Sciencemadness Discussion Board

Silicon tetraiodide (and other uses for elemental silicon)

Melgar - 13-6-2016 at 13:03

I just dissolved some gold off of a bunch of gold-plated scrap silicon wafers, and somewhat to my surprise, the silicon didn't appear to be dissolved in the slightest by the aqua regia. So now I'm trying to come up with interesting things to do with elemental silicon. One idea I had was reacting it with elemental iodine, to get silicon tetraiodide. The logistics of this proved difficult though. Elemental iodide doesn't like to stay in whatever part of the flask it's being heated in, and tends to vaporize and then crystalize wherever the temperature is lowest in the flask. By the time the silicon was hot enough to react with the iodine, the vapors had all been driven away.

I suppose I could have just used the process mentioned here, in an old Encyclopedia Britannica article. Essentially, pack a tube with broken-up silicon shards, then heat iodine in a test tube and direct the vapor through the also-heated silicon-packed part of the tube. But I don't have that sort of glassware right now, so instead, I tried heating the silicon red-hot in a test tube, then dropping it into another test tube where iodine was being heated, though just enough to make it produce a thick vapor. There seemed to be an exothermic reaction, and there seemed to be a new chemical in the tube afterwards that behaved a lot like iodine, but was slightly different. (Silicon tetraiodide is 95% iodine by mass, which is kind of impressive.) However, there was still mostly iodine in the test tube, so that wasn't a very good synthetic route.

My next idea was to dissolve as much iodine as I could in the least reactive, least polar solvent I could think of, which would probably be mineral oil, and heat that. That should reduce the vaporization of the iodine considerably.

My other idea was to combine the two solids in a flask, and heat the mixture in a microwave that didn't belong to someone else. Silicon absorbs microwaves like a metal, but iodine wouldn't, so there should be plenty of hot spots on the silicon for iodine to absorb enough heat from to react. Silicon tetraiodide would presumably not be affected by microwaves? IIRC microwaves only affect certain types of polar bonds.

As for what could be done with silicon tetraiodide, supposedly it hydrolyzes into SiO2 and HI the instant it comes into contact with water. Maybe once the DEA learns of this, it'll give them reason to make a third element illegal?

Any other interesting chemistry that can be done with elemental silicon?

[Edited on 6/14/16 by Melgar]

clearly_not_atara - 13-6-2016 at 13:20

MeXSiCl(4-X), and Cl3SiH are accessible by reaction with gaseous MeCl or HCl respectively.

http://en.wikipedia.org/wiki/Direct_process

http://en.wikipedia.org/wiki/Trichlorosilane#Production

The latter compound, trichlorosilane, is capable of reducing phosphates. All silicon halides are extremely toxic and volatile, and many silicon hydrides are pyrophoric, so this is not a sort of chemistry that can be taken lightly. One of the byproducts of Cl3SiH production, dichlorosilane (Cl2SiH2), is pyrophoric.

If you can make calcium silicide, CaSi2 -- which is sort of a calcium di-silaacetylide -- this hydrolyses to siloxene, which is mildly interesting.

blogfast25 - 13-6-2016 at 13:44

Quote: Originally posted by Melgar  

My next idea was to dissolve as much iodine as I could in the least reactive, least polar solvent I could think of, which would probably be mineral oil. Then throw in a few silicon shards and stick the flask in a microwave on low power, so it gets bombarded by microwaves for a few seconds several times a minute. It'd probably be a good idea to break up the pieces to reduce the voltage that could be generated across each shard. We wouldn't want mineral oil tar to deal with. Or, actually, I could just heat the mineral oil directly, since that should reduce the vaporization of the iodine considerably.



Unlikely to work, IMHO but likely to create a real mess.

You just have to find a way to lead I2 vapours over hot Si, I guess.

clearly_not_atara - 13-6-2016 at 16:07

Use CCl4 or something as solvent; don't heat a fucking halogen in mineral oil you crazy bastard. I2 should be ok in refluxing p-dichlorobenzene, maybe (suggest trying this at small scale WITHOUT Si present to be sure)

[Edited on 14-6-2016 by clearly_not_atara]

Melgar - 13-6-2016 at 20:03

I dunno. Iodine is like the fat kid of halogens. Even when it does react, it doesn't expend a lot of energy doing it, and it can get knocked off of a carbon chain by just about anything. It's the only halogen that sometimes looks and reacts like a metal. I couldn't even get iodine radicals to pull a hydrogen off of toluene, let alone a straight-chain hydrocarbon. I'd be more worried about the silicon getting so hot it would pyrolyze the mineral oil.

Actually, that microwave synthesis sounded stupid, and now I remember why. Silicon absorbs microwaves like a metal would, but iodine doesn't. So when I originally thought of the microwave idea, the plan was to heat both solid elements together in a flask in the microwave, at which point the silicon should get hot enough to react, and the iodine would only get hot enough to react when it was in contact with the silicon. Silicon tetraiodide should be inert to microwave radiation, I'd assume. Of course, I don't currently own a microwave, and wouldn't want to put my theories to the test with my roommate's. I'll edit my original post to reflect that.

I actually DID heat a test tube with iodine dissolved in it with silicon shards, but the iodine darkened the solution so much I couldn't tell if there was any reaction. I guess I could shake it with water and test for acid? I'm kind of low on iodine though, and if there wasn't a reaction, I might want to try and reclaim it from the mineral oil once I think of a way how that doesn't involve sodium thiosulfate and separating elemental iodine from elemental sulfur.

