Emil Erlenmeyer

Who Drew the Second Line · Chemist · 1825–1909

Who in 1862 first wrote a double line between two carbon atoms, to account for the fact that ethylene takes up two atoms of bromine where ethane takes up none.

Who was Erlenmeyer?

Erlenmeyer trained as an apothecary before turning to chemistry, studied under Liebig at Giessen, and came to academic life late and by an indirect route, eventually holding the chair at Munich. His contribution belongs to the years immediately after Kekulé's and Couper's proposals of 1857–58, and it is a contribution to notation, which is a way of saying that it is a contribution to how chemists think. Kekulé had established that carbon is tetravalent and that carbon atoms link to one another; Erlenmeyer asked what follows for a compound like ethylene, which has two carbons and only four hydrogens. If each carbon must account for four units of affinity and only three are used, one is left over on each — and they face one another. In 1862 he wrote them as a second bond between the two carbons, and in the same spirit gave acetylene a third. The objection made at the time was that he had invented lines to balance a book. His answer was that the notation predicts: a compound written with a double line ought to be able to take up two more atoms and become a singly bonded one, and a compound written with single lines ought to take up nothing at all. Ethylene and ethane do exactly that, which is why bromine goes into one and not the other, and the prediction is what carried the notation. Erlenmeyer also worked on naphthalene, established the constitution of several natural products, formulated the rule that bears his name about hydroxyl groups on doubly bonded carbons, and edited a major journal for years. He is, to his own probable irritation, best known outside chemistry for the flat-bottomed conical flask he designed in 1860 because a round-bottomed one will not stand on a bench.

Major achievements

• First wrote the carbon-to-carbon double bond, for ethylene, in 1862
• Extended the same reasoning to the triple bond of acetylene
• Argued that additive capacity is the evidence for the extra bonds, which is why the notation was accepted
• Established the constitution of naphthalene and of several natural products
• Formulated the Erlenmeyer rule on hydroxyl groups attached to doubly bonded carbon

Life in brief

• 1825 — Born at Wehen: Son of a pastor; trained as an apothecary.
• 1860 — The conical flask: Flat-bottomed, so it will stand still.
• 1862 — The double bond: A second line drawn between the carbons of ethylene.
• 1866 — Professor at Munich: Where he stayed for two decades.
• 1880 — The Erlenmeyer rule: On hydroxyl attached to a doubly bonded carbon.
• 1909 — Dies at Aschaffenburg: The notation long since universal.

Explore the life of Erlenmeyer — five chapters

1. An Indirect Route — Apothecary, medical student, chemist, late
2. One Affinity Left Over — The problem of ethylene
3. Let the Line Predict — Addition, and how a notation earns its keep
4. The Rule That Bears His Name — Naphthalene, natural products, and a journal
5. Known for a Flask — What a notation actually does

Read the five-chapter life of Emil Erlenmeyer →

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