On this page
Open the NCERT chapter on amines and the name Hofmann appears more than once. The Hofmann bromamide degradation, the isocyanides behind the carbylamine test, and much of what we know about primary, secondary and tertiary amines trace back to one German chemist who spent twenty years in London. August Wilhelm Hofmann also trained the students who launched the synthetic dye industry. This post covers his life, his work on aniline, a worked example of his bromamide reaction, common mistakes, and what made him such an effective teacher.
Who was August Wilhelm Hofmann?
An 1892 obituary in Science by his former student Morris Loeb records that Hofmann was born on 8 April 1818 in Giessen, where his father was an architect, and that news of his death in Berlin reached New York on 6 May 1892. A second obituary, by Charles Loring Jackson for the American Academy of Arts and Sciences, says he became professor in Berlin in 1865 and was "the father of the German Chemical Society, founded in 1867".
He did not start in chemistry. Lyon Playfair, who knew him at Giessen, recalled in the Chemical Society's Hofmann memorial lectures that Hofmann first studied philology and law before entering Justus von Liebig's laboratory. On Liebig's recommendation he became the first professor of the new Royal College of Chemistry in London, which opened in 1845.
What did Hofmann discover about aniline?
William Perkin, his most famous student, described this in the same 1896 lectures. Before Hofmann, one compound had been found four times and given four names: Unverdorben's "crystalline" from distilling indigo, Runge's "kyanol" from coal-tar oil, Fritsche's "aniline", and Zinin's "benzidam" from nitrobenzene. In his first published paper Hofmann showed these were all the same compound, and he adopted the name aniline. Perkin wrote that Hofmann sometimes called aniline his "first love".
In London he and his students went further. Perkin describes Hofmann's "researches regarding the molecular constitution of the volatile organic bases", in which he replaced the hydrogens of the NH2 group with alkyl groups using alkyl iodides and bromides, making ethylaniline, diethylaniline and methylaniline and eventually quaternary ammonium compounds. Jackson's obituary credits this work with revealing secondary and tertiary amines and substituted ammonium compounds as classes.
Loeb lists the isonitriles among the compounds studied by Hofmann and his pupils, and adds that azo dyes were discovered in his laboratory by his pupil Peter Griess. The story of his student Perkin and mauve is in our post on William Perkin.
Worked example: the Hofmann bromamide reaction
Late in his career Hofmann published "Ueber die Einwirkung des Broms in alkalischer Lösung auf Amide" (on the action of bromine in alkaline solution on amides, 1881). Jackson's obituary describes this as a new way of preparing primary amines. The NCERT Class 12 Amines unit explains that an alkyl or aryl group migrates from the carbonyl carbon to nitrogen, so the amine has one carbon fewer than the amide.
Take benzamide to aniline:
C6H5CONH2 + Br2 + 4 NaOH → C6H5NH2 + Na2CO3 + 2 NaBr + 2 H2O
Check the balance. Left: C 7, H 7 + 4 = 11, N 1, O 1 + 4 = 5, Na 4, Br 2. Right: C 6 + 1 = 7, H 7 + 4 = 11, N 1, O 3 + 2 = 5, Na 2 + 2 = 4, Br 2. Balanced.
Count carbons. Benzamide has 7 carbons; aniline has 6. The lost carbon leaves as carbonate.
Theoretical yield. Molar mass of benzamide, C7H7NO = 84 + 7 + 14 + 16 = 121 g/mol; aniline, C6H7N = 72 + 7 + 14 = 93 g/mol. From 12.1 g of benzamide (0.100 mol) you can get at most 0.100 × 93 = 9.3 g of aniline.
NCERT's own example runs this in reverse: to make propanamine (3 carbons), start from butanamide (4 carbons).
What mistakes do students make with Hofmann's reactions?
Keeping the carbon count. The bromamide product always has one carbon fewer than the amide.
Using it for secondary amines. It gives primary amines only.
Mixing up the carbylamine test. NCERT states that only primary amines, aliphatic or aromatic, give the foul-smelling isocyanide with chloroform and ethanolic KOH; secondary and tertiary amines do not.
Forgetting aniline's quirks. It is too reactive for controlled nitration and does not undergo Friedel-Crafts reactions, which is why chemists protect it first, as in our post on acetylating aniline.
For the wider family of amine reactions, see amines and diazonium salts. If you lose track of carbons in a conversion, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student learn from how Hofmann worked?
Hofmann's first result was not a new substance at all. It was the realisation that four known substances were one, which came from comparing carefully what others had already made. He then built a school around research: Jackson describes him lecturing, visiting each advanced student in the morning, and writing late into the night. Before memorising a reaction, check whether it is really the same as one you already know under a different name.
Write the amide needed to make ethanamine by the Hofmann bromamide reaction, then balance the equation and count every atom.
