Some more radicals bond strongly with hydrogen: °C6H5, °C6F5... They could abstract H
from cyclopropane, spiropentane or others and leave the rest ready to bind with the desired species, for instance a halogen. From Yu Ran Luo's "Bond Dissociation Energies":
BDE in kJ/mol
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452 ? H-sPen
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The energy balance (but it isn't an activation energy) of hydrogen transfer suggests that:
- Nitrile is very efficient everywhere (but toxic).
- Pentafluorophenyl may act on spiropentane as easily as chlorine atoms on isobutane.
- Phenyl on cyclopropane would be as easy too.
- Ethenyl and trifluoroethenyl are less brutal.
- Trifluoroethyl would be a bit stronger than chlorine. No data for H-C(CF3)3, worth a try.
Ethynyl precursors would detonate easily. Does HOCF3
If the transfer proceeds at RT or even below, it may preserve the spiropentane skeleton. For the products I seek (you know) I need no selectivity.
by a halide shall split the halogen from the sought radical. It's a safe assumption for °CH2
. Hopefully some unsaturated compounds in the list do it too. No chain reaction is expected, so the light source provides over one photon per product molecule.
Even (cold) atomic chlorine doesn't abstract hydrogen from the considered targets, so elemental halogen can be a reactant, if it doesn't absorb too much light; a pending carbon bond breaks the halogen molecule. If the haloatom separated from the hydrogen-greedy radical shall be the substitute at the target, then it replenishes the reactor naturally.
I've added NO2
to the halogens since obtaining nitro compounds in one step
can be useful. Other functional groups must be possible.
Light may split the produced halides too. They would form again from the available halogen molecules, but the effectiveness drops. The reactor shall separate continuously the species to maintain good proportions
- Target molecule.
- Halogen or similar. Less abundent so radicals meet the target most often.
- Precursor (often halide) or the hydrogen-greedy radical. Less abundent so the stripped target meets the halogen most often.
- All radicals are naturally scarce.
My next goal is to couple the target hydrocarbons by halide abstraction, so I'd have nothing against a variant where the hydrocarbyl radicals recombine with an other rather than with a halogen. Short concentrated light pulses and halogen elimination would help, but it's uneasy.
Regenerating a halide of the hydrogen-greedy radical means a higher energy barrier than halogenating the target, but higher temperature and less caring reactants like hypohalites are possible, and some reactions are known for instance for benzene. For nitriles it involves the salts, yuk - or does ArF light split C2
Br is one sound example in the spectra
appended in two messages. At KrCl's 222nm, its section is 2*10-19
, so 10-2
bar partial pressure at 298K attenuate by exp(1) in 0.2m, and CF3
I would enable lp-Hg lamps. The target hydrocarbon at 1bar and byproduced CF3
attenuate nothing. If the substituent is chlorine, both it and the produced chloride can stay at nearly 1bar in the reactor. If it's bromine, just the product must be kept around 10-1
bar. Iodine must be kept a bit under 10-1
bar but the produced iodide is less critical. A nitro produced that way must be removed quickly, or rather, it would use a lp-Hg lamp with CF3
What does light to other hydrogen-greedy radical precursors do? I've no data. For instance C6
-Cl, -Br and -I absorb 222nm light 1000× better than C6
does, but do they separate the haloatom, or is some aromatic transition shifted to longer waves? BrCN and ICN seem clearer, with absorption peaks at wavelengths known from other X-C bonds and where HCN is transparent.
Marc Schaefer, aka Enthalpy