[Enthalpy]

Hello dear friends!

In rocket fuels, only amines are more efficient than alkanes (and Al and B compounds but with drawbacks). Nitrogen increases the gas exhaust speed and the fuel's density. Amines are also easier to synthesize. Here I'd like to discuss the synthesis of strained amines, stable and saturated, not too toxic (which eliminates most aziridines and hydrazines), not easily flammable.

---------- Azetidines ----------

The parent azetidine is efficient and stable but volatile hence dangerous: the molecule must be bigger, and the amine better tertiary for a wide liquid range.

Plain azetidine doesn't seem available in quantity, as opposed to cyclopropylamine for instance. Synthesis from propyldiamine in known. Dihalopropane can close a ring, but only with primary amines to give N-substituted azetidine; if cyclopropylazetidine is obtained that way, it's efficient but still a bit volatile with C+N=7. Vinylamine may provide vinylazetidine giving diazetidylcyclobutane by irradiation (or not); with C+N=12 it would be blended if freezing too easily. And would H₂C=NH combine with ethylene by irradiation to yield azetidine? The reactant's instability improves if substituted.

Cyclobutylazetidine would result from azetidine and halocyclobutane. A bit volatile with C+N=8, can be blended with a bigger molecule, for instance diazetidylcyclobutane made simultaneously.

Possibly the easiest fuel would be diazetidylcyclopropane, from bromoform via carbene and dibromocyclopropane reacted with azetidine. C+N=11 suggests a safe liquid range. Muuuch more hypothetic: use the unstable cyclopropanone instead of dibromocyclopropane, in case it's easily produced
http://www.chemicalforums.com/index.php?topic=78226.msg286255#msg286255

Diazetidylmethane may result from azetidine and formaldehyde - maybe: dimethylamine reacts efficiently that way. Or dihalomethane then. C+N=9 suggests to blend it with a heavier molecule.

I've tried to estimate the density and heat of formation, by hand comparison with similar compounds; while less bad than software, such estimates are imperfect. I have take no interaction at all between geminal amines, because some data suggests a lower heat of formation
http://www.chemicalforums.com/index.php?topic=46407.msg302977#msg302977
while other suggests a higher one. The boiling boint is estimated by the Mpbpvp software.

 m/s    +s    O₂:100    kg/m³   Bp °C
--------------------------------------------------------------
3366    +5      213     945     +174    Diazetidylmethane
3364    +5      215     953     +212    Diazetidylcyclopropane
3363    +5      234     904     +142    Cyclobutylazetidine
3374    +6      226     875     +117    Cyclopropylazetidine
3313    REF     283     834     +197    Rg-1 as reference
--------------------------------------------------------------
 m/s    +s    O₂:100    kg/m³   Bp °C

Comments and suggestions welcome, even dissent and objections!
Marc Schaefer, aka Enthalpy

[Enthalpy]

---------- Diazaspiroheptanes ----------

Derivates of 2,6-diazaspiroheptane show promising estimated performance and some synthesis path is known, as reported by Burkhard and Carreira
http://pubs.acs.org/doi/abs/10.1021/ol801293f (thanks Discodermolide!)
The dimethyl-, cyclopropylmethyl- and dicyclopropyl- shall be less viscous than primary amines. C+N=9, 11 and 13 adjust the flash point, and the cyclopropyl-azetidyl rotation is to lower the freezing point. A mix of isomers is welcome, a eutectic of methyl- and cyclopropyl- too.

These amines perform well with the same volume proportion of oxygen as Rg-1 "kerosene", easing an upgrade of existing stages. Their density and mix ratio help pressure-fed stages even more.

