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Molecular structure[edit]

I've searched crystallographic databases for Et3Al several times previously, but never found quite what I was looking for — the structure of the pure substance, as opposed to complexes or derivatives thereof.

Now I've really focussed my efforts on searching thoroughly, including a few Web of Knowledge searches, and I've located the following useful information:

I'll make some images based on these references, and clear up the confusion over the structure once and for all. If anyone can find any secondary or tertiary sources that cite or confirm the findings of these papers, they'd make a better source for the article.

Ben (talk) 19:52, 16 May 2010 (UTC)

Nice find. I only looked at your Organometallics ref, which has a nice intro/overview. I didnt check its refs but probably the structure was done by electron diffraction. My Ullmann source and this paper both point to the fact that AlEt3 is mainly a dimer in the gas and liquid phase. The paper mentions that the monomer, always a minority for the straight-chain trialkyls, becomes (slightly?) more favored for the Bu>Pr>Et>Me series, as expected. Only with branching do the monomers become predominant in the condensed phase. If we wanted to do this right, we could find the ΔH and ΔS for the dimerization, then we could show the ratio vs T, at least for a given phase. Of course eventually every dimer breaks at sufficiently high T. --Smokefoot (talk) 20:20, 16 May 2010 (UTC)

Yes, the Organometallics paper is almost a review in itself, a nice little source of further information and history on the topic. If I've understood the paper correctly, the structures of trimethylaluminium they draw are calculated (using the HF/6-31G** level of theory). The paper cites "Structure of the Trimethylaluminum Dimer As Determined by Powder Neutron Diffraction at Low Temperature", Organometallics (2000) 19, 4398–4401, too. Image forthcoming!

In the results and discussion section, the authors conclude the following:

To obtain reasonable signal-to-noise ratio, the sample has to be heated at least to 50−60 °C. By heating the path between the sample and the ionization chamber up to 200 °C no change of the photoelectron spectrum was experienced. This observation confirms that already at 50−60 °C the pure monomer spectrum was recorded (Figure 4).

They also note that AlEt3 is a dimer in solution in benzene, according to J. Am. Chem. Soc. (1946) 68, 2204–2209 and Liebigs Ann. Chem. (1960) 629, 104–121.

Here's the geometry of the Et2Al-Si(SiMe3)3 dimer in the crystalline phase:


I'll make a 3D picture (and accompanying Jmol model) if anyone's interested.

Ben (talk) 20:53, 16 May 2010 (UTC)

isn't the molecular structure shown on the page that of trimethylaluminium....zontraS (talk) 13:18, 10 December 2011 (UTC)

No. --Leyo 11:35, 11 December 2011 (UTC)
No on what grounds? The diagram depicts C6H18Al2 but lists the molecular formula as C6H15Al. The latter is the one depicted as the "Monomeric form found in the gas phase" as well as the chemspider page (talk) 12:30, 19 August 2013 (UTC)
Dimeric form
How do you count atoms? The figure on the right depicts the dimeric form. Its molecular formula is C12H30Al2. --Leyo 13:17, 19 August 2013 (UTC)

The structure shown above, by conventional standards, shows CH3CH as a bridge since the connectivity for carbon is assumed to be 4 unless shown otherwise. Also it implies that one Al-C bond is much weaker than the other. Which is incorrect, I am pretty sure. Looking forward to sorting this out.--Smokefoot (talk) 14:01, 19 August 2013 (UTC)