Melgar - 13-6-2016 at 20:41

Quote: Originally posted by clearly_not_atara  
MeXSiCl(4-X), and Cl3SiH are accessible by reaction with gaseous MeCl or HCl respectively.

http://en.wikipedia.org/wiki/Direct_process

http://en.wikipedia.org/wiki/Trichlorosilane#Production

The latter compound, trichlorosilane, is capable of reducing phosphates. All silicon halides are extremely toxic and volatile, and many silicon hydrides are pyrophoric, so this is not a sort of chemistry that can be taken lightly. One of the byproducts of Cl3SiH production, dichlorosilane (Cl2SiH2), is pyrophoric.

If you can make calcium silicide, CaSi2 -- which is sort of a calcium di-silaacetylide -- this hydrolyses to siloxene, which is mildly interesting.


I don't exactly have any chloromethane, although trichlorosilane sounds interesting. On the other hand, the majority of damage my chemistry experiments have done have been from HCl vapors, (I sort of discovered a bromide-catalyzed chlorination reaction that gave off HCl vapors as a byproduct) and generating HCl gas and heating the vapors to 300C sounds like an accident waiting to happen. I'd at least need more/better glassware, but perhaps in the future.

Reacting with chlorine directly seems doable though, and the wikipedia page implies it can be done in a lab to form silicon tetrachloride. I just noticed that silicon tetrachloride has a boiling point that makes it liquid at room temperature, which makes the synthesis and purification a lot easier than I initially thought.

j_sum1 - 13-6-2016 at 20:52

https://www.youtube.com/watch?v=wU77anzCzxg
procedure for SiCl4

I don't remember it well except that it seemed within the realms of reasonable doableness.

Melgar - 13-6-2016 at 21:23

Quote: Originally posted by j_sum1  
https://www.youtube.com/watch?v=wU77anzCzxg
procedure for SiCl4

I don't remember it well except that it seemed within the realms of reasonable doableness.

Actually, the iodine reaction would be even easier than that, since there wouldn't be any need to generate any gas, except that silicon tetraiodide has a melting point of 120C. It's very soluble in polar solvents though, so if I made it, that's probably how I'd try and
capture it. I'd be hesitant to mix halogens and use DCM, but heptane or cyclohexane seems doable. What I'm less sure about, is how to separate iodine from silicon tetraiodide. Their boiling points aren't that different, and we all know iodine vaporizes far below its boiling point.

[Edited on 6/14/16 by Melgar]

Tsjerk - 13-6-2016 at 23:15

Does SiI4 sublime? If not you have a nice way to separate the two.

Melgar - 14-6-2016 at 05:46

Quote: Originally posted by Tsjerk  
Does SiI4 sublime? If not you have a nice way to separate the two.

Yes, but I just realized that it's the melting points of the two that are similar (113C vs 120C). Their boiling points are 180C for I2, 280C for SiI4. I guess that's what the inert gas is for though; moving the vapors through the apparatus so that they can be collected in different parts of it.

I guess that just leaves me with a need for an easily-controlled source of dry CO2. A quick google search shows that a 5-lb CO2 tank can be purchased for about $50, probably less if purchased used. I know I could figure out some chemical way of doing it more cheaply, but that's probably just one less thing that can go wrong, and experience has taught me to value that.

halogen - 15-6-2016 at 22:56

hot silicon might reduce co2, actually, it certainly will, but to which extent? possibly even solid carbon, but probably just surface silica with the relatively low hear.

atara hydrogen iodide was used to reduce benzene to cyclohexane, Melgar's was an understatement

woelen - 15-6-2016 at 23:15

At temperatures of 200 C or so, the CO2 will not be reduced by Si. You need MUCH higher temperatures for that.

unionised - 16-6-2016 at 12:58

Quote: Originally posted by Melgar  
I dunno. Iodine is like the fat kid of halogens. Even when it does react, it doesn't expend a lot of energy doing it, and it can get knocked off of a carbon chain by just about anything.

It rather depends on the hydrocarbon.
https://www.youtube.com/watch?v=3sgCJaICcaY

Melgar - 18-6-2016 at 08:56

Quote: Originally posted by unionised  
Quote: Originally posted by Melgar  
I dunno. Iodine is like the fat kid of halogens. Even when it does react, it doesn't expend a lot of energy doing it, and it can get knocked off of a carbon chain by just about anything.

It rather depends on the hydrocarbon.
https://www.youtube.com/watch?v=3sgCJaICcaY

At the risk of straining an analogy beyond its breaking point, that reaction would be more analogous to a fat kid stepping on a land mine.

Panache - 23-6-2016 at 07:30

Quote: Originally posted by Melgar  
I dunno. Iodine is like the fat kid of halogens. Even when it does react, it doesn't expend a lot of energy doing it, and it can get knocked off of a carbon chain by just about anything. It's the only halogen that sometimes looks and reacts like a metal.


I once needed 100g of HgI2, which forms from the elements I think at a mass ratio of 10:12g, Hg:I.
I first ground up a 10ishg batch in a mortar and pestle, no problems, then I tipped the other 90 g in and ground again, and an iodine/Mercury gaseous volcano ensued about a metre high, before cooling and mushrooming over the lab, under control (as in not an explosion) but definitely not desirable
I sealed the glass walled lab and watched it, then let it settle for a few hours before spending two days decontaminating the lab.
So yeah cf/Cl and Br its less aggressive but it's still very active.

Why not just seal freezer cold silicon and iodine in a evacuated tube then drop it in an oil bath and start heating?

[Edited on 23-6-2016 by Panache]