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#### Enthalpy

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##### Azetidine and relatives
« on: January 13, 2011, 09:00:12 PM »
Hello you all!

Many satellites, space probes and rockets currently use hydrazine N2H4, methylhydrazine (mmh), 1,1-dimethylhydrazine (udmh) and their mixtures as a fuel in combination with N2O4. Agencies and companies would like to replace these hydrazines, which are meanwhile known carcinogens, by fuels as efficient, preferably hypergolic with N2O4, and liquid in a wide range around 300K.

One interesting candidate is azetidine (aza-cyclobutane if you like).
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2372#p27240
http://de.wikipedia.org/wiki/Azetidin
http://en.wikipedia.org/wiki/Azetidine
efficient, dense, stays liquid at cold... But it boils at +63°C already, with 176hPa vapour pressure at +19°C, and a flash point of -20°C.

How bad is it to work with in tons or hundreds of tons? A volatile amine is something you dislike, isn't it? Merck's MSDS states Xi.

A fuel would rather be a relative of azetidine. An N-methyl would improve the liquid range, the heat of formation, ignition behaviour... And then a tail of butyl maybe, at the other end, to lower the ignition temperature and raise the boiling point for safer vapour pressure and flash point. Suggestions, data?

Or could (= aza-cyclopropane) aziridine become less harmful if getting an N-methyl and an alkyl tail? It would be more efficient than azetidine, but genuine aziridine is a mutagen and a possible carcinogen, yuk.

Thank you!

#### 408

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##### Re: Azetidine and relatives
« Reply #1 on: January 14, 2011, 09:26:24 AM »

How bad is it to work with in tons or hundreds of tons? A volatile amine is something you dislike, isn't it? Merck's MSDS states Xi.

Far, far safer than hydrazine.

#### AndersHoveland

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##### Re: Azetidine and relatives
« Reply #2 on: January 14, 2011, 04:55:26 PM »
I have two sites related directly to advanced and lesser known energetic compounds and their associated chemistry. Perhaps you can find something there that will be helpful.

#### Enthalpy

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##### Re: Azetidine and relatives
« Reply #3 on: January 14, 2011, 08:34:52 PM »

About those energetic molecules: I'm interested in rocket propellants (especially liquid ones), and these are supposed to burn more slowly than the three first molecules suggested; moreover, bi-liquids, which separate specialized fuel and oxidizer (=liquid oxygen), have better performance than the all-in-one molecules. The fourth family, metal hydrides, are known in rocketry as tempting but difficult fuels. Most are pyrophoric, all are too unstable for use in hundreds or thousands of tons. The latest proposal I read was to encapsulate AlH3 in tiny metal spheres.

Well, compositions designed for slow burn don't always burn smoothly neither.
Enjoy chemistry the big way...

More opinions, suggestions welcome! Less volatile relatives of azetidine, harmless parents of aziridine?

#### AndersHoveland

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##### Re: Azetidine and relatives
« Reply #4 on: January 16, 2011, 06:51:57 PM »
AlH3 is actually considered to only be a mildly reactive reducing agent.
It will not reduce nitromethane, for example. Burning in oxygen, it is slightly more energetic than aluminum (I think about 10% more).

Here is a list of potential fuel-oxidizer combinations
lithium and fluorine (lithium is a fairly rare and expensive element, fluorine is difficult to liquify)
pentaborane and oxygen difluoride
diborane and chlorine pentafluoride
aluminum borohydride and nitrogen trifluoride mixed with sulfur hexafluoride
magnesium aluminum hydride and nitric acid, enriched with 20%N2O5 and NO2
trimethyl aluminum and tetrafluorohydrazine

I also have the idea that hydroxylammonium nitroformate would be a good solid rocket oxidizer, being both more energetic than ammonium perchlorate, and having acceptable resistance to detonation. Like almost all other solid oxidizers, however, it would not be able to compete with cryogenic oxidizers in terms of specific impulse.