Replacement by Smokefoot
As noted in my edit comment, the replacement has several issues:
  • Technical quality: There is no/not enough space between the 2 and the bond on the upper left corner; On the top, there is no need to write H3C instead of CH3 since the bond is straightly upwards.
  • The way the ethyl groups are drawn, it is hard to see for lay readers that they are in fact all ethyl groups. At least the four non-bridged ethyl groups should not be abbreviated as CH2CH3.
  • The styles of the dimeric and the monomeric form shown in the article are now quite different, which is confusing for readers.
--Leyo 11:54, 21 August 2013 (UTC)
Thanks for the feedback and giving me a chance again to achieve some consensus. I am only in intermittent access now, but here are two new versions.
one depiction of Al2Et6 structure
another depiction of Al2Et6 structure
--Smokefoot (talk) 02:26, 22 August 2013 (UTC)
If you flip H3C to CH3 on the top of the last version, I agree on using this one. --Leyo 07:25, 22 August 2013 (UTC)
Looks fine to me. --Smokefoot (talk) 12:45, 30 August 2013 (UTC)
Is there a way to avoid having 5 "normal-looking" bonds to each of the bridging carbons? Maybe if it were rotated 90° about the Al---Al axis, so that the terminal ethyls were wedge/dot and the bridging ligands (an article which focuses almost exclusively on inorganic ligands and 2c-2e bonds!) in-the-plane, those Al–C–Al could be dotted lines to distinguish them as 3c-2e bonds (an article that highlights the Al–C–Al of trimethylaluminium)? DMacks (talk) 13:45, 30 August 2013 (UTC)
Well as we know, 2c/2e bonds are a kind of fiction, since we are talking MO's. What you are discussing would be pretty disruptive since there are many bridging hydrides (most boranes), alkyl (all nonmonomeric trialkyl Al cmpds), etc. It is a long, long list. Then we may need to rethink how to re-represent bonds SF4 and all hypervalent compounds since these are not normal bonds. Interstitial carbon is typically six coordinate. ... whew! --Smokefoot (talk) 13:55, 30 August 2013 (UTC)
Triethylaluminium dimer 3D ball.png
I know I know:) It's a great diagram for illustrating the MOs and connectivity, but I think the MO idea as illustrated is either too advanced or too confusing to use as a standalone image. For the lede, how about a ball&stick, so that we get away from simple lines as different kinds of bonds (bonus: pretty!)? Unrelatedly, the hashed bonds in the proposed diagrams are backwards from IUPAC recommendations...should have point at Al not C. DMacks (talk) 14:12, 30 August 2013 (UTC)
Figured you had been through all this before... The ball and stick is fine with me. Thanks for tip on the hashed bonds, always been confusing to me. Should read more IUPAC, I guess.--Smokefoot (talk) 14:36, 30 August 2013 (UTC)
I know this isn't helpful of me, but I have to say that I'm not a fan of only showing the dimer of something in its chembox. The structure doesn't match the stated molecular formula or mol weight. — Preceding unsigned comment added by Project Osprey (talkcontribs) 15:17, 30 August 2013 (UTC)
Well, we cant really show the monomer that does not exist in appreciable amounts and we cant change a lot of history on naming. But maybe you have a new angle ... --Smokefoot (talk) 17:15, 30 August 2013 (UTC)

Use as rocket fuel[edit]

"It also can be used as a rocket fuel, but has not been for any production vehicle."

I dispute that T. can be used as a rocket fuel. One of the fundamental Properties of a rocket fuel is, that it reacts with an oxidizer to a gas. T. reacts in a large part to a solid, aluminum oxide. Solids can not expand in a nozzle and they clog feed lines. Therefore T. would be a very bad rocket fuel. The sentence is misleading at least. T. is no more a rocket fuel than wood or a large number of other substances that can be ignited. If there is no objection, I will remove it in a week. -- (talk) 15:32, 4 February 2012 (UTC)

 Done I removed the sentence that indicated the use as a fuel. it is used as an ignitor or can be.--Smokefoot (talk) 15:44, 4 February 2012 (UTC)
The part with the igniter is true. It's usually mixed with Triethylborane. Both are hypergolic (react without ignition) with liquid oxigen, but Triethylborane reacts slowly and as mentioned before Triethylaluminium reacts to solid aluminum oxide. So basically, you use Triethylaluminium to produce enough heat so that Triethylborane can ignite the engine faster. The downside is of course, that you have to get the oxide out of the engine. If something gets clogged, disaster ensues.-- (talk) 17:01, 5 February 2012 (UTC)