Benzene

Benzene is sometimes referred to as an aromatic compound as opposed to the majority of other organic molecules that you have so far studied that are found in chains and sometimes referred to as aliphatic compounds.

Aromatic substances are said to have an unsaturated ring of atoms whereas aliphatic substances open chains. The word ‘arene’ is sometimes used in place of aromatic.

Benzene has the formula C₆H₆. Its structure was first proposed by the German chemist Friedrich August Kekulé in 1865. Its molecular formula had been known for some time, but he was the first chemist to propose a structure for it.

This was ground breaking at the time as up to this point, organic chemists had only thought of organic substances existing as aliphatic compounds, i.e. chains of atoms. However, it is not possible to have an arene of C₆H₆ – it just doesn’t work.

Kekule proposed that benzene was not, in fact, an arene but was found as a ring structure. He proposed an unsaturated molecule consisting of six carbon atoms in a ring, connected together by alternating double and single bonds:

Kekule was working on this problem for a long time and not having any luck. At the 25^th anniversary of the discovery of its structure he claimed that on the day he worked out the structure, he was daydreaming about the possible structure of the molecule and imagined a snake eating its tail, forming a ring structure, allowing him to realise the molecule was aromatic as opposed to aliphatic.

This was groundbreaking at the time and opened up a whole new branch of chemistry. Don’t underestimate the importance of this discovery!

Of course now, we know much more about benzene. It does not contain alternating double and single bonds but a system of delocalised electrons above and below the ring structure (that’s why it is drawn with a circle and not alternating double and single bonds). These delocalised electrons mean that it is difficult to get it to react (strong nucleophiles such as NO₂⁺ are needed) and it is a remarkably stable substance, no reacting by addition (as it were if it were unsaturated) by substitution. It is also a planar molecule with a bond angle of 120^o between a H-C-C.

There are many organic and inorganic molecules out there that have good stories attached to them. Do you know any? If you do, please feel free to share them below.

References

https://en.wikipedia.org/wiki/August_Kekule was used to help write this blog post.

Image credit

Friedrich August Kekulé von Stradonitz (1829–1896). Derived from Kekule_-_Ueber_einige_Condensationsproducte_des_Aldehyds.pdf, via Wikimedia Commons