benzyme wrote:Hailstorm wrote:benzyme wrote:reduction is a hydrogenation reaction... that's not going to happen in alkaline conditions.
Yes, I made sure the reduction was done in strongly acidic conditions (+50ml 2M HCl, pH ~ 1 tested with a pH paper) before the final basification and pulling.
ahh ok. that makes sense
Technically its the other way around, hydrogenations are reduction reactions. Chemical reactions are described by the movement of electrons i.e. bonds, vs the movement or change in atoms or molecules.
Reductions as well as hydrogenations can certainly happen in alkaline conditions, acids provide a source of protons (H+), which is not the same thing as a source of hydrogen. H+. Acids are not a source of hydrogen but a source of protons. Protons are a more common form of hydrogen due to the low electronegativity of the atom (2.1), meaning it holds its electrons very weakly, and can stabilize a positive charge effectively due to it's small size and spherical orbital shape.
The only purpose of having protons present in the reduction, is so that after your substrate (harmaline) picks up the electron from the reducing agent (zinc) it will exist transiently as a highly basic charged carbanion, it will pick up a proton from the solvent to be neutralized, as long as it is capable of giving one. Any polar protic solvent is sufficient, methanol, ethanol, acetic acid, water, even alkaline water.
H+ is not a reducing agent but actually an oxidant. It possesses no electrons and is capable of gaining some, carrying a charge makes it favorable to do so. The result is a bond, usually form of a dative bond, accepting a pair of electrons from a negatively charged species into it's empty s orbital. This however is neither oxidation or reduction, since the electron count remains the same, it just participates in energy lowering interaction (hydrogen bonding).
When H+ gets reduced by zinc you generate H2 gas, and that is all the bubbling you will see during the reaction. This is actually a bad thing, because you want the zinc to react with your harmaline, not with protons. This is why you need a very large excess of zinc metal to actually complete the reaction. If you wanted to optimize this, you would change the conditions so that the redox potential of harmaline is as low as possible, and the redox potential of H+ is as high as possible. This can be altered by changing the pH or the solvent.
Dissolving metal reductions as this one are by the same principles in how batteries work. The electrolyte in batteries can be acidic, like your lead-acid car battery, or they can be basic, like your good ol' alkaline AA batteries.
Redox potential is directly related to electronegativity, and the electronic orbital structure of the elements.
http://hyperphysics.phy-...se/Tables/electpot.html
The table is measured in relative difference in the reduction half reaction potentials
The reduction of H+ to generate molecular hydrogen is actually used as the universal reference, standard hydrogen electrode (SHE), and so is set as the reference point (0.00 V) the absolute is something like 4.44 V ircc
2H+(aq) + 2e- -> H2(g) 0.00 V
For zinc:
Zn2+(aq) + 2e- -> Zn(s) -0.76 V
These tables are always written in their reduction form (Gaining electrons), but since we use zinc as the reducing agent, we want to flip the equation so we see the redox potential when zinc is oxidized.
Zn(s) -> Zn2+(aq) + 2e- +0.76 V
Now if we add the two half reactions together, the reduction half from H+ and the oxidation half from zinc, it should be balanced. The electrons cancel out, and the charges all add up, we get:
2H+(aq) + 2e- -> H2(g) 0.00 V
Zn(s) -> Zn2+(aq) + 2e- +0.76 V
Zn(s) + 2H+(aq) -> Zn2+(aq) + H2(g) +0.76 V
So zinc metal reacts with protons to generate H2 gas and +0.76 volts, and so a spontaneous process. This is essentially a battery. The protons/acid (H+) are acting as the electron acceptor here, but any other ion, molecule, could act as well. The protons serve other purposes as well, since zinc can react with oxygen and water in the air to form Zn(OH)2 as a passivating layer. The acid with appropriate counter ion can remove the passivating layer to form soluble zinc salts (ZnCl2, ZnSO4, Zn(OAc)2), so that fresh zinc metal is exposed and continue reacting.
A good example is to use another metal that possesses a lower redox potential than zinc, like copper or silver, which gives an additive voltage output. this is a zinc-copper battery you can make for kids, and also this is what is commonly done in gold refining to precipitate or dissolve specific metals. These redox reactions are leaving out the counter-ions, which are important for solubility and side reactions.
half rxns
red: Cu2+(aq) + 2e- -> Cu(s) +0.34 V
ox: Zn(s) -> Zn2+(aq) + 2e- +0.76 V
redox: Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s) +1.10
Enough tangents,
Now to reiterate, and get back to the original point.
