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Organic chemistry students draw curved arrows all day and rarely ask where they came from. One early source is a 1922 paper by Robert Robinson, the chemist who five years earlier had made a complicated alkaloid in a single pot. This post covers both pieces of work, checks the chemistry, and looks at what made his approach different.
Who was Robert Robinson?
The Nobel Prize biography records that Robinson was born at Rufford, near Chesterfield, on 13 September 1886. He graduated from Manchester University in 1906 and in 1912 became the first Professor of Pure and Applied Organic Chemistry at the University of Sydney. After posts at Liverpool, St Andrews, Manchester and London, he held the Waynflete chair of chemistry at Oxford from 1930 to 1955. He was President of the Royal Society from 1945 to 1950, a keen mountaineer and President of the British Chess Federation. His wife, Gertrude Maud Walsh, a fellow student at Manchester, worked with him on plant pigments called anthocyanins. He died on 8 February 1975.
He received the 1947 Nobel Prize in Chemistry "for his investigations on plant products of biological importance, especially the alkaloids".
How did Robinson make tropinone in one pot?
Tropinone is a two-ring molecule related to atropine and cocaine. Richard Willstätter had already made it, but by a long route. In A Synthesis of Tropinone (1917), Robinson described Willstätter's method as "of too complicated a character" to become an economical process. (A scan of the paper is in the Internet Archive's copy of the Journal of the Chemical Society, Transactions, volume 111.)
Robinson then did something students can copy. He looked at the structure and imagined breaking it apart. "By imaginary hydrolysis at the points indicated by the dotted lines," he wrote, "the substance may be resolved into succindialdehyde, methylamine, and acetone." So he tried mixing those three in water, and tropinone formed in small yield. Using a salt of acetonedicarboxylic acid in place of acetone worked better. The first product was a dicarboxylic acid, which lost two molecules of carbon dioxide when the solution was acidified and heated. In one experiment the yield of a derivative used to detect tropinone was 42 per cent of the theoretical amount.
The Nobel presentation speech described this as a case where "three rather simple molecules spontaneously unite into a complicated system", and linked it to Robinson's idea about how plants themselves build alkaloids.
Worked example: balancing the one-pot reaction
Take the version with acetone. Formulas: succindialdehyde C₄H₆O₂, methylamine CH₅N, acetone C₃H₆O, tropinone C₈H₁₃NO.
Add the reactants: C 4 + 1 + 3 = 8; H 6 + 5 + 6 = 17; N 1; O 2 + 1 = 3.
Tropinone has C 8, H 13, N 1, O 1. The difference is H 4 and O 2.
That is exactly two molecules of water: C₄H₆O₂ + CH₅N + C₃H₆O → C₈H₁₃NO + 2H₂O.
Check masses (C 12.01, H 1.008, N 14.01, O 16.00): reactants 86.09 + 31.06 + 58.08 = 175.23 g/mol; products 139.20 + 2 × 18.02 = 175.23 g/mol.
Water leaving at each new bond tells you these are condensation steps. A common mistake is to balance carbon and forget that nitrogen enters only from methylamine; count every element.
Where do curly arrows come from?
In 1922 Robinson and William Kermack published An explanation of the property of induced polarity of atoms, trying to explain organic reactions in terms of shared electrons. On page 433 of the paper, in the scanned 1922 volume, a chain of three conjugated double bonds is drawn with small curved arrows under it, showing pairs of electrons shifting along the chain to leave a positive charge at one end and a negative charge at the other. The same page discusses butadiene.
Martin Saltzman's history in the Bulletin for the History of Chemistry says Robinson introduced the idea of mobile electron pairs in conjugated systems and published a general theory of reactivity in 1926, and that his rival Christopher Ingold came round to these ideas. That rivalry is covered in our post on Christopher Ingold and reaction mechanisms.
Today the rules are fixed: an arrow starts at a lone pair or a bond and ends where the electrons go. The most common mistake is drawing it backwards, from the electrophile to the nucleophile. Our guide to learning organic mechanisms instead of lists has practice.
What can a student take from Robinson's way of working?
Look for symmetry. He noticed the shape of tropinone suggested a simple origin.
Test the simple idea first. Mixing three cheap chemicals in water is a quick experiment.
Invent notation when you need it. The arrows were a tool for thinking, not decoration.
If you find arrow-pushing hard to follow on a page, Fermi's AI tutor writes on a board and draws the figure as it explains, and checks your understanding as it goes.
Try it now: split tropinone into its three pieces on paper, then draw arrows for the first bond that forms.
