Organic Chemistry/Introduction to reactions/Alkyne hydrogenation

From Wikibooks, open books for an open world
Jump to navigation Jump to search

Alkynes are hydrogenated in generally the same way as alkenes. However, standard catalysts like Pd/C will not allow hydrogenation to stop before the alkane stage. A "poisoned catalyst" will permit the reduction of a triple bond to a double bond by syn addition, but no further. A common poisoned catalyst is Lindlar's catalyst: Pd/CaCO3 treated with quinoline and lead acetate.

Hydro2.GIF


Anti addition of hydrogen atoms can be achieved with a dissolving metal reduction.

Anti.GIF



Dissolving Metal Reduction[edit]

The dissolving metal reaction takes place in a solution of sodium or lithium metal dissolved in liquid ammonia. An alkali metal is dissolved in liquid ammonia and forms a solution containing solvated electrons.

Solution.GIF

Once an alkyne is added to the solution, the electrons add to the antibonding π molecular orbital and produce a radical-anion intermediate. This intermediate has one unpaired electron and has a negative charge.

Stepone.GIF

The radical anion then reacts with a proton source (generally ethanol or tertiary-butyl alcohol which is added to the ammonia) to yield a vinyl radical. The radical then accepts another electron and forms a carbanion.

Step2&3.GIF

The alkene is formed when the vinyl carbanion is protonated. Protonation normally occurs from the side of the double bond that is least sterically hindered, so a trans-alkene is produced.

Step4.GIF