Here are some basic calculations I have done, while thinking about energetic rocket fuels:

The most energetic reaction that is possible for a rocket is a mix of OF2 with a Li-Be alloy. Beryllium and Oxygen together release 24.36 kJ/g. Lithium and Fluorine release 23.75 kJ/g.
While using ozone instead to burn Be would yield 26.26kJ, ozone is sensitive to detonation. Oxygen difluoride will yield 23.8 kJ/mol extra energy than a comparable mixture of O2 and F2. There are 54g/mol for OF2, thus it is calculated that the use of OF2, rather than O2 and F2, should provide an extra 0.44 kJ/g. Lithium is fairly expensive (US$95/kg). Beryllium is much more so ($745/kg). While lithium-Al alloy is already used (as structural material) for that large central brown colored exterior fuel tank on a rocket, this tank is recoverable. The fuel inside the tank has far more mass than the tank itself, so using Lithium as rocket fuel is not economical.

Using a new article: "A New Determination of the Heat of Formation of Oxygen Difluoride" by WARREN R. BISBEE, Rocketdyne Division, North American Aviation
The heat of formation of OF2, with this newly determined value, is 16.994 kJ/mol (0.3147 kJ/g); this calculation does not agree with my earlier one. Wikipedia claims a 24.5 kJ/mol heat of formation, which gives a value of 0.454 kJ/g. Thus, three different values have been calculated for the additional energy OF2 will contribute.

#### Enthalpy

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##### Re: Azetidine and relatives
« Reply #5 on: January 18, 2011, 09:48:04 PM »
All these belong to the common substrate of rocket designers' remote desires ...
The latest report I've seen mentioning such propellants was meant for a probe meant to orbit Uranus (?), needing small amounts of more efficient propellants.

In all other cases, this profusion has boiled down to very few guidelines (I don't consider the more diverse solids here).

Metals are generally bad. They improve heat per mass unit over hydrocarbons only if the oxide is solid (at >3500°C), which limits to Al2O3 (and maybe BeO and LiAlO2). But a solid doesn't accelerate by itself when "expanding" in the nozzle: it only transfers heat to the expanding gas, making expansion inefficient. Thus plain aluminium worsens good fuels and improves only bad propellants as solids are. Hydrides or organometals would improve fuels but are too difficult to use up to now. Liquid aluminium is too difficult for a tiny improvement.

Because of the amounts involved, all propellants too explosive, toxic, expensive are excluded. Ozone, fluor, beryllium, boron, OF2, NF3... are all abandoned. Figure that nitromethane was excluded as unstable. Small leaks do happen, often, including at 500 bar or from 100m height, and shall not have catastrophic consequences.

Because of low performance compared to good liquids, HNO3, boron and its hydrides, sulphur and its fluorides, are abandoned.

Which leaves few combinations in present times.

With N2O4, liquid at room temperature, usually supplemented with NO to avoid freezing, people use liquid fuels. MMH (CH3-NH-NH2) and variants are common because they ignite on contact and burn easily; this is the toxic one I want to replace (as many people). And with an igniter, hydrocarbons are good as well.

N2O4 is not favoured now, because liquid O2 isn't very difficult to handle and improves performance so much. With (my) cryocooler we will even be able to store it for years in satellites and probes.

Then we may feed the engine with pressurized tanks or with pumps. Only if pressure-fed, the fuel's temperature and density matters, to save mass at its tank and at the helium tank. In this case, exotic fuels with nitrogen or strained cycles bring some advantage as they use less of the cold oxygen that makes the helium tank heavier: the other use of azetidine, diazetidine, and aziridine (too toxic as is) variants.

If pump-fed, alkanes are more efficient than but the most energetic strained rings, as they contain more of the light hydrogen. Double and triple bonds produce polymer dirt in the engine's cooling jacket, excluded. This reduces to:
- Hydrogen, but it's difficult to pump;
- Methane in the future. First trials were conducted badly;
- Improved kerosene. Though branched alkanes would be better;
- Aza and polyrings (see Syntin at Wikipedia) aren't really worth it here. They don't improve over methane, which should replace kerosene.

Not very varied, is it?

#### AndersHoveland

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##### Rocket Fuels
« Reply #6 on: January 20, 2011, 05:18:38 PM »
While you are technically correct about the metal fuels as propellents, if liquid hydrogen is also being injected into the combustion, then the most important factor becomes the heat produced from the fuel.
The weight contribution from liquid hydrogen is very negligable, and for large scales, this also includes the storage tank for the cryogenic liquid.

