Question 1

How many equivalents of H2 are required for complete alkyne reduction?

Accepted Answer

One C≡C bond requires two equivalents of H2 for complete reduction. The first equivalent gives the alkene stage, and the second reduces the remaining C=C bond to produce an alkane.

Question 2

Why is the product from Lindlar reduction cis?

Accepted Answer

For 2-butyne, a poisoned palladium catalyst stops the reduction after one equivalent of H2, producing cis-2-butene. The two methyl groups are therefore on the same side of the remaining C=C bond.

Question 3

Is a fully hydrogenated product cis or trans?

Accepted Answer

No. Cis and trans describe the relative positions of substituents across a C=C bond. Once the double bond has been completely reduced to a single bond, the product is an alkane and must not be labelled cis or trans.

Question 4

How does C-X reduction differ from alkene hydrogenation?

Accepted Answer

In this alkyl-halide example, X is replaced by H at the same sp3 carbon, so it is not an addition of two hydrogen atoms across a C=C or C≡C bond. Aryl and vinyl halides are different cases because their C-X bonds are attached to sp2 carbons.

Question 5

Which metals can catalyse complete hydrogenation?

Accepted Answer

Unpoisoned Pt, Pd or Ni can completely hydrogenate an alkyne with sufficient H2 to form an alkane. A poisoned Lindlar catalyst can stop the reduction at the cis-alkene stage instead.

Reductions of C=C, C≡C and C-X: Syn Addition and cis-Alkenes

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