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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.
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