Lindlar and Dissolving-Metal Reduction

A carbon-carbon triple bond is a synthetic chameleon: from a single internal alkyne you can dial up a cis alkene, a trans alkene, or a fully saturated alkane just by changing the reducing conditions. This is one of the most useful stereochemical switches in the alkyne toolkit, and it works because two of the three reagents deliver hydrogen through completely different mechanisms.

The headline reaction: 2-butyne plus hydrogen over Lindlar catalyst stops at (Z)-2-butene, the cis alkene.

1. The same alkyne can give a cis alkene, a trans alkene, or the alkane

Catalytic hydrogenation of an alkyne is a two-stage process. The triple bond is reduced first to a double bond and then, if nothing stops it, on to the single bond. The art is in controlling where the reaction halts and which face the hydrogens arrive on. Three standard reagent systems each pin down a different answer.

• H2 + Lindlar catalyst stops at the alkene and gives the cis (Z) product.
• Na (or Li) in liquid NH3 stops at the alkene and gives the trans (E) product.
• H2 + ordinary Pd/C does not stop; it runs all the way to the alkane.

Because the first two land on opposite geometries, the pair is the workhorse method for setting alkene geometry from an alkyne. If you need a review of how we name and rank those geometries, see E/Z notation and alkene stability.

Three products from one starting alkyne (2-butyne), selected purely by reagent choice.

2. Lindlar catalyst is a poisoned catalyst that stops at the alkene

Lindlar catalyst is palladium deposited on calcium carbonate and then deliberately "poisoned" with lead (as lead acetate) and quinoline. The poison blunts the metal surface just enough that it will bind and reduce the more reactive triple bond but will no longer grip and reduce the resulting double bond. In practice the reaction stalls cleanly at the alkene stage instead of overshooting to the alkane.

2-Pentyne plus H2 over Lindlar gives (Z)-2-pentene; the reaction stops at the alkene.

Think of Lindlar as a throttled version of ordinary palladium. Plain Pd/C would keep going, but the poisoned surface trades away that extra activity in exchange for selectivity, and selectivity is exactly what we want here.

3. Lindlar delivers both hydrogens to the same face, giving the cis alkene

The geometry comes from the mechanism. On the metal surface the alkyne lies flat and binds to the palladium; the two hydrogen atoms are then transferred from that same surface to the same side of the pi system. This is a syn addition: both new C-H bonds form on one face, so the two substituents that were on the carbons are pushed to the same side of the new double bond. That is the definition of a cis (Z) alkene.

Syn delivery on the catalyst surface converts 3-hexyne into (Z)-3-hexene.

Syn addition on a metal surface is the same idea behind alkene hydrogenation more broadly; here it is simply arrested at the halfway point. For the broader picture of how reagents add across the triple bond, see addition reactions of alkynes.

4. Dissolving-metal reduction runs through a radical anion and gives the trans alkene

Sodium or lithium metal dissolved in liquid ammonia takes a completely different route. There is no metal surface and no syn constraint. Instead the alkyne is reduced one electron at a time in solution.

Na in liquid ammonia reduces 2-butyne to (E)-2-butene, the trans alkene.

The sequence is: the metal donates an electron to the alkyne to make a radical anion; ammonia protonates it to give a vinyl radical; a second electron converts that into a vinyl anion; and a final proton from ammonia delivers the alkene. The stereochemistry is decided at the vinyl-anion stage. That carbanion can equilibrate between two geometries, and it settles into the arrangement where its two bulky substituents sit far apart, on opposite sides. Protonation of that more stable, trans-configured anion locks in the (E) alkene.

5. Ordinary Pd/C is not poisoned and reduces all the way to the alkane

If you use hydrogen with an unpoisoned catalyst such as palladium on carbon, nothing stops the second stage. The alkyne is reduced to the alkene and then the alkene is reduced again to the fully saturated alkane. Stereochemistry no longer matters because the product has no double bond left.

With ordinary Pd/C the reduction does not halt; 3-hexyne is taken to hexane.

This is the "off" position of the switch: choose Pd/C when you want to erase the unsaturation entirely, and reserve Lindlar or Na/NH3 for when you want to keep a double bond of a defined geometry.

6. Summary

One internal alkyne, three destinations. H2 with Lindlar catalyst is a lead/quinoline-poisoned palladium that stops at the alkene and, because it delivers both hydrogens to the same face (syn addition on the surface), gives the cis (Z) alkene. Na or Li in liquid ammonia reduces through a radical-anion / vinyl-radical / vinyl-anion sequence in solution; the vinyl anion adopts its more stable trans geometry before the final protonation, so the product is the trans (E) alkene. H2 with ordinary Pd/C is not poisoned and reduces the alkyne all the way to the alkane. Remembering "poisoned metal surface = syn = cis" and "dissolving metal = thermodynamic anion = trans" lets you write down the right product every time.