Hydrogenation of alkenes can be carried out in the presence of

Correct — D, nickel. Adding hydrogen across a carbon-carbon double bond needs a metal surface that can hold both the alkene and the hydrogen molecule long enough for them to combine, and finely divided nickel does that at modest temperature and pressure. The reaction is the industrial basis of vanaspati — liquid vegetable oils rich in unsaturated chains are hydrogenated over nickel until they set to a semi-solid fat. Palladium and platinum catalyse the same reaction and are used where selectivity matters; nickel is the cheap workhorse and the one the examiner wants.

• (a)copper — Heated copper is the classic catalyst for the reverse operation — removing hydrogen from an alcohol to give an aldehyde. It does not add hydrogen across a double bond.
• (b)zinc — Zinc is used with dilute acid to generate hydrogen gas, not to catalyse its addition. Producing the reagent and catalysing the reaction are different jobs.
• (c)aluminium — Aluminium has no catalytic role in this reaction. Its oxide serves as a catalyst support in other processes, which is a different function altogether.

Hydrogenation of an alkene is an addition reaction — H₂ adds across the double bond and the unsaturated compound becomes saturated. It will not proceed usefully on its own, so a transition-metal surface is supplied to dissociate the hydrogen and lower the activation energy.

The reaction is where an organic chapter meets a food-technology fact, and questions come from either side. Full hydrogenation gives saturated fat; partial hydrogenation is what generates trans fats, which is why the Food Safety and Standards Authority of India has been tightening limits on industrial trans fat in vanaspati and bakery fats. Nickel, palladium and platinum are the three catalysts worth remembering, with Raney nickel the finely divided form used in the laboratory.

• CH₂=CH₂ + H₂ over nickel at about 150 °C gives CH₃-CH₃; the general reaction converts an alkene to the corresponding alkane.
• Vegetable oils contain unsaturated chains and are liquid; hydrogenating them raises the melting point and gives the semi-solid vanaspati.
• Paul Sabatier's work on metal-catalysed hydrogenation won the Nobel Prize in Chemistry in 1912.
• Palladium and platinum catalyse the same addition; a poisoned palladium catalyst is used when only partial hydrogenation is wanted.

• Choosing zinc because it is associated with hydrogen gas in the laboratory.
• Confusing hydrogenation with dehydrogenation, where heated copper is the standard catalyst.
• Assuming the catalyst is consumed and must be supplied in stoichiometric amount.

Asked as the catalyst for hydrogenation, as the reaction type when vegetable oil becomes vanaspati, or as the product formed when hydrogen adds to a named alkene.

• practice — not a real PYQ

  The conversion of vegetable oil into vanaspati is an example of

  • (a)substitution
  • (b)addition
  • (c)decomposition
  • (d)neutralisation

  Answer(b) addition — hydrogen adds across the carbon-carbon double bonds of the unsaturated chains, saturating them.

• practice — not a real PYQ

  Ethene reacting with hydrogen in the presence of nickel gives

  • (a)ethyne
  • (b)ethanol
  • (c)ethane
  • (d)ethanal

  Answer(c) ethane — the double bond is saturated, turning C₂H₄ into C₂H₆.