The above is a copyrighted picture reproduced here by permission. © 2009 by Jochen Wilke.
In Silber auf eisernem Dreifuss in flammendem Kohlenbecken ein
natürlicher Siedekolben,
in den vier Schildesecken begleitet von je
einer goldenbesamten roten Rose.
Auf dem Helm mit rot-silbernen Decken,
3 golden-besamte rote Rosen an grünen Blätterstengeln.
Hofmann coined the term aromatic to denote a class of organic compounds with high carbon content, which are now known to include a phenyl ring (like benzene). The word aromatic is indeed first used (with a chemical meaning compatible with the modern definition) in Hofmann's report (1855) on his own extension to an independent work (c. 1845) of Jean-François Persoz (1805-1868) on the oxidation of cuminic acid C10H12O2 or (CH3)2CHC6H4COOH
This marked the discovery of what Hofmann himself called insolinic acid C9H8O4 or CH3C6H3(COOH)2 which is now known to consist of one methyl group and two carboxyl groups attached to a phenyl ring. The only other "bibasic" (i.e., diprotic or dicarboxylic) aromatic acids known to Hofmann at the time were the phthalic acid discovered in 1836 by Auguste Laurent (1807-1853) and the isomer known as terephthalic acid discovered by Caillot, in 1847. Caillot found it among the products of the action of dilute nitric acid on oil of turpentine whose French name (térébenthine) provided him with the prefix he used to name the new compound. Both isomers consist of two carboxyl groups attached to a phenyl ring, respectively in the ortho and para configurations, and they share the formula C8H6O4 or C6H4(COOH)2. The third isomeric configuration (meta) dubbed isophthalic acid wasn't known to Hofmann.
As shown at right, Hofmann attempted to match those diacids to the related monoacids he knew. He called all those compounds aromatic, without stressing that he was thus coining a new name... (Hofmann's formulas would actually represent the number of atoms in two molecules if the symbol H stood for two atoms of hydrogen; modern formulas are thus obtained by halving the indices of C and O.) His simple arithmetic didn't yield good predictions for the missing compounds (marked "?"). True understanding would have to wait until the discovery of the ring structure of benzene by Kekulé, ten years later (1865).
Benzoic acid
C6H5COOH
was discovered in 1556 by Nostradamus (1503-1566)
as he described the dry distillation
of styrax
or gum benzoin
(benjoin in French).
Hofmann is commonly credited for being the first chemistry professor to use (primitive) molecular models in public lectures. The color coding he introduced for that purpose became the basis for the scheme used in most modern sets, which was tentatively standardized by the Institute of Physics in 1947: carbon=black, hydrogen=white, oxygen=red, halogens=greenish, nitrogen=blue, sulfur=yellow, phosphorus=purple...
Institute of Physics, "The standardization of crystal structure models" Journal of Scientific Instruments, Vol. 24, 9 pp. 249-250 (1947).
The British Museum has several molecular models that had been used by earlier chemists, including John Dalton (1766-1844). The use of polyhedral models predates the theoretical framework which made them legitimate for chemistry: Johannes Kepler (1571-1630) used them to speculate about the microscopic structure of crystals as early as 1611...
Wikipedia | Hofmann elimination | Hofmann rearrangement | Hofmann-Löffler reaction | Bibliography on Molecular Models (NBS, 1960)
