[Enthalpy]

Thanks wildfyr!

Err, yes, H are implied - but I had to write them in CH₂O to distinguish from carbon monoxide.

[Enthalpy]

TheUnassuming hinted (thanks!) at efficient N-N coupling by O₂+catalyst or by electrolysis
http://www.chemicalforums.com/index.php?topic=97919.msg345959#msg345959
https://pubs.acs.org/doi/10.1021/jacs.8b05245 and https://pubs.acs.org/doi/10.1021/ja5013323
which achieves molecules as crowded as tetraphenylhydrazine, so cyclopropylmethylhydrazines look accessible. This needs a good path to dicyclopropylamine, as some previous compounds here do.

The dicyclopropyldimethyl would be slightly flammable but a mix with heavier compounds shall improve it and lower the freezing point, the isomers being welcome too.

The Osiris applet estimates that these hydrazines are harmless (but it considers Udmh as slightly carcinogenic).

The seemingly cheap N-N coupling would apply to other compounds here like diazetidyl.

Cyclopropyls are expected to weaken the N-N bond further. Bad for the heat stability, but it may improve the hypergolic ignition with N₂O₄.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Careful hand estimations of the heats of formation, as liquids at 298K, give all cyclopropylmethylhydrazines +7s specific impulse advantage over RG-1 "kerosene" rocket propellant.

The Osiris applet can't give certainty about the health risks of cyclopropylmethylhydrazines, which are 3s short of dangerous Mmh and Udmh but only 2s better than the supposedly safe diazetidylcyclopropane, so alternatives exist.

Marc Schaefer, aka Enthalpy

[Enthalpy]

I tried to estimate the heats of formation of cyclopropylated and cyclobutylated diazetidines, but insufficient data lets cumulate many uncertainties.

• Dicyclobutyl was measured by Katorgyn, so 1,2-dicyclobutylcyclobutane is accessible.
• To 1,2-dicyclopropylcyclobutane, I applied the same +46.2kJ/mol as from methylcyclobutane to methylcyclopropane.
• From tertiary C-C to N-N, I applied +251.4kJ/mol because it's +175.1 from propane to Mmh, +200.0 from butane, +203.1 from isobutane.
• And I supposed that the ring strain acts on diazetidine as on cyclobutane.

N,N'-Dicyclopropyl-1,2-diazetidine is then 10s better than RG-1, as efficient as Mmh. The Osiris applet doesn't consider it badly toxic, but that's only software. C+N=10 makes it flammable if lukewarm, similarly to turpentine.

Mixing some cyclobutylcyclopropyldiazetidine raises the flash point and depresses the melting point. Being 8s better than RG-1, it wastes little performance.

While matching the perfomance of Mmh would be an achievement, dicyclopropyldiazetidine can't replace it on existing rockets, as the mix ratios differ too much. The photocoupling synthesis I suggested on June 23, 2018 is hypothetic too.

Marc Schaefer, aka Enthalpy

[hollytara]

So the diazetidines:  typical C-C bond strength is 350 kJ/mol. typical N-N bond strength is much less - 165 kJ/mol.  Meanwhile, cyclobutane ring has strain energy of 110 kJ/mol - so the bond strength in the diazetidines will only be about 55 kJ/mol - that is 13 kcal/mol.  Bonds this weak can't be isolated at room temperature. 

I would be vaery careful with all of these compounds - put me in mind of mustard gases. 

[rolnor]

I dont think they are alkylating but hydrazines are never non-toxic. They could be impossible to synthesize and non-stable.

[Enthalpy]

Hi Rolnor, thanks for your interest!

One nice improvement here is that the hydrazines are far less volatile than Mmh or Udmh. If a leak occurs at a satellite or rocket, people won't lick it, but Mmh is as volatile as ethanol, ouch.

At least N, N'-dimethyl-1,2-diazetidine was synthesized, although in small amount using a path unsuitable to rocket amounts: ethylene bromide and sym-dimethylhydrazine, big dilution, low yield. It was stable enough to measure the 70°C boiling point. Appended is a screenshot of page numbered 28 in the report, is 39/123 in the Pdf
https://apps.dtic.mil/dtic/tr/fulltext/u2/093924.pdf

The page 80 of the document uses an estimated heat of formation for dimethyldiazetidine. I get far better figures for the estimated dicyclopropyl.

[Enthalpy]

Hi hollytara, nice to read you!

