In organic chemistry "reduction" is defined as the addition of hydrogen.
In my benzidine thread UC235 makes the following comment:
"That being said, I think it is extraordinarily unlikely that the hydrogen itself is the reducing agent but rather that the magnesium reduces
nitrobenzene through a single electron transfer mechanism..."
Is it correct to say that if the reduction is not by H on a catalytic surface like that of platinum, Raney nickel, etc, that it is by electron
transfer?
Is this true even though a metal that donates the electron is being dissolved by a strong acid and producing H2? Nicodem - 1-4-2016 at 14:03
In organic chemistry "reduction" is defined as the addition of hydrogen.
No, there is no special definition for organic chemistry. It is the same as for any chemistry. The species/atom that gains electrons is being reduced.
Nothing to do with hydrogen addition, except that H2 addition on multiple bonds is a reduction.
Quote:
Is it correct to say that if the reduction is not by H on a catalytic surface like that of platinum, Raney nickel, etc, that it is by electron
transfer?
In the case of hydrogenation, the electrons come from H2, or in the case of CTH from a hydrogen donor (HCOO-,
BH4-, H2PO2-, cyclohexene...).
Quote:
Is this true even though a metal that donates the electron is being dissolved by a strong acid and producing H2?
In the case of metal dissolving reductions it is another thing. Here the electrons come from the metal and there is no H2 involved at all.
The formation of H2 is just a parasitic reaction that has no relevance to the reduction of the substrate. In fact, if you see a lot of
hydrogen gas evolution, this generally means the reduction is not going well and the conditions might not be optimal. The reduction mechanism is a SET
and hydrogen atoms, when incorporated into the substrate, come from the solvent or added acids (water, alcohol, acetic acid, HCl,
NH4[sup]+...).
There are threads dedicated on the topic of dissolving metal reductions and review articles posted. UTFSE for more info.
magpie
Sulaiman - 1-4-2016 at 14:21
OIL RIG
Oxidation Is Loss, Reduction Is Gain (of an electron)
neither oxygen nor hydrogen need to be involved
the only benefit of being 'new' to chemistry is adoption of current terminology,
unfortunately it's not much use compared to experience
[Edited on 1-4-2016 by Sulaiman]Darkstar - 1-4-2016 at 17:49
Is it correct to say that if the reduction is not by H on a catalytic surface like that of platinum, Raney nickel, etc, that it is by electron
transfer?
Not if by "electron transfer" you mean one-electron reductions that involve single-electron transfer. Dissolving-metal reductions and catalytic
hydrogenations are not the only kinds of reductions in organic chemistry. There are also reductions that proceed through hydride transfer, such as
those where NaBH4 or LAH are the reducing agent. Organic substrates can also be reduced through disproportionation as well, for example in
free-radical reactions where organic radicals are "reduced" via hydrogen abstraction.
Quote:
Is this true even though a metal that donates the electron is being dissolved by a strong acid and producing H2?
As Nicodem has already pointed out, the production of H2 is an undesirable side-reaction that has nothing to do with the actual reduction
of nitrobenzene itself. What's likely going on is that the methanol is mediating outer-sphere electron transfer from magnesium to nitrobenzene by
partially solvating the metal's valence electrons, which are then picked up by the substrate. In other words, the "dissolving" phenomenon is primarily
the magnesium metal ionizing as its valence electrons are lost and quickly surrounded by methanol molecules.
Unlike ammonia, which gives extremely stable solvated electron solutions due to ammonia's strong resistance to reduction, methanol--like most
alcohols--makes for a rather poor solvent as it's quite susceptible to reduction itself. Thus the methanol is effectively acting as an additional
electron scavenger and competing with the nitrobenzene for reduction.
Edit: While I can certainly imagine other electron-transfer mechanisms that do not involve solvated electrons, it is known that polar solvents can
pretty readily trap electrons. And though I can't find much regarding magnesium in alcohols, it is now known that at least the alkali metals in
alcohols (namely sodium in ethanol) do, in fact, generate solvated electrons.