The reaction between Beryllium and oxygen, with liquid hydrogen present, has a higher specific impulse than
liquid hydrogen burning with oxygen. The highest specific impulse for a chemical propellant ever tested in a rocket engine was lithium, fluorine, and hydrogen (a tripropellant), which gave a value of 542 seconds (5320 m/s).

Methyl hyrazines actually haver lower specific impulse values than plain hydrocarbons, such as kerosene.
The reason that hydrazines are used is because they are hypergolic with N2O4 or HNO3. Hydroxylamine compounds, which are very reactive reducing agents and have much lower toxicity then hydrazine, could potentially replace hydrazine as a hypergolic fuel. For example, N,O-diethyl-hydroxylamine is a liquid. While hydroxylamine itself has an explosion hazard, many of its derivitives have no danger.

While it is quite understandable that ozone, and fluorine, boron-based compounds were abandoned, in my opinion
these fuels should still be regarded as potentially valuable for the upper stages of rockets.
Because of the risk of explosion ozone might b more suitable for use to send up low value objects, such as reserve fuel, into space. Even using a fuel combination with only a slightly higher specific impulse can allow significant cost savings. It requires fuel to lift unburnt fuel, so more energetic fuel can lead to exponential weight reductions.
If I remember correctly, using a fuel that is 15% more energetic can potentially reduce the required fuel by 50%. Considering how expensive it is to lift weight towards space, small weight savings could easily be worth the extra complexities and hazards associated with exotic fuels. Of course, for the non-professional experimenter, the gains would be very insignificant compared to the added difficulty and danger.

At present, azetidines are too difficult and expensive to produce to be suitable for large scale rocket use.

It is unfortunate that fluorine-based oxidizers tend to produce toxic HF in their exhaust. This is the main reason why fluorine oxidizers are not used. One note to mention. The US navy has experimented with SF6 burning with metalic lithium to propel torpedoes. SF6 is unusually inert, and when mixed with oxygen, it is even safe to breathe. If the fuel consists only of a metal, the metal fluorides, while still creating a hazardous smoke, are far less toxic than HF gas. There has already been much experimentation with oxygen and nitrous oxide burning solid HTPB or even blocks of wax. These have the advantage of the fuel having little danger of combustion during storage, and such engines have been proposed for future commercial use. I would think that liquid pressurized SF6 could be used to burn an aluminum core. I am not sure if this would be more energetic than nitrous oxide burning with methane, but this combination could potentially be very energetic. SF6 and Al would both be extremely safe (compared to other oxidizers and fuels) before combustion. Ideally the combustion products would form only sulfur and AlF3, which is not extremely toxic. Aluminum trifluoride has a surprisingly low heat of formation, which should not be surprising, considering the high 1291°C melting point of the compound.
"The Enthalpy of Formation and Entropy of Aluminum(I) Fluoride(g)" Hon Chung Ko, Michael A. Greenbaum, Jay A. Blauer, Milton Farber. J. Phys. Chem., 1965, 69 (7), p 2311–2316 (1965)
There would, unfortunately, also be traces of toxic SF4 produced during combustion that would inevitably escape into the exhaust

I agree, methane would be a very conventient fuel, with good specific impulse, and not extremely difficult for storage.
« Last Edit: January 20, 2011, 06:11:14 PM by AndersHoveland »

#### AndersHoveland

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##### TNAZ
« Reply #7 on: January 20, 2011, 06:48:03 PM »
(first see my post about rocket fuel directly above)

On the subject of azetidines, I had an idea for easily producing 1,3-substituted azetidines as a precursor to the high performance explosive TNAZ. TNAZ is meltcastable with excellent properties, but unfortunately the current synthesis is too complex, and thus TNAZ is too expensive for commercial use.

I showed this idea to the dean of the math/science division at the college, who has a doctorate in organic chemistry. He said he did not know enough about these type of ring expansions to give an opinion.

