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Portrait of Benzene

Science · Chemistry

Benzene

1825 (isolated); 1865 (ring structure proposed)

Chemistry — aromatic organic chemistry

Benzene is the simplest aromatic hydrocarbon, C6H6: six carbon atoms in a flat regular hexagon, each carrying one hydrogen. Michael Faraday isolated it from the oily residue of illuminating gas in 1825, but its structure baffled chemists for forty years, because no chain formula could account for a compound so hydrogen-poor yet so unreactive. August Kekulé's 1865 proposal of a closed ring — which he later, and controversially, said had come to him in a reverie of a snake seizing its own tail — founded the whole of aromatic chemistry. The six carbon–carbon bonds are all identical, 140 picometres long, because six pi electrons are delocalised around the ring; that delocalisation is what "aromatic" now means.

How to spot them

  • ▸C6H6 — six carbons, six hydrogens, one flat hexagon
  • ▸All six carbon–carbon bonds identical in length, between a single and a double bond
  • ▸The archetype of aromaticity and of Hückel's 4n+2 rule
  • ▸A sweet-smelling liquid that is also a proven cause of leukaemia
  • ▸Drawn as a hexagon with a circle inside it

Key facts

Isolated by Michael Faraday in 1825 from the oily residue of gas used for street lighting, and named by him 'bicarburet of hydrogen' (1825)

Faraday obtained it from the compressed oil gas supplied to London by the Portable Gas Company.

Its structure is said to have come to Kekulé in a reverie of a snake seizing its own tail (1890)

He told the story at the Benzolfest in Berlin in 1890, twenty-five years after the discovery, and had never mentioned it in print before — so many historians treat it with suspicion.

The simplest aromatic hydrocarbon, formula C6H6

Four degrees of unsaturation in a molecule that stubbornly refuses to behave like an alkene.

Its six carbon–carbon bonds are all 140 picometres long, because six pi electrons are shared around the whole ring

Intermediate between a C–C single bond (154 pm) and a C=C double bond (134 pm).

In 1925 Armit and Robinson replaced the alternating double bonds in its symbol with a circle inside the hexagon (1925)

The circle stands for the delocalised sextet of pi electrons.

Eduard Mitscherlich made it from benzoic acid in 1834 and named it after gum benzoin, the resin the acid came from (1834)

He called it 'benzin'; Liebig preferred 'benzol', which is still the German name.

Removed from petrol, from decaffeination and from shoe glue once it was shown to cause leukaemia

It is a Group 1 human carcinogen; cigarette smoke remains most people's main exposure.

Key ideas

  • Aromaticity: a flat, fully conjugated ring holding 4n+2 pi electrons is far more stable than the alternating-double-bond picture suggests. The extra stability, about 150 kJ/mol, is why it reacts by substitution rather than addition.
  • The word 'aromatic' originally just meant these compounds smelled; it now refers to the electronic structure, and plenty of aromatic compounds have no smell at all.

Quick-fire facts

  • •Michael Faraday isolated it in 1825 and worked out the ratio of carbon to hydrogen; Eduard Mitscherlich gave it the name in 1834.
  • •Josef Loschmidt drew ring-like structures for it in 1861, four years before Kekulé's paper, which has fuelled a long priority argument.
  • •Kekulé published the ring proposal in French in 1865, in the Bulletin de la Société Chimique de Paris, and in German the following year.
  • •The 1890 Benzolfest was held in Berlin's city hall to mark twenty-five years of the ring structure; the snake story dates from that speech.
  • •Kekulé taught three future Nobel laureates — van 't Hoff, Fischer and Baeyer.
  • •It boils at 80.1 °C and freezes at 5.5 °C, so it can solidify in a cold laboratory.
  • •It is a Group 1 carcinogen linked to acute myeloid leukaemia, and is capped at around 1 per cent in petrol in the EU and UK.
  • •Industrially it is the starting point for styrene, phenol, nylon and aniline dyes; it is produced by the tens of millions of tonnes a year.
  • •Common misconception: the molecule is not flipping between two alternating-bond forms. It is a single structure with one kind of bond all the way round.

Drill it — 9 cards

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Sources