Activity 1 – Introduction to Benzene
You will probably be familiar with the structure of benzene, a simple, and the most common,
aromatic structure. It has the formula C6H6 and can be shown in many different ways, some
of which are shown in Figure 1.
Figure 1. Three of the possible ways to represent benzene
The Discovery
Benzene was discovered in 1825 by Michael Faraday when he separated an oily, and
aromatic, liquid from the by-products of the production of lighting fuel which was made by the
distillation of fish oil [1]. Faraday distilled, and redistilled, the oily residue, each time
separating out the products more clearly; for example a fraction which initially distilled over
at between 160 and 170 degrees C was found, upon a second distillation, to split into two
fractions with one coming over at 130 degree C whilst a second did not distil until 200
degrees C. By going through this process a number of times Faraday separated a novel
compound, bi-carburet of hydrogen, the compound we now call benzene.
Faraday’s description of this new compound was as follows:
“Bi-carburet of hydrogen appears in common circumstances as a colourless transparent
liquid, having an odour resembling that of oil gas, and partaking also of that of almonds. …
When cooled to about 32 degrees it crystallises, becoming solid.” He also went onto state
that “its boiling point in contact with glass is 186 degrees.” [2]
All of this data is in surprisingly good agreement with todays accepted data on benzene.
By carrying out a number of chemical analyses including combustion and reaction with
copper oxide he concluded that the mass ratio of Hydrogen to Carbon within the structure
was 1:12, giving the molar ratio of 1:1, a ratio which is, of course, in agreement with the
structure we know today (interestingly, at the time it was thought that the mass of Carbon
was 6, and so Faraday actually suggested a structure with the empirical formula C2H).
The Name
The name benzene came along about 10 years later on and was supplied by Eilhard
Mitscherlich who found that “benzin” could be isolated from the reaction of benzoic acid (also
known as gum benzoin, from where the name benzin originates) with lime. When translated
into English we obtained the name benzene.
Mitschwelich also gave us the formula of benzene, showing that it has the formula C6H6,
although at this point how this formula could be true was unclear. The low ratio of hydrogen
to carbon suggested that the structure must contain double, or perhaps even triple, bonds,
however this did not tally with the reactivity which was being observed at the time.
The Structure
The idea of the structure of benzene is one of the favourite stories in Chemistry. The Kekulé
structure, according to August Kekulé himself, came to him in a day-dream when he saw a
snake seizing its own tail forming a ring structure. The ring system had alternating double
and single bonds to allow for the C6H6 structure previously accepted. Initially, these single
and double bonds were thought to be fixed (1865), but on further inspection of the structure
and reactivity of benzene derivatives Kekulé further refined his structure suggesting the
double and single bonds are continuously changing place, making the all the carbon-carbon
bonds equivalent (1872) (see Figure 2).
Figure 2. Two resonance forms of benzene, and, on the right, the common
representation of benzene showing all of the bonds are equal.
Spectroscopic analysis of the structure of benzene showed that it had six equal carboncarbon bond lengths of 1.39Å (this is compared to a normal C-C of 1.47Å and C=C of
1.34Å). One way of explaining this is indeed to use the idea of a resonance structure which
exists in a state between the two extremes shown on the left in Figure 2 above. A nice way
to think about this idea is shown in Figure 3.
Figure 3. Showing the idea of resonance structures.
This structure is normally shown as a six membered cyclic carbon structure with a ring in the
centre showing that all the bonds are the same, and that there is some double bond
character to all of the bonds (see Figure 2, right). This structure agrees with the
experimentally determined bond angles of 120 degrees, and the increased stability supplied
by the resonace structures, the resonace energy, explains the difference in reactivity which
is observed compared to what would be expected from a structure containing isolated C=C.
[1] Lynman C. Newell; J. Chem. Educ., 1926, 3 (11), p 1248
[2] M. Faraday; Philosophical Transactions of the Royal Society of London, 1825, p 440-466
(http://www.jstor.org/stable/107752)
Kekulé (1865). "Sur la constitution des substances aromatiques". Bulletin de la Societe
Chimique de Paris 3 (2): 98–110.
Kekulé (1866). "Untersuchungen uber aromatische Verbindungen". Annalen der Chemie und
Pharmacie 137 (2): 129–196. doi:10.1002/jlac.18661370202.