You might have heard that this year marks 200 years of the Ri’s two major lecture series: the CHRISTMAS LECTURES and Discourses. But in 1825 something else quite extraordinary was going on, not in our iconic Lecture Theatre, but in our basement laboratories. If you’ve wandered through the exhibition corridor in the lower ground floor of the Ri, you might have spotted it: a tiny, quite unassuming, clear vial of fluid.

Why do we care so much about a few millilitres of liquid?

Michael Faraday In His Basement Laboratory, 1852 (cropped). Credit: Royal Institution)

Whales, Gas, and Hard Work

In the early 19th century, gas fuel illuminated London. But it wasn’t the natural gas we know today, it was oil gas. Oil gas was a smelly business, typically made from whale or fish oil dropped into a hot furnace. The resulting gas was compressed into copper cylinders which were then transported around London to illuminate homes and businesses. But after a while, people found that the gas from the cylinders seemed to lose some of its illuminating power. In 1825, the Portable Gas Works asked Faraday to find out why.

Faraday found that in the process of converting oil to gas, some portion of it was condensed into an oily liquid residue. What was this strange substance? He recorded this carefully in his diary. On 26 April, some oil gas was delivered to Albemarle Street, and Faraday immediately began exploring different distillates and their properties. Weeks later, on 9 May, Faraday visited the gas works himself. After some initial tests, on the 16 May the real work began.

211 volumes of oil gas were delivered the Ri for Faraday to experiment on. It was a gruelling process of fractional distillation, aiming to remove water and other undesirable compounds, to determine the core components of this residue. Rounds of freezing, boiling, condensing, and squeezing with a Bramah press followed. But by 18 May Faraday had a hunch he had isolated something entirely new, so he continued tirelessly for the next few weeks until he was certain of the purity of this compound. In his diary during this time he often noted: “Peculiar Substance.” Today, we know that peculiar substance as benzene. Faraday announced this substance to the Royal Society on 16 June 1825. But nowhere in this paper does Faraday mention benzene. Why?

Michael Faraday’s diary from May 1825 (cropped). Credit: Royal Institution

The Birth of Benzene (Sort of)

If you look carefully at the vial, you can see that Faraday etched the name of this substance on the glass: “bicarburet of hydrogen,” *not *benzene. You can see this name in the margins of Faraday’s diary too. Faraday’s understanding of chemical formulae was different to those we use today. After making some calculations, he presented the substance in terms of its proportional weights: 2 parts carbon for every 1 part hydrogen.

This name didn’t stick. As new routes to deriving benzene were discovered and other chemists attempted to place benzene into existing understandings of chemical substances, after a few permutations, the name benzene stuck. Fortunately, Faraday approved!

19th-century chemists were also now exploring a whole new family of chemicals that were related to benzene. These became known as “aromatic compounds” due to their shared sweet-acrid smell. While today aromaticity has nothing to do with odour (instead defined as a cyclic compound), for chemists of the past their noses were a vital tool for identifying benzene-like compounds.

These aromatic compounds have had a huge impact on modern life. The first major innovation happened in 1856, when an 18-year-old chemistry student named William Henry Perkin was attempting to synthesize artificial quinine, a valuable antimalarial drug. Perkin accidentally created something quite different. Working with aniline, an aromatic coal tar derivative containing benzene, he produced a dark precipitate that, when dissolved in alcohol, revealed a beautiful purple colour. This was mauveine, the first synthetic dye.

Mauvemania raged across Victorian society. This new synthetic dye was vivid and unsusceptible to fading unlike natural dyes. Suddenly aromatic chemistry was fashionable, commercial, and visibly transformative. And crucially, the science around benzene became incredibly profitable, and other uses of benzene-like compounds began to be developed.

At the Ri, Perkin’s former teacher, the famous chemist August Wilhelm Hofmann, gave a Friday Evening Discourse on April 11th, 1862, titled “On Mauve and Magenta.” He brought Faraday’s original benzene vial up to the lecture theatre and pointed to Faraday’s “pure research” as the gold standard of science. This was not a particularly well-veiled attack on Perkin, who had now made significant wealth from his mauveine synthesis. The Ri’s journal, The Proceedings, included dyed fabric samples of various new dyes. You can see how their vibrancy persists even today!