#### AndersHoveland

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##### Rocket, excess liquid hydrogen
« Reply #8 on: January 21, 2011, 04:46:19 PM »
For some reason, this topic will not let me modify any of my posts
yet my other posts in other topics are allowed to be changed

also to mention, in normal rockets that burn H2 with O2, extra liquid hydrogen is used beyond the stoichiometric
quantity. The heat released from formation of water molecules causes the excess liquid hydrogen to boil, and this is what provides the thrust. Liquid oxygen is much heavier than liquid hydrogen, and this is why using extra hydrogen has a higher specific impulse than burning all the hydrogen. It is interesting that many people wrongly assume that all the hydrogen is reacted.

#### Enthalpy

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##### Re: Azetidine and relatives
« Reply #9 on: January 21, 2011, 10:37:54 PM »
Propellants are interesting. AndersHoveland, why shouldn't you open one or more threads? I'd prefer to stay focussed on the Azetidine family here, and would be glad to discuss the other propellants there.

At present, azetidines are too difficult and expensive to produce to be suitable for large scale rocket use.
Up to now, I've found prices only for 1mg and 1g, both amounts cost 100€, so maybe 1000t cost 100€ as well?
If rockets want to burn azetidine and there is not enough supply, it will be produced locally, as are hydrazines or perchlorate. Then the difficulty of synthesis, not previous market prices, makes the cost.

In a 1968 report, the authors synthesized N,N'-dimethyl-1,2-diazetidine with a limited effort:
maybe I'm wrong, but azetidine should be easier to produce. Or not?

Other physical data for relatives of azetidine and aziridine? Melting and boiling points for instance...

Thanks!

#### AndersHoveland

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##### Re: Azetidine and relatives
« Reply #10 on: January 23, 2011, 04:20:51 PM »
“A One-Pot Preparation of 1,3-Disubstituted Azetidines”
Michael C. Hillier, Cheng-yi Chen

HOCH2CH(R’1)CH2OH reacted with CF3SO2OSO2CF3 to form
CF3SO2OCH2CH(R)CH2OSO2CF3. This then reacted with (R’2)NH2 to form
R’2(NC3H5)R’1.    where R'1 amd R'2 are side groups

A straightforward synthesis of 1,3-disubstituted azetidines has been accomplished via the alkylation of a primary amine with the bis-triflate of a 2-substituted-1,3-propanediol species. This transformation is carried out in one reaction vessel. N,N-Diisopropylethylamine
((CH3)2CH)2NCH2CH3 was used as the solvent in both steps of the reaction.

R’2 could be acetamide CH3C(=O)NH2, the acetyl group could later be hydrolyzed off, leaving a plain NH in the square ring. R’1 could be an ether, such as --OCH2CH3, which could later be hydrolyzed off (with 20% HCl) leaving a hydroxyl group, then oxidized with a mild oxidizing agent such as pyridinium chlorochromate to the ketone (=O). Thus, azetidine-3-one could potentially be made in a one pot reaction, if triflic anhydride was available.

Of course, if one is not interested in a TNAZ precursor, both R'1 and R'2 could be methyl groups, thus forming 1,3-methylazetidine. Putting a methyl group on the amine in the ring will make the compound much more volatile. It seems that azetidine itself is a volatile liquid, boiling at only 61degC.

I am uncertain of the boiling points of azetidine derivitives, but comparisons can be made to more familiar compounds, to give some indication of what the boiling points would probably be. N-ethyl-N,N-dimethylaminoethane, with the structure
CH3CH2N(CH3)2, has a boiling point of 36-38 °C, meaning it is an extremely volatile liquid that would boil from the warmth of being held in your hand. Whereas, triethylamine has a boiling point of 89degC.
« Last Edit: January 23, 2011, 04:46:58 PM by AndersHoveland »

#### Enthalpy

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##### Re: Azetidine and relatives
« Reply #11 on: January 23, 2011, 10:10:46 PM »
Meanwhile I've seen very encouraging documents as well. Especially if one prefers N-methylazetidine, which is the case for a rocket fuel: better ignition, lower melting point, better heat of formation...

