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Topic: Strained Amines Fuels  (Read 3868 times)

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Offline Enthalpy

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Strained Amines Fuels
« on: July 24, 2016, 01:53:59 PM »
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

Offline Enthalpy

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Re: Strained Amines Fuels
« Reply #1 on: July 31, 2016, 10:50:32 AM »
---------- 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

Offline Enthalpy

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Re: Strained Amines Fuels
« Reply #2 on: May 20, 2018, 09:12:48 AM »
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

Offline Enthalpy

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Re: Strained Amines Fuels
« Reply #3 on: February 23, 2020, 08:01:37 PM »
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 ΔHf=+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

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