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Blue jeans are dyed with indigo, and for most of history that dye came from plants, much of it from India. By 1905 a synthetic version was already competing successfully with the plant dye. The chemistry behind that change, and a theory you still meet when you study cycloalkanes, came from the German chemist Adolf von Baeyer. This post covers his life, how he approached indigo, a worked example using the angle numbers from his 1885 strain theory paper, common mistakes, and what his attitude to experiments can teach you.
Who was Adolf von Baeyer?
His Nobel Prize facts page records that he was born in Berlin on 31 October 1835 and died in Starnberg on 20 August 1917. He received the 1905 Nobel Prize in Chemistry alone, while at the University of Munich, "in recognition of his services in the advancement of organic chemistry and the chemical industry, through his work on organic dyes and hydroaromatic compounds".
His Nobel biography adds that at twelve he had already found a new double salt of copper. He studied with Bunsen in Heidelberg, worked in Kekulé's laboratory, and received his doctorate in Berlin in 1858. In 1873 he succeeded Justus von Liebig at Munich and built a new laboratory there. The biography notes that he trained a school which alone produced fifty future university teachers, and that he was raised to the hereditary nobility on his fiftieth birthday.
How did Baeyer make indigo in the laboratory?
The biography says he began work on indigo in 1865, a dye that had fascinated him since his youth, and that it soon led him to indole. The 1905 Nobel presentation speech describes indigo as one of the hardest problems of its time, needing "years of work", and lists three routes he found: from ortho-nitrophenylacetic acid, from ortho-nitrocinnamic acid, and from ortho-nitrobenzaldehyde and acetone. The last route was published with his co-worker Drewsen in 1882 as Darstellung von Indigblau aus Orthonitrobenzaldehyd.
The speech notes that "the annual tribute which the West used to pay India" for natural indigo had been large, that synthetic indigo's price had fallen to a third, and that Germany exported over 25 million marks' worth of it in 1904. It also stresses that turning the laboratory result into an industrial process took further years of work.
What is Baeyer strain theory?
In an 1885 paper, Ueber Polyacetylenverbindungen, Baeyer added a seventh rule to the accepted principles of carbon chemistry. He wrote that the four valencies of carbon point towards the corners of a tetrahedron, at 109°28' to each other, and that bending them away from this direction produces a strain that grows with the size of the bend.
He then imagined each ring as a flat regular polygon. If the ring's internal angle differs from 109°28', each of the two bonds at a carbon must bend by half the difference.
Worked example: Baeyer's angle deviations
Deviation d = ½ × (109°28' − ring angle)
Cyclopropane (triangle, angle 60°): d = ½ × (109°28' − 60°) = ½ × 49°28' = 24°44'
Cyclobutane (square, 90°): d = ½ × 19°28' = 9°44'
Cyclopentane (pentagon, 108°): d = ½ × 1°28' = 0°44'
Cyclohexane (hexagon, 120°): d = ½ × (109°28' − 120°) = ½ × (−10°32') = −5°16'
These are exactly the values in Baeyer's paper. He also treated ethylene as a two-membered ring in which both bonds bend until parallel, ½ × 109°28' = 54°44'. He linked the numbers to behaviour: ethylene, his two-membered ring, is opened by hydrogen bromide, bromine and even iodine; trimethylene (cyclopropane) only by hydrogen bromide; and tetramethylene and hexamethylene hardly at all.
A quick check in decimals: 109°28' = 109.47°, and (109.47 − 60) ÷ 2 = 24.73°, which is 24°44'.
Where did Baeyer's theory go wrong?
The theory assumed every ring is flat. Rings of six or more carbons are in fact puckered, not flat, so a real cyclohexane can keep its angles close to tetrahedral and is not strained, even though the flat-ring calculation gives −5°16'. The question of how such rings could bend out of a plane was already being raised in 1890, in Hermann Sachse's paper on geometric isomerism in hexamethylene (cyclohexane) derivatives. Baeyer himself had noticed one awkward fact, that six-membered rings were far more common in known compounds than five-membered ones, and explained it away; the flat-ring assumption was the real issue.
What mistakes do students make with strain theory?
Halving twice or not at all. The deviation is half the difference, because two bonds at each carbon share the bend.
Mixing degrees and minutes. 109°28' is not 109.28°. Convert minutes by dividing by 60.
Treating large rings as strained. The negative values for six or more carbons come from the flat-ring assumption, not from real strain.
Forgetting the sign. Positive means bonds squeezed together; negative means pushed apart.
For more on the ring that started organic structure theory, see our post on August Kekulé and benzene. If angles and hybridisation still feel disconnected, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise the geometry behind them.
What can a student learn from how Baeyer worked?
The Nobel biography quotes Baeyer as saying: "I have never set up an experiment to see whether I was right, but to see how the materials behave". It also describes him as careful never to overestimate the value of a theory. His own strain theory shows why that matters: it was useful, it matched several facts, and it still rested on an assumption that turned out wrong. When you solve a problem, run the experiment or the calculation to find out what happens, not to confirm what you hoped.
Calculate Baeyer's deviation for a seven-membered ring, whose regular polygon has an internal angle of about 128.6°, and explain why a real cycloheptane is not as strained as the number suggests.
