alt Hacker News> Its peculiar behaviour, such as its surprising stability despite being highly unsaturated, hinted at a deeper mystery that would not be fully resolved until the mid-19th century with the proposal of its cyclic structure.
How were chemists in the early 19th century able to determine benzene must be highly unsaturated without knowing its structure? Did they simply combust it and measure the amount of water vapor and carbon dioxide produced?
Replies
isoprophlex • 06/16/2025
What these early chemists accomplished with, to our eyes, extremely crude methods is astounding. Physical methods like weighing, burning and collecting residue; describing crystallization and precipitation behavior, even smelling (and sometimes tasting) was at one point a routine thing to do.
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Horffupolde • 06/16/2025
Yes.
There's a two step-process to stochiometry.
The first step, as people have elaborated below, is combust the compound and measure the weights of various oxides, which (after the atomic masses of the relevant elements were settled around the 1820s) lets you work out the empirical formula of an unknown molecule. For benzene, this would tell you that there is 1 C : 1 H, but this doesn't tell you if it's C₄H₄ or C₆H₆ or C₁₁₁H₁₁₁.
The second step is to determine the molar mass of your compound, which requires finding something that depends on the amount of substance but not the mass directly. (In modern times, this is primarily mass spec). Back in the 19th century, this is probably abusing the ideal gas law, which lets you compute the number of moles in a gas given the pressure, temperature, and volume of a vessel. Combine this with the mass of that container, and you know how much a mole weighs. If you get out, say, 77g/mol, and you know that the ratio is 1 C : 1 H, well, the only formula that makes sense is C₆H₆ (which should ideally have 78g/mol, but you might not get the right answer for various experimental reasons).