12.4 Reduction of Alkynes

12.4 Reduction of Alkynes

  • The first two steps are used to install one ethyl group, and the next two steps are used to install another ethyl group.
  • This dianion does not form because it is too high in energy.
    • If we treat one mole of acetylene with two moles of sodium amide, we will find that one mole of amide remains in the mixture.
    • When ethyl iodide is introduced into the reaction flask, the excess amide ions would react with it.
    • If we want to install an alkyl group on each side of acetylene, we need to separate them.
  • Hydrogenation is also observed.
  • The starting alkyne is more reactive toward hydrogenation than the alkene.
    • A partially deactivated catalyst is called a poisoned catalyst if we want the alkene as our product.
    • Lindlar's catalyst is a Pd catalyst prepared with CaCO and traces of PbO.
  • The reagents for this reaction are Na and NH.
  • The steps of the accepted mechanism for dissolving metal reductions can be seen.
  • An electron is transferred to the alkyne.
    • The second step of the mechanism is a protons transfer.
    • The two steps install a hydrogen atom.
    • Before we move on to the last two steps of the mechanism, we should look at some important features of the first two steps.
    • The first step uses curved arrows.
    • Each of the arrows is called a fishhook arrow because it resembles a fishhook, and it represents the motion of only one electron.
    • A fishhook arrow is used to show the motion of a single electron in the first step.
    • The fishhook arrows in step 1 represent one electron.
    • Double-barbed arrows are used in the second step of the mechanism.
    • The motion of two electrons is represented by the curved arrows.
    • The mechanism uses both types of curved arrows.
  • There is a feature of the first step that we should pay attention to.
  • This intermediate has a negative charge and an unpaired electron, which makes it a radical anion.
    • In order to minimize their repulsion, the unpaired electron and lone pair are positioned as far apart as possible.
  • The last two steps install a hydrogen atom.
    • There are four steps in the mechanism.
    • The first two steps install one hydrogen atom, and the last two steps install another hydrogen atom.