Author Topic: Question about Methyl Mans writeup  (Read 7313 times)

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Flip

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Also
« Reply #20 on: July 13, 2002, 06:58:00 AM »
Can't really top the previous advice but I did notice:

Wonder if theres a way to speed up the amalgamation time?

Little newbee tip here, not sure how widely accepted it is, is to kickstart the reaction with a small amount of (aq)NaOH.  Can't say I know why but this speeds up the amalgamation substantially, although I believe that it burns out much quicker, and may effect yeilds (can someone confirm?)  But anyway thats an option.

Also, excess mercury salts are simply reduced to the element.  You might try more foil, or better, thinner aliminum cut in smaller peices, to increase surface area which would speed up the reaction quite a bit as well.

Just make sure you got icewater recirculating through a condenser of adequate length and this reaction is very forgiving of these minor details.

Of course the big super secret clandestine trick is having good ketone in there in the first place.  Someone who is really not myself and someone who is also not myself learned this the hard way awhile back.

Flip

goiterjoe

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it's not just an initiator
« Reply #21 on: July 13, 2002, 08:34:00 AM »
water is consumed throughout the entire reaction.  The only step that produces any water is the imine formation, which produces one mole of water for every mole of imine formed.  The other two reactions require water to run.

All paths are the same: they lead nowhere

Flip

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I think Osmium explained it best
« Reply #22 on: July 13, 2002, 08:56:00 AM »
Osmium
(Stoni's sexual toy)
02-13-02 05:49

Post 269032

(Osmium: "Re: Hg Amalgamation rates", Chemistry Discourse)



An amalgam is an alloy containing Hg having the interesting property of overvoltage. This means the Al wants to react and dissolve badly with alcohols and water producing H2 in the process, but this reaction is greatly hindered (NOT completely stopped, just slowed down a lot!) since H2 doesn't like to be produced on Hg surfaces.

When there is anything reducable around, like a nitro compound or an imine the Al can dissolve without having to produce H2 gas, and it readily will do so.
This reaction involves electron transfers between the reaction partners, which is just another way of saying the Al is dissolving and gives up its electrons to the Hg which in turn transfers them to the organic reactand to be reduced. It's not the Hg salt that breaks up the Al. If there is nothing around to be reduced then the Al can't get rid of those electrons, and it will not dissolve. Al/Hg in an inert solvent like hexane or toluene should be quite stable for a while. Al/hg will react with oxygen because Al is a very reactive metal which is only protected by its oxide coating (passivation), once that coating is removed it will oxidise in air, but that is usually not an option when covered by a solvent since O2 solubility is very low.

The solvolysis reaction with an alcohol will be very slow with thick Al because it has a much higher Hg loading on its surface and a much lower exposed surface area than foil.

Al/Hg in a pure alcohol: only very slow dissolving via solvolysis. Greatly enhanced by water presence (since Al2O3 formation is very exothermic and the driving force) and change to basic pH

Al/Hg in water: exothermic, will heat up considerably and might eventually react violently

Al/hg with nitro in alcohol: very exothermic reaction due to water formation, and since nitro is hot stuff

Al/Hg with nitro in alcohol containing considerable amounts of water and a base: let's not go there, runaway reaction with foil, very exothermic with sheet

Al/Hg with an imine in alcohol with a littlewater: rather slow and gentle reaction

Foil reacts much faster than sheet!

Don't try to tweak the reaction too much and reinvent the wheel! Al/hg works, in up to 90% according to the literature and some bees experience. And when done without haste and with thick Al it can even be very volume efficient.




This is perhaps the best explanation I have found of the properties of the reaction.  It seems to suggest, and please correct me if I am wrong, that the Al/Hg does not by itself break up into slush because H2, the product of the reaction, does not like to be produced on Hg surfaces, and that simply the prescence of the reducable compound and the presence of oxygen(proton source) allow the reaction to take place, ultimately with water as a byproduct of imine formation.  Osmium does however state that water is a  initiator as well.  I think the main function of water is to make absolutley sure you have availiable oxygen to start the reaction, but if for some reason it was present anyway in the solvent I believe that would substantiate Chromic's statement about water's neccessity.  I think this would be correct anyway, but i've been up 38 hours.

Or perhaps the solvent alone, rate depending on how strong of a proton donor the alcohol is?

No, Seriously, Chromic, Man, would this interpretation be correct?

Or do I need to read up on redox?




BTW For those of you who didn't know Osmium is obviously a genius.  ;)  My hat is off to you.  Keep the wisdom coming.

Flip

Chromic

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ugh
« Reply #23 on: July 13, 2002, 02:00:00 PM »
Ugh, I take back that initiator comment.

Chromic

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Agreed.
« Reply #24 on: July 14, 2002, 09:12:00 PM »
Agreed, Osmium did explain the best. That post is awesome and concurs with all the experimental observations I've made.  :)

scram

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"Re: 1) let the foil amalgamate until it...
« Reply #25 on: January 09, 2003, 11:21:00 PM »
"Re: 1) let the foil amalgamate until it starts to vigorously bubble"
"Why does it always take 30-45 minutes for this to happen!? Yields are usually 10% less when rushed and started prematureley."
You have to reduce the amount of solvent covering your al. You will notice a big reduction in amalgamating time if the bare minimum is used to cover the foil. Mine jumped from taking 40 to 55 minutes to about 15. Initiating temp makes a big differnce in time too.