As the spiro atom adds little strain energy to the ring pair, azetidine synthesis paths are tempting for diazaspiroheptane. Though, ammonia elimination suggested by
https://de.wikipedia.org/wiki/Azetidin
may be history only. Orgsyn doesn't mention it among the decent methods
http://www.orgsyn.org/demo.aspx?prep=CV6P0075

I've re-estimated the heat of formation and density by hand and get now slightly better performance than for azetidines, but the uncertainty is big.

 m/s    +s    O2:100   kg/m3    Bp °C   kJ/mol  -diazaspiroheptane
-------------------------------------------------------------------
3370    +6      215     998     +241     +398    dicyclopropyl-
3373    +6      213     972     +200     +308    cyclopropylmethyl-
3376    +6      211     939     +154     +218    dimethyl-
3313    REF     283     834     +197             Rg-1 as reference
-------------------------------------------------------------------
 m/s    +s    O2:100   kg/m3    Bp °C   kJ/mol  -diazaspiroheptane

Marc Schaefer, aka Enthalpy

[Enthalpy]

Dave et al synthesized a diazaladderane by photodimerization of an azetine:
 "Synthesis of syn-Diazatricyclooctane and anti-Diazatricyclooctane", reported in
 "Recent Advances in the Synthesis of Cyclobutanes by Olefin [2+2] Photocycloaddition Reactions"
 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5025837/pdf/cr5b00723.pdf

Acetyls from the original synthesis are undesired in a fuel, but hopefully methyls can replace them before or after the cycloaddition.

The di(methylaza)-3-ladderane gains 5s over RG-1, according to my hand-estimated ΔHf. That's only as good as diazetidylcyclopropane, which seems to be easier to produce, more stable at heat and less flammable, but the azaladderane may be denser.

Dave et al used a Hg lamp and a sensitizer. A Xe excimer lamp at 172nm may give the cycloaddition a decent quantum efficiency.

The pure syn compound is only 0.5s better than the mix, so the ease of production or the melting point can decide.

The cyclopropyl methyl is 1s worse than the dimethyl but less flammable. A mix should favor the liquid. The cyclopropylaza (single aza) is as efficient as the di(methylaza), less flammable, but less easy to produce.

The toxicity is unknown. Aziridine is ugly and azetidine far less, hence the aza at the less strained positions here.

Marc Schaefer, aka Enthalpy

[Enthalpy]

In the synthesis of diazetidylcyclopropane of May 16, 2016 here, the second HX elimination from dihalocyclopropane and azetidine is uncertain, as halogens bind strongly to cyclopropyl but the cycle may open. The more reactive gem-dihalo give the first HX elimination better chances.

In case this step stops at azetidylhalocyclopropane, I suggest to continue to bis(azetidylcyclopropyl) C1CCN1C2(CC2)C3(CC3)N4CCC4 as in the appended sketch. From my hand estimation of ΔH_f=+342kJ/mol, the compound is almost as efficient. It is very little volatile (bp +256°C estimated by software) hence little flammable. The melting point of the heavy (C+N=14) symmetric molecule with easy rotations is unclear.

I've written iodine for reactivity. Bromine too would be recycled for cost. Coupling is unclear: by a metal, it should be chosen for easy recycling. Gaseous potassium seems caring, as it closes very fragile molecules. Or break the C-X by light?

Marc Schaefer, aka Enthalpy

[Enthalpy]

Diazetidylcyclobutane must have nearly the performance of diazetidylcyclopropane and improve the flash point and vapor pressure, with excellent density too, and possibly an easier synthesis. The melting point is unpredictable, just C+N=12, asymmetry and easy rotations are promising, and diazetidylcyclobutane with diazetidylcyclopropane might build a eutectic.

What do you think of my synthesis ramblings, especially the dehydration of an amine? Dessicated alumina, dry base? Replace OH by Br first, then eliminate HBr? If a dehydrating acid neutralises the amine, I hope the amine can be freed at the end.

I haven't found neither when dehydration of alcohols makes an alkene and when it makes an ether. Different solvents?

I believe amination across acetylene proceeds directly to the unwanted diaminoalkene and is no alternative, hence the dihaloethanes prior to HX elimination or the haloethenes, where both amination or HX elimination are acceptable as the first step.

