Palladium on barium sulphate (Pd/BaSO 4 ) is also known as:

Correct — C. Palladium supported on barium sulphate is the Rosenmund catalyst, which is option (c).

It is used in the Rosenmund reduction, which converts an acyl chloride into an aldehyde:
RCOCl + H₂ → RCHO + HCl, over Pd/BaSO₄

The barium sulphate is the whole point. It is a deliberately poor support, so the palladium is active enough to reduce the acyl chloride but not active enough to carry the aldehyde on to the primary alcohol. The catalyst is often poisoned further with sulphur or quinoline for the same reason.

Why the others are wrong

• (a)
  an aldol reaction catalyst — The aldol reaction is a base-catalysed (or acid-catalysed) condensation of two carbonyl compounds through an enolate. It uses dilute alkali, not a metal catalyst.
• (b)
  a Cannizzaro reaction catalyst — The Cannizzaro reaction needs concentrated alkali acting on an aldehyde with no alpha-hydrogen, such as benzaldehyde. It is a disproportionation, not a hydrogenation, and no palladium is involved.
• (d)
  a Hillman reaction catalyst — The reaction the paper calls Hillman is the Baylis-Hillman reaction, driven by a tertiary amine such as DABCO adding to an activated alkene. Palladium on barium sulphate plays no part in it.

A catalyst can be tuned down on purpose, and two named reductions exist only because chemists learned to do it.

Rosenmund: Pd on BaSO₄, poisoned with sulphur or quinoline, stops the reduction of an acyl chloride at the aldehyde stage.

Lindlar: Pd on CaCO₃, poisoned with lead acetate and quinoline, stops the reduction of an alkyne at the cis-alkene stage.

Both are the same idea — a weak support plus a poison — applied to different substrates. What to memorise is the pair: the reagent, and the product the reaction stops at.

Key facts

• Palladium on barium sulphate is the Rosenmund catalyst.
• The Rosenmund reduction converts an acyl chloride (RCOCl) to an aldehyde (RCHO).
• The weak BaSO4 support and a sulphur or quinoline poison stop the reduction short of the alcohol.
• Lindlar's catalyst is the parallel case: Pd on CaCO3, poisoned, giving a cis-alkene from an alkyne.

Common traps

• Swapping Rosenmund (Pd on BaSO4) with Lindlar (Pd on CaCO3), since both are poisoned palladium.
• Assuming a named reaction with a metal in it must be a coupling reaction.

No directly related past PYQ was found.