The one book that lets suppose mass-production would be realistic:
Heterocyclic chemistry, by Malcolm Sainsbury
citations:
- Azetidine has been known since 1899
- Azetidines are often synthesized by reacting 1,3-dihalogenopropanes with an amine (ammonia gives poor yields), or from propane-1,3-diamines where one N-substituent can function as a leaving group.

The N position shouldn't carry an alkyl bigger than methyl, or this will eclipse the N, which is supposed to determine hypergolic ignition.

-----

Presently, synthesis methods concentrate on putting exotic functions to make drugs, and obtaining the right enantiomer:
http://www.organic-chemistry.org/synthesis/heterocycles/azetidines.shtm

-----

Mpbpvp gives reasonable estimates of boiling points, free there as the autonomous module MPBPNT.exe of EpiSuite:
http://www.epa.gov/oppt/exposure/pubs/episuitedl.htm
http://www.epa.gov/oppt/exposure/pubs/episetup_v400.exe
some operations would be necessary for Vista.

Where I find experimental data, Mpbpvp is consistently within 10K of the boiling point. But the melting point is commonly 80K away, seen 200K error once.

#### Enthalpy

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##### Re: Azetidine and relatives
« Reply #12 on: January 25, 2011, 10:41:57 PM »
Beyond Azetidine, Burkhard and Carreira achieved to close two aza rings sharing a carbon, thus obtaining a diaza spiro:
"A concise and scalable synthesis of a 2,6-diazaspiro[3.3]heptane building blocks"
http://pubs.acs.org/doi/abs/10.1021/ol801293f

Their method is (to my eyes!) similar to closing azetidine, but they start with neopentane instead of propane, and carrying four (Br Br Br OH) things instead of two, to react with the amine.

As the result would be a nice rocket propellant, especially with methyls or cyclopropyls at N and N', I like to interpret the authors'
"concise and scalable synthesis"
as
"produce in rocketry amounts"
should I?

-----

As well, azetidine seems easier to synthesize if the amine reactant puts many carbons close to the nitrogen, isn't it? Then, I'd like to use cyclopropylamine, to obtain the more energetic cyclopropylazetidine...

-----

Drawings are there, clearer than names:
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2372#p30326
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2372#p30332 (or scroll down)

#### AndersHoveland

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##### Re: Azetidine and relatives
« Reply #13 on: January 26, 2011, 04:25:50 PM »
First, I want to suggest that any further posts be made in very clearly understandable terminology, with explanations; and second, that explanations be provided, in addition to any links, because links have a nature of eventually not working after some time. Make it possible for future readers to understand.

I found a mention to 1,2-diazetidine-1,2-di-N-oxides, which seems potentially interesting. Oxidizing polynitrogenous rings can actually increase stability of the molecule. I would be very interested in 1,3-diazo bridge substituted cyclobutane cages, if they exist. (with a formula C4H6N2).

I think it should be realized that any of these azetidines will not be more energetic than methane or aluminum powder. While bond strain certainly adds energy, it is difficult to compete with heat of formation of formation of Al2O3, or the light weight of hydrogen atoms in a molecule.

#### AndersHoveland

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##### 1,3-diazo cyclobutane
« Reply #14 on: January 26, 2011, 08:24:16 PM »
The name for the cage would be 2,3-diazobicyclo[2.1,1]hexane.
The structure of bicyclo[2.1,1]hexane can be seen here:
http://elchem.kaist.ac.kr/jhkwak/OkanaganPdb97/nomenclature/pdb/al0059.gif
(the two carbon atoms connected to 3 other carbons are in the 1- and 4- positions.
The 2- and 3- positions are in the ethylene (--CH2CH2--) group which bridges the 1- and 4- positions.
The 5- and 6- positions are in the methylene bridges (--CH2--))
The molecule could best be described by plain text as:
..CH2CH2
./..l
HC-CH2-CH
...l.../
..CH2

Enthalpy, could you provide more details about the article that suggests that triethylamine is more hypergolic with nitric acid than hydrazine? You mentioned this in the other forum, but the link is inaccessable. If this is true, then one would wonder why hydrazine is being used.
« Last Edit: January 26, 2011, 08:35:52 PM by AndersHoveland »