C) act as the donor orbital in many reactions.
Aromaticity of Benzene Rings
Due to the cyclic, contiguous, and planar structure of benzene, as well as its odd number of pi electron pairs, this molecule is classified as aromatic. In essence, this means that
Because of this, benzene won’t typically invoke its pi bonding orbitals for the purpose of bonding. The only instance of this aromatic ring reacting will come from an interaction with an incredibly good electrophile. Friedel Crafts Acylation and Friedel Crafts Alkylation are both great examples illustrating this exception. In Friedel Crafts Acylation, the acylium ion(a resonance stabilized carbocation) is such a good electrophile that it is energetically favorable to temporarily break the aromaticity of the phenyl group. A similar concept is present in Friedel Crafts Alkylation where the presence of a tertiary carbocation fulfills the same function.
In essence, phenyl groups are incredibly stable and won’t react like regular alkenes. Due to this stability, it is actually rather difficult to cleave aromatic rings and require very specific mechanisms to do so.
Benzene Molecule in Spectroscopy
Due its unique symmetrical structure, the benzene molecule is easily identified using various techniques. Some of the most common techniques seen in organic chemistry are discussed below.
Benzene Rings NMR Spectroscopy
The proton and carbon NMR spectroscopy of benzene is unique compared to other aromatic rings. The NMR of both produce only 1 signal(1 singlet) due to all the protons and carbons being chemically equal. This chemical equality also leads to the absence of the effects of coupling and splitting as seen in the NMR of other aromatic rings.
Benzene Rings IR Spectroscopy
The absorption bands for aromatic rings are also unique. Aromatic rings have particularly strong peaks in the fingerprint region of the spectrum. These peaks create overtones(harmonics) signature to aromatic rings in the 1700-2000V(cm¯¹). Below is an image of the IR spectrum of toluene, a close relative of benzene in structure. Highlighted is the aromatic overtones both molecules share.
Benzene UV/Vis Spectroscopy
Benzene absorbs strongly in the UV region. Below is the UV/Vis spectrum for benzene around 200 nm.