You can do hydrogenations in alkaline conditions, the half rxn reducion for water is :
2H2O(l) + 2e- -> H2(g) + 2OH-(aq) -0.83 V
This is electrolysis of water, the addition of 2 electrons generates H2(g) and hydroxide ions. it costs more energy than the reduction of H+. Which makes sense, H+ is more electrophilic. If we disregard the problem of passivation, combing the half reactions we get:
red: 2H2O(l) + 2e- -> H2(g) + 2OH-(aq) -0.83 V
ox: Zn(s) -> Zn2+(aq) + 2e- +0.76 V
redox: 2H2O(l) + Zn(s) -> H2(g) + 2OH-(aq) Zn2+(aq) -0.07 V
So this shows that the reaction actually requires a voltage input, neutral or alkaline conditions significantly lower the redox potential of H+, because it is more tightly bound to water molecules. So if passivation and Zn(OH)2 weren't problems of themselves, then raising the pH might actually be more efficient.
But again, no data here on the redox potential of harmalas, but there are some papers out there. We do know that the redox potential of zinc (0.76 V) is high enough to reduce harmaline, but not harmine.
Aluminum is a pretty damn powerful reducing agent, double the potential of zinc at 1.66 V
Al(s) -> Al3+(aq) + 3e- +1.66 V
It however suffers the same problems as zinc, it is even more susceptible to passivization, and the hydroxide salts (Al(OH)3) are god awful. It will react with protons to generate H2 much much faster than it will react with harmalas. Mercury amalgam is toxic and shuold be avoided, but historically the most effective method to use aluminum in organic reductions/hydrogenations.
For example, one of the oldest old school reductions is using sodium metal dissolved in some alcohol.
Very powerful electron donor, with a standard potential of 2.71V. It was also the first material used to reduce harmalas, and it is capable of reducing both harmaline and harmine to THH.
Na(s)-> Na(s) + e- +2.71 V
In this example, acidic conditions are not necessary at all, and the reaction conditions are extremely alkaline. It however suffers the same problem, it will preferentially react protons before it reacts with your substrate. This can be controlled by keeping a lower temperature, and choosing a suitable protic solvent. Especially with pure sodium, aqueous conditions are to be avoided entirely. Sodium will abstract a proton from water to make hydroxide, or from alcohols to make alkoxide.
Na(s) + EtOH -> Na+ + EtO- + H2
To reiterate again, the idea is to strike a balance between having enough protons that they can be incorporated into the reduced THH (hydrogenation) but not so much that the reducing agent just reacts with the protons alone to generate H2. Excess almost always needs to be used. The side reactions can be inhibited by using a bulky alcohol, like isopropanol, or tert amyl alchol, this steric bulk makes it challenging to abstract the proton. And the other option is by creating a mercury sodium amalgam. You can make a 1-10% w/w alloy of sodium and mercury, which reduces the surface area and the reactivity of the sodium. But as you can imagine, it uses a hell of a lot of mercury. This is 1900's chemistry, better methods exist. Dissolving metals is old school.
Aside from dissolving metals as electron source, Hydrogen as a reducing agent requires a source of molecular hydrogen, hydride, or a hydrogen radical. The hydrogen atoms must be bonded to elements it possess electrons from molecules in which it is bound to having a lower electronegativity than hydrogen, i.e. aluminum (1.5) or boron (2.0). In these molecules electron density is concentrated around H in the Al-H bonds. However the mechanism in how these reducing agents is different, and related to the structure of the group 12 elements, i.e. the empty p-orbital. AlH3 for example, possesses a Lewis acid capability (just like a H+), which can activate the substrate to facilitate hydrogen transfer. These are infact, much better agents for hydrogenation or reduction. They are highly selective to certain functional groups, double bonds, ketones, etc. Excess does not need to be used, and most of the time you can use just 1 molar equivalent. Protic solvents are not necessary, but can be used without problem in many cases.
It all depends on redox potential. For instance sodium metal is a powerful reducing agent, but sodium hydride is not. So even though you have an extremely low electronegativity of sodium (0.9) bound to hydrogen, providing an extremely polarize bond, ionic interaction with electrons localized on hydrogen.
NaH -> Na+ + H-
you end up with powerful hydride source. However, since sodium has already been ionized, it actually has very little redox potential.
So you've got your hydride, but you have no driving force to move those electrons into your desired molecule. In addition, hydride is also poorly nucleophilic, so hydride tends to act as a selective and strong base. Which is useful in its own right. It will violently react with water or other acids (Proton source H+) to form hydrogen gas and a sodium salt. It has a pKa of about 35, so you can deprotonate things like amides, pyrroles or indoles. So you are doing a reduction, reducing protons to make H2. But not really useful for hydrogenation or other types of reduction on small organic molecules.
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