Samples of mauveine and other aniline dyes from the Ri’s Proceedings 1862, Credit: Royal Institution

Molecular Mysteries

But where isolating benzene had been hard, understanding its behaviour was even harder. A key question remained unresolved: what did it actually look like? Benzene’s molecular formula, 6 carbons and 6 hydrogens, suggested that it should be highly reactive and unstable, with multiple carbon-carbon double bonds. Yet benzene was remarkably stable and behaved quite differently from other compounds with similar empirical formulas.

Various chemists tried to crack the code. The most famous theory came from August Kekulé: that having dreamt of a snake biting its own tail inspired him to propose a ring structure for benzene in 1865. But even this couldn’t fully explain benzene’s unusual stability, and structural proof remained elusive.

At the Ri, Professor James Dewar (of Dewar flask fame) proposed alternative structural possibilities. Others weighed in with their own theories. The exact arrangement of benzene’s atoms remained frustratingly uncertain, and scientists had no means to directly observe molecular structure. Even at benzene’s centenary celebration in 1925, the structure hadn’t been conclusively proven. It had transformed industry but remained mysteriously elusive.

Various proposed structures for benzene from: Linus Pauling “The nature of the chemical bond. V. The quantum-mechanical calculation of the resonance energy of benzene and naphthalene and the hydrocarbon free radicals.“ March 21, 1933. J. Chem. Phys. 1

Seeing the Unseeable

Three years later, in 1928, scientists in the Ri’s basement were once again trying to solve the structure problem. Under William Henry Bragg, a pioneering laboratory dedicated to the new field of X-ray crystallography was using X-rays to probe different molecular structures. By firing X-rays at crystals, they could analyse the diffraction patterns to infer atomic arrangements. This state-of-the-art technology changed everything.

You might say: but benzene is a liquid, not a crystal! The crystallographers had a cunning solution to this: hexamethylbenzene, a crystalline relative of benzene. The crucial work on hexamethylbenzene was undertaken by Kathleen Lonsdale, a young crystallographer working in Bragg’s lab. Using X-ray photography and sophisticated space group calculations, Lonsdale made a ground-breaking discovery.

Structure Factor Tables “ (1936) was published in her own handwriting, and contained tables of structure factors that she calculated for 230 space groups. Credit: Royal Institution

In a paper published in 1929, Lonsdale definitively showed that benzene was a flat, hexagonal ring. More importantly, her calculations revealed that all the carbon-carbon bonds were of equal length, and the electrons were delocalised around the ring, explaining benzene’s curious stability at last. It had taken just over 100 years since Faraday’s initial isolation, but finally, just meters away from his laboratory, the structure problem was solved.

Model of Benzene’s structure from 1929 showing its delocalised electrons and flat structure. Credit: Royal Institution

The Story in a Vial

Today, pure benzene is used only in controlled industrial processes, as its carcinogenic effects are now well known (no more using it to decaffeinate coffee, degrease engines, or in shaving cream, thankfully!) But its structure, the benzene ring, is pervasive in things we use every day, including plastics, pharmaceuticals, perfumes, and pigments.

Isolating benzene marked a huge turning point in chemistry, but it had consequences far beyond that too. It was a moment when pure research, undertaken without immediate application, led to an extraordinary legacy. So, while the bi-centenaries of the CHRISTMAS LECTURES and Ri Discourses may take the headlines, we are delighted to also be celebrating Faraday’s and Lonsdale’s labours.

Benzene is about the importance of pursuing science for science’s sake. When our Lecture Theatre was last refurbished, the team chose a deep, purply-pink colour for the upholstery; a subtle nod to benzene’s story at the Ri, its impact on our world, and the unexpected consequences of pure research.

So the next time you visit the Ri, head downstairs and look for the vial. Inside that liquid is a story of curiosity, confusion, controversy, and clarity. And don’t miss our temporary exhibition on benzene in the exhibition corridor, celebrating 200 years since Faraday’s remarkable work.