The halogenation of spiropentane and cyclopropane
is difficult, with yields of 1/3 reported by Houben-Weyl and by
Applequist, Fanta and Henrikson in "The chlorination of spiropentane".
The C-H bond is strong in cyclopropane and spiropentane, 7kJ stronger than in methane according to Neumann, stronger than H-H and H-Cl.
Consistently, the reactions see a significant barrier. Nist publishes an activation energy for spiropentane and atomic oxygen to spiropentyl, thankshttp://kinetics.nist.gov/kinetics/ReactionSearch?r0=157404&r1=17778802&r2=0&r3=0&r4=0&p0=3352576&p1=83321226&p2=0&p3=0&p4=0&expandResults=true
from which I "deduced" other activation energies in the appended table - shifted according to the bond energies to hydrogen, which shouldn't be done.
The ring-opening addition to cyclopropane is exothermal. If this were an indication for the ease of reaction:
-212kJ, HBrO -187kJ approx, BrCl -184kJ, Br2
-125kJ, HCl -93kJ, H2
and ring-opened dichloro uses to be as abundent as chlorocyclo in the products.
Cyclopropane absorbs light at 174nm from Xe2
lamps. 193nm light from less good ArF lamps acts less at the target species, and could even harm bigger cyclopropanes, while 222nm from KrCl seems impossible.
These difficulties don't apply to cyclobutane, for which I still hope Xe2 light and water
achieve the oligomershttp://www.chemicalforums.com/index.php?topic=81721.msg297374#msg297374
among other potential methodshttp://www.chemicalforums.com/index.php?topic=50579.msg296973#msg296973
In the photochlorination process with least drawbacks
I saw, at least on paper (whatever meaningful this is), the species in the reactor are by decreasing abundance:
- 1. Spiropentane (or cyclopropane), for instance at atmospheric pressure - hopefully transparent.
- Gaseous HCl kept around 0,05bar. Absorbs 63% of ArF's 193nm in 100mm.
- The produced chloride, to be removed continuously since it absorbs light as much as HCl does. It condenses 50K earlier than the hydrocarbon.
- By-produced H2, to be kept lower than HCl.
- An optional bit of Br2 to quench chain reactions.
- Possibly some hydrocarbon dimer, and some self-destroying Cl2 that doesn't hurt in small amount.
- H°, Cl° and spiropentyl radicals.
The reaction shall proceed this way:
- 193nm light splits efficiently HCl into H° and Cl°.
- H° and Cl° abstract each an H from spiropentane to make HCl and H2. The activation energy would be estimated to 16kJ and 20kJ, and the reaction endothermal by 10kJ and 13kJ approximately, but the radicals are emitted excited.
- The spiropentyls react with HCl exothermally to make spiropentane back and the product chlorospiropentane.
If some bis-spiropentyl appears, fine! Strong concentrated light should favour it.
At best every second radical creates a chlorospiropentane, I take a third - but one 6.4eV photon makes two radicals, so it needs 14MJ of light to get 1kg of spiropentyl. The 15% power-efficient lamp uses then 25kWh/kg, and alone the electricity costs 2.5€/kg
Some attempts I set aside:
- Added Br2, more abundant than HCl, would absorb little light and react with the spiropentyl to yield bromide - but only if H° and Cl° didn't react first with Br2, which is easier than with spiropentane.
- H2O2 (towards alcohol), Cl2+Br2 and BrCl (towards bromide), HBrO (towards bromide) supposedly open the cycle like Cl2 does.
Also, photohalogenation papers are often older and use little light at Hg wavelengths to trigger long chain reactions. Present exciplex lamps can provide a strong photon per molecule, which creates excited radicals, so direct bromination or quenched chlorination could be tried again.
Marc Schaefer, aka Enthalpy