Or should azetidine be cyclized around etheneamine?

Marc Schaefer, aka Enthalpy

[Enthalpy]

[...] when dehydration of alcohols makes an alkene and when it makes an ether [...]

Neuman 9.4A: use a low concentration of alcohol.

I'm sure you all knew it but consider me a desperate case.

[wildfyr]

A major problem is that alkyl halides are expensive and generally nasty

[Enthalpy]

I suggested here on 31 July 2016 to close diazaspiroheptanes rings from dibromo and a primary amine, and this is what reports
  Burkhard's thesis scheme 8 page 36
for the second ring of diazaspiroheptane, with 90% yield, wow.

So the N, N' dicyclopropyl is a quite serious fuel candidate, optionally mixed with some cyclopropylmethyl and not too much dimethyl (C+N=9 is flammable). Production just like the scheme I proposed then, without my "wild extrapolation" comment.

==========

Hi Wildfyr, a a rocket fuel production wouldn't throw expensive bromine away, you're right. The by-produced HBr wouldn't even be transported to a supplier, but recycled on-site to make pentaeerythrityl tetrabromide from pentaeerythritol. The added material cost is zero, it means only additional operations.

[Enthalpy]

The idealized path for Br or Cl recycling in diazaspiroheptanes production is shown here under. But some halogen will exit as other compounds, needing more subtle processing.

Far better: react the amine with pentaerythritol with no halogen step (same drawing). Known reaction, stable compounds, heat is cheap, so try hard.

==========

Rocket design may prefer a mix of dicyclopropyl-diazetidine with some cyclopropylmethyl-:

• If the dicyclopropyl has a bad melting point. Cyclopropylmethyl- must improve it, a possible eutectic even more.
• Because methylamine is cheaper than cyclopropylamine.

Just mixing methylamine with cyclopropylamine would also produce volatile and flammable dimethyl-diazetidine, of which I prefer none. Better than removing produced dimethyl-, I'd (second drawing) first react with cyclopropylamine, remove the unreacted pentaerythritol, then react with methylamine.

Can the amination stop at a secondary amine? Four-membered ring close uneasily, but secondary amines are more reactive. The reported 90% yield suggests swift ring closure.

==========

This makes a small research project, possibly for students.

• Synthesise the dicyclopropyl-, cyclopropylmethyl- and dimethyl-diazetidine.
• Produce multi-kg amounts to convince rocket designers.
• Characterize the compounds: Mp, Bp, Ig, reliable Fp, accurate ΔHf, ρ, η at cold too, odour.
• Based on Mp, Fp, η, odour, decide if mixing cyclopropylmethyl-, maybe dimethyl-.
• Search for eutectics, characterize.

And Diazetidylcyclobutane too makes a project. Path proposed here on 03 Aug 2023, needs light sources. Same tasks but no mix.

Diazetidylcyclopropane can make an even smaller research project. Buy dihalocyclopropane and azetidine, synthesize in one step as on 16 May 2016. Same tasks. Try also and characterize mixes with diazetidylcyclobutane.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Among the characterized attributes, I forgot the sensitivity to shocks. Toxicity can come later.

==========

Bis(azetidylcyclopropyl) proposed here on 24 Feb 2020 should belong to an investigation of diazetidylcyclopropane, Stop earlier the second amination to diazetidylcyclopropane, don't separate, switch to Wurtz, measure each product and their mixes.

[Enthalpy]

A bit of nitrogen improves a fuel's performance and can make it hypergolic, so here are more azaladderanes.

I suggested there how to produce dihydroazete
  chemicalforums
but a volatile amine is nasty, so a heavier variant is depicted.

Depicted lower is a partial idea to prevent N-N bonds using the intermediate obtained after Kharash addition. Maybe.

In such syntheses, perhaps dilute hydrogen peroxide can selectively destroy the hydrazines.

Marc Schaefer, aka Enthalpy