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"Add H₂, Ni catalyst" appears so often in organic chemistry that it is easy to forget someone had to discover it. That someone was Paul Sabatier, working in Toulouse. This post tells how he found it, explains what the catalyst is doing, works one calculation, and ends with what you can take from his way of working.
Who was Paul Sabatier?
According to his Nobel Prize facts page, Sabatier was born on 5 November 1854 in Carcassonne and died on 14 August 1941 in Toulouse. He shared the 1912 Nobel Prize in Chemistry "for his method of hydrogenating organic compounds in the presence of finely disintegrated metals". His Nobel biography notes that the other half went to Victor Grignard for the Grignard reagent.
The biography also says he was "ever faithful to Toulouse". He became Professor of Chemistry there in 1884, held the post until he retired in 1930, and turned down offers elsewhere, including the succession to Henri Moissan at the Sorbonne in 1908.
How did Sabatier discover catalytic hydrogenation?
Sabatier tells the story in his Nobel lecture. Moissan had seen acetylene decompose with a glow on freshly reduced nickel and had explained it as a physical effect. Sabatier suspected a real chemical combination between the metal surface and the gas. Once he was sure Moissan was not continuing the work, he and his collaborator Senderens tried ethylene.
Ethylene passed over nickel at about 300 °C glowed and dropped carbon, but the gas coming out was mainly ethane, not hydrogen. That could only come from hydrogenation. So they passed a mixture of ethylene and hydrogen over reduced nickel, and the ethylene became ethane, with "the same metal" usable "indefinitely" (June 1897). Nickel was acting as a catalyst.
At the end of 1900 they turned benzene into cyclohexane over nickel at about 180 °C, a reaction other hydrogenating agents had failed at. In 1901 they stated the general method: pass the vapour of the substance with excess hydrogen over freshly reduced nickel, generally between 150 and 200 °C.
What does the nickel actually do?
Your NCERT Class 11 hydrocarbons chapter explains it as Sabatier argued: dihydrogen adds to alkenes and alkynes in the presence of finely divided platinum, palladium or nickel, because "these metals adsorb dihydrogen gas on their surfaces and activate the hydrogen – hydrogen bond." It adds that platinum and palladium work at room temperature, while nickel needs higher temperature and pressure, and that benzene gives cyclohexane under vigorous conditions with a nickel catalyst.
Sabatier's own picture, from his lecture, was that hydrogen forms an unstable compound with the metal surface, which then hands the hydrogen to the organic molecule and regenerates the metal. For more on how catalysts lower the barrier to reaction, see our post on collision theory and catalysts.
Worked example: hydrogen needed for benzene
How much hydrogen converts 7.81 g of benzene to cyclohexane?
Equation: C₆H₆ + 3H₂ → C₆H₁₂.
Molar mass of benzene: 6 × 12.01 + 6 × 1.008 = 78.11 g/mol. Moles: 7.81 / 78.11 = 0.100 mol.
Hydrogen needed: 3 × 0.100 = 0.300 mol.
Mass: 0.300 × 2.016 = 0.605 g. Volume at STP, using the 22.7 L/mol that your NCERT Class 11 chemistry book uses: 0.300 × 22.7 = 6.81 L.
An alkene needs only one H₂ per C=C bond. Ethene takes 1 mol of H₂ per mole; benzene's ring takes 3.
Common mistakes students make with hydrogenation
"The catalyst is used up." Sabatier's point was that the same nickel kept working. A catalyst appears on the arrow, not in the stoichiometry.
Ignoring poisons. In his lecture Sabatier warned that traces of sulphur, bromine or iodine poison nickel, and that benzene with any thiophene left in it would not hydrogenate.
Assuming all double bonds are equally easy. He found alkenes easy to hydrogenate and the benzene ring much harder. NCERT says the same thing with its "vigorous conditions".
Practise the reactions around this one in our guide to hydrocarbon reactions. If your hydrogen counts keep coming out wrong, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
Why did the method matter so much?
The lecture records that liquid oleic acid could be hydrogenated to solid stearic acid, and that the same reaction applied to oils turned them into solid fats, already "the basis of a great industry in Britain and Germany" by 1912. The Nobel biography adds that his finely divided metal catalysts formed the bases of the margarine, oil hydrogenation and synthetic methanol industries.
What can a student learn from Sabatier's way of working?
Sabatier noticed that an explanation did not fit, then looked at the products rather than assuming. The gas leaving his tube was ethane, not hydrogen, and that small fact opened a whole field. When an answer surprises you, check what actually came out of the problem before you trust the theory you started with.
Today, write the hydrogenation equations for ethene, ethyne and benzene, and count the H₂ each one needs.
