On this page
Friedel-Crafts alkylation and acylation are two of the first reactions you learn for benzene, and two of the most tested. They were found almost by accident, by a French mineralogist-chemist and an American visiting Paris, in a few weeks of work in 1877. This post tells their story from their own published notes, works through the chemistry with numbers, covers the classic mistakes, and ends with what their method of working can teach you.
Who were Charles Friedel and James Crafts?
Charles Friedel's tomb inscription, quoted in Crafts's Friedel Memorial Lecture to the Chemical Society of London in 1900, gives his dates: born in Strasbourg on 12 March 1832, died at Montauban on 20 April 1899. He worked with Adolphe Wurtz, curated the mineral collection of the School of Mines in Paris, became professor of mineralogy at the Sorbonne in 1876, and later succeeded Wurtz in organic chemistry.
James Mason Crafts was born in Boston on 8 March 1839 and died on 20 June 1917, according to his National Academy of Sciences memoir. He studied under Bunsen in Heidelberg and Wurtz in Paris, where he met Friedel, taught at Cornell and MIT, and returned to Paris from 1874 to 1891 to do research, mostly with Friedel at the School of Mines. He was later president of MIT, from 1898 for two years.
How was the reaction discovered?
Their first note to the Paris Academy, read on 11 June 1877, tells the story. They were studying what aluminium metal does to organic chlorides. The reaction started slowly, then sped up violently, giving off hydrogen chloride, and it seemed faster the more aluminium chloride had formed. That made them ask whether the real agent was the chloride, not the metal. It was: adding a little anhydrous aluminium chloride to amyl chloride set off brisk gas evolution even in the cold.
Next they mixed amyl chloride with excess benzene and added aluminium chloride in small portions. Hydrogen chloride came off steadily, and they isolated amylbenzene, boiling between 185 and 190 °C. Ethyl iodide gave ethylbenzene. Methyl chloride gave toluene, along with higher-boiling products they took to be more heavily methylated benzenes.
In his 1900 lecture Crafts summed it up: "The discovery sprang from an accidental observation of the action of metallic aluminium on chloride of amyl."
What else did their second note show?
The second note, a week later, widened the method. Benzyl chloride with benzene gave diphenylmethane. Chloroform gave triphenylmethane, and carbon tetrachloride gave the previously unknown tetraphenylmethane. Acid chlorides behaved the same way: benzoyl chloride with benzene gave benzophenone, and acetyl chloride gave acetophenone (which they called methylbenzoyl). That is acylation.
They also reported a negative result: chlorobenzene, heated with benzene and aluminium chloride to 200 °C, gave no hydrogen chloride. A chlorine attached directly to a benzene ring did not react.
Worked example: amylbenzene from amyl chloride
Use C = 12, H = 1, Cl = 35.5. The reaction Friedel and Crafts wrote is:
C6H6 + C5H11Cl → C6H5–C5H11 + HCl
Molar masses: benzene 78 g/mol; amyl chloride, C5H11Cl, 60 + 11 + 35.5 = 106.5 g/mol; amylbenzene, C11H16, 132 + 16 = 148 g/mol; HCl 36.5 g/mol.
Mass check: 78 + 106.5 = 184.5 g on the left; 148 + 36.5 = 184.5 g on the right.
From 10.65 g of amyl chloride: 10.65 ÷ 106.5 = 0.100 mol, so at most 0.100 × 148 = 14.8 g of amylbenzene, with benzene in excess as they used it.
Why excess benzene? Their methyl chloride run shows the answer. An alkyl group makes the ring more reactive, so the product can be alkylated again. Plenty of benzene makes it more likely that the electrophile meets fresh benzene.
NCERT's Class 11 Hydrocarbons unit covers both Friedel-Crafts alkylation and acylation with anhydrous AlCl3 as a Lewis acid. Its Class 12 Amines unit adds that aniline does not undergo the reaction, because it forms a salt with aluminium chloride.
What mistakes do students make with Friedel-Crafts?
Using a haloarene as the alkylating agent. Chlorobenzene does not react this way, as Friedel and Crafts found in 1877.
Ignoring polyalkylation. The first alkyl group activates the ring, so more substitution follows unless benzene is in excess.
Forgetting water kills the catalyst. The AlCl3 must be anhydrous.
Trying it on aniline. The amino group binds AlCl3 and the reaction fails, as NCERT notes.
For the full mechanism, read our post on electrophilic substitution in benzene. If you get stuck predicting products, 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 Friedel and Crafts worked?
They noticed that a reaction ran faster as one product built up, and asked whether that product was the real cause. They then tested the idea directly, and within weeks tried it on as many compounds as they could, including one that failed. Write down your negative results too: the chlorobenzene line is still one of the most useful facts about the reaction.
Write the products of toluene with acetyl chloride and AlCl3, and explain why acylation stops after one substitution while alkylation often does not.