To my understanding, the strain energy spreads on four bonds (N-N may reacts differently to it, but I have no data), so with your figures, N-N would retain 137kJ. Accordingly, 1,2-diazetidines were already synthesized, see page numbered 28 in
https://apps.dtic.mil/dtic/tr/fulltext/u2/093924.pdf

The authors of the report didn't investigate this compound more. They boiled the dimethyl at 1atm and 70°C without detonation, but for instance sensitivity to shocks would be no-no.

[hollytara]

If they made it, they made it. I read about the cyclic hydrazine with one of the N's in a 4 membered ring - it was pretty reactive, and ring opened quite easily.   

[rolnor]

OK, the 3-membered is reactive I know, its maybe different when the nitrogen in the 4-membered ring is connected to another nitrogen atom. The mustard gas type of compounds are different with the ability to cross-link DNA etc.

[hollytara]

There is this issue of "neighboring group participation" which is what makes the mustards so reactive. 

[Enthalpy]

Synthesis of 1,2-diazetidines: a thesis exists
http://wrap.warwick.ac.uk/38106/7/WRAP_THESIS_Brown_2011b.pdf
especially page 18 of the pdf, marked page 8, gets 60% yield with
RNNR+BrCCBr -> R-diazetidine-R
where R is isopropyl, giving me hope for cyclopropyl.
Whether ethylene oxide is better?
I've seen no mention of photoaddition there.

Just below,
"These  diazetidines  were  found  to  be  very  stable  compounds."
but this is on a chemists' scale. Rockets are more demanding.

[sriram]

what if we try TMEDA or also TEMED as hypergolic with N2O4

[Enthalpy]

Hi sriram, thanks for your interest!

Most amines are hypergolic with tetroxide, tertiary amines better so, hydrazines even better. A rocket engine a very fast ignition by contact, and this is uncommon.

One reason is that accumulation of propellants in the chamber before ignition is a show stopper. An other reason is that hypergolic propellants don't demand a perfectly stable flame at the beginning of the chamber, since they can catch fire downstream occasionally if needed, and this eases the design of the chamber, and can make it smaller.

Additional choice criteria are (1) safety (2) safety and (3) safety. TMEDA with C+N=8 is as volatile and flammable as gasoline, nothing desired. PMDETA is good for this with C+N=11.
https://www.chemicalforums.com/index.php?topic=79637
Diamines mass-produced for polyamines are a good start too. Permethylating improves the liquid range, important.

Though, my goal is not hypergolicity, because nobody wants to keep the toxic and inefficient tetroxide. The future is liquid oxygen. I have interest in amines because they bring 2-3s better performance than the alkane homologues and are easier to produce. Among these, PMDETA is my standard choice for safety, easy production, liquid range, and some performance improvement over RG-1 "kerosene" (it's nearer to Diesel oil). More exotic compounds, including the hydrazines of doubtful harmlessness, are a speculative attempt at higher performance, exceeding the strained but probably sound amines like diazetidylcyclopropane
https://www.chemicalforums.com/index.php?topic=86972.0

From my ramblings, you can trust my estimates for the heat of formation and the performance. The boiling and flash points are reasonable. The melting points are always doubtful. The toxicity of the hydrazines is essentially unknown, most are strongly undesired, but the present thread exists because aminoguanidine was tested as a drug and the volunteers survived. And from the synthesis paths I propose, don't believe a word, I'm not a chemist.

Whether engines and stages will exist with such propellants? They gain 4-7s while methane gains 11s. I'd prefer a safer strained amine, storable and denser than methane, but SpaceX and Blue Origin have already adopted methane.
https://www.chemicalforums.com/index.php?topic=79637.msg337420#msg337420
The other option is to cool the engine with the liquid oxygen rather than the fuel: this enables better fuels and more pumping cycles where ethylene is a trivial choice, but other fuels can be more interesting.

[Enthalpy]

Synthesis of 1,2-diazetidines: [...]
RNNR+BrCCBr -> R-diazetidine-R
Whether ethylene oxide is better? [...]

Ahum. No, ethylene oxide doesn't react with both amines at once, quite the opposite.

However, I disagree with the usual "bromide hence expensive". To produce tons, the plant would catch the by-produced HBr and recycle it to produce the dibromoethane reactant, either by anti-Markovnikov addition on acetylene feed, or over Br₂ added to a safer ethylene feed.