Benzene

Benzene is such a unique chemical that when it's bonded to organic compounds, chemists classify them as "aromatic". To be specific, we have aliphatic compounds, which consist of single, double or triple bonds between carbon atoms (think basic alkanes, alkenes, alcohols, aldehydes, ...), as well as alicyclic compounds, which are much the same except carbon atoms are bonded in a cycle.

Benzene is unique as it sort of combines all three of these categories - that is if you use one of the models. This is one of the models, with alternate single and double carbon bonds:

 

Its chemical formula is C₆H₆, and since Michael Faraday discovered it in 1825, chemists wondered what benzene could be like molecularly. August Kekulé was the person whose model (as seen) brought him to fame, all thanks to a supposed dream he had. Many years later, Linus Pauling came up with a newer model, one I personally dislike when depicted as skeletal:

 

Resembling a nut, this model incorporates the idea of delocalised electrons which form an electron cloud in the centre of a benzene molecule. Simply put, Pauling realised that the enthalpy of hydrogenation of benzene, since it had three C=C bonds, should be three times as big as that of cyclohexene, which has one C=C bond, because all the C=C bonds must open up and admit hydrogen. However, research showed this wasn't the case - the enthalpies didn't line up exactly. Pauling regarded benzene's missing enthalpy to be its "resonance energy", which should enable benzene to be stable - the energy comes through the overlap of orbitals, hence the delocalised electrons which are shared between the carbon atoms. It's notable, however, that Pauling wrote a paper which also included an alternative model of benzene, and I don't want to discuss the models anymore because there are so many of them. 

 

Benzene is a component of petrol, indeed many languages derive their word for petrol from benzene, but makes up only 1% of it. Either way, benzene is also a carcinogen, so this poses a risk to people, as this paper on petrol station workers discusses, and petrol is one of the biggest contributors to benzene's presence in the environment. 

Most organic compounds are aromatic, and, owing to their stability, can be found in countless other fields. Many drugs and painkillers, such as aspirin, include benzene, and solvents such as phenol often include benzene too. In these instances, benzene thus acts as a functional group and these compounds are also called arenes. 

Benzene's stability is the main reason as to why it's so prevalent. Estradiol is a steroid that stimulates the production of estrogen, and benzene is part of it. Thanks to its stability, the region surrounding a benzene ring is planar, unlike the rest of the molecule - due to this, estradiol can more easily bind to a protein as the ring can slot more easily into the protein, which can then go on to trigger blood clotting, for instance¹.

Phenol in particular is quite interesting. It forms when a hydrogen atom in benzene is substituted for a hydroxy group, thus its formula is C₆H₅OH.

It's also used as a disinfectant and in mouthwash, albeit in low volumes, as well as in the production of herbicides and slimicides (which act against fungi). It's often used in plastic production, with phenolic resins coming about through the reaction of phenol with formaldehyde. Bakelite is one such example, having been used in circuitry and games pieces. It's also used to produce nylon, a tough elastic polymer which is used in clothing and fabrics or to mould products. When added to a phthalein dye, you get the indicator phenolphthalein, hence its convoluted name.

Similarly to benzene, phenol is also harmful and crops up in petrol as well. Reassuringly, though, it's not thought to be a carcinogen, yet that's not without forgetting its ability to cause liver and kidney damage.

 

Benzene undergoes substitution reactions for the most part. Indeed, this is also another reason why it can't contain C=C double bonds - if it did, it should undergo electrophilic addition reactions, yet it won't. React benzene with nitric acid, and you'll form nitrobenzene, or C₆H₅NO₂ - it's used in polish and solvents (ranging from paint to cleaning gun barrels), and can be used to synthesise aniline, used in thermal insulation foam. Nitrobenzene began as a perfume, but it's toxic. It affects your blood's ability to carry oxygen and can lead to liver damage and potentially death. Similarly, if the substituent in the reaction is an alkyl group, we get toluene, chemical formula C₆H₅CH₃. It's present in crude oil and is used in making paint, as well as lacquers and polish. It can also cause nerve damage, skin inflammation, and liver damage.

If we took these substitution reactions to their limit, we may end up with graphite, which can be used as an electrical conductor as well as pencil lead. Graphite has no hydrogen atoms, indeed it's layers of hexagonal rings of carbon atoms throughout, yet the cloud of delocalised electrons from before have remained. It's used as electrodes thanks to its conductivity, and is part of the steelmaking industry. It's also proven to be beneficial in the enzyme industry - attach three enzymes to a graphite bead, and catalysis can occur at a far greater rate¹.

One layer of graphite is called graphene, which seemingly is the future though I've not heard about it for a while. Benzene's stability means graphene is light and yet durable, with a tensile strength far greater than steel. It's thus been used in electronics, medicine, and in coating materials, as well as in solar cells and hydrogen fuel cells, which are likely to become omnipresent. The University of Manchester are very proud of graphene, it was first isolated there after all, and you can read more about it here.

I've learned that benzene is essential, useful, life-saving, deadly, and most importantly, a very unique chemical that seems to be everywhere. Its impact cannot be denied.

¹ Source on benzene's role in estradiol and graphite's use in enzymes - Organic Chemistry A Very Short Introduction by Graham Patrick