[Enthalpy]

Hello the experts!

You all know the halogenation by free radical reaction:
(1) X-X -> X° + °X by light
(2) R-H + °X -> R° + H-X
(3) R° + X-X -> R-X + X°
nice, fine, but with some limits. Chlorine makes subsequent reactions difficult, iodine doesn't react, and bromine can be slow or impractical at some substrates R (...like cyclopropane and the assimilated spiropentane for instance).

Still according to textbooks, this is because the endothermal limiting step (2) is too difficult when the R-H bond is strong. The appended tables give bond energies by the halogens (where I included hydrogen) and between C and H in varied cases, which explain why t-butane (38kJ short) is brominated quickly but methane (72kJ) slowly.

----------

For difficult brominations, I suggest use hydrogen radical as the step (2) halogen, and keep Br₂ at step (3):
(2) R-H + °H -> R° + H-H
(3) R° + Br-Br -> R-Br + Br°
H° is marginally stronger than Cl°, just 9kJ short on cyclopropane, so it should create R° quickly, and the exothermal step (3) is easy anyway.

H° shall result from HI photolysis. I seek no chain reaction, just one photon to start one bromination. The adjusted relative abundances are R-H > Br-Br > H-I > R-Br, I-I >> H°, I°, R°.

• H° disappears quickly in H-H thanks to R-H.
• H° reacts faster with Br-Br but the abundances shall compensate. It would also react with I-I.
• H° encouters seldom H°, I°, R°.
• H-H may recreate some R-H (39kJ short).
• I° is unreactive anyway. It may form a little R-I which doesn't hurt and probably disappears. I-I is the expected fate.
• Br-Br becomes R-Br and Br°.
• Br° doesn't react with H-H nor this R-H, finds little H°, may create some Br-I, but more probably ends as Br-Br or a second R-Br.
• H-I is kept less abundant than Br-Br so it doesn't preeempt R°.

The proportions of R-H, Br-Br, H-I must be adjusted and R-Br, H-H, I-I removed as the reaction proceeds, which is normal at a production site but a drawback in a lab.

To compare with, Cl° reacts a few times more with (CH₃)₃C-H (-28kJ or -43kJ) than with C₂H₅-H (-9kJ or -22kJ) so I hope the selectivity of H° towards Br-Br (-173kJ, react every time) versus C₃H₅-H (+9kJ, exp(-E/kT)=0.08) remains manageable through the Br-Br abundance.

Chemists don't need the warning: this is paperwork, take with due mistrust.

UV absorption spectra are to come, enlightening the choice of the reactants.
Marc Schaefer, aka Enthalpy

[Enthalpy]

The absorption spectra of chlorides, bromides and iodides, compared with good light sources, are appended.

• Hg lamps are powerful (>10kW), reliable (16,000h) and efficient (40%).
• Among Hg, high-pressure lamps have a broad spectrum.
• Xe₂ is efficient (40%) but cyclopropane absorbs the light.
  
• Some excimer lamps like KrCl are decent.
• GaI seems good.
• Low-pressure Na is excellent but at 589nm.
• Powerful (60W light) UV Led panels (Ushio) are recent and promising.

The superior low-pressure Hg lamps provide H° from H-I, don't touch the alkane, excite the bromoalkane little (it recreates anyway) and Br-Br little (wastes light); I-I is 8 times less sensitive than H-I and should be removed. Good combination.

Could H-BR provide the H°? Uneasy. The produced bromoalkane absorbs light like H-Br does; the absorption by Br-Br suggests a Xe₂ lamp, or even ArF at 193nm. Less seducing, but at least I-I doesn't bother.

Could Cl° make the hard step instead of H°? Yes, if obtained from I-Cl (LP Hg lamp) or Br-Cl (Hg lamp but Br-Br absorbs, or XeBr, XeCl but Cl-Cl absorbs). Or just from Cl-Cl, with XeCl or filtered HP Hg lamps, both inefficient; future Leds may solve it. Less seducing because of the lamps, but Cl₂+Br₂ is known for difficult photobrominations, allegedly through BrCl.

Could we iodinate that way? H-I and I-I with a LP Hg lamp, or I-Cl and I-I with a filtered HP Hg lamp or Leds? I suppose not. Cl₂+I₂ is known to make only the chloroalkanes; the iodides are so unfavourable that I believe every H°, Cl°, Br° destroys them as they form (other explanations exist). But if the bromination works, the iodination is a trial worth.

Alternatives are known, like hypobromites.

Comments desired as usual!
Marc Schaefer, aka Enthalpy

[pgk]

It seems a nice idea.
As an additional information, bond dissociation energies correspond to characteristic radiation wave lengths that can be taken from tables or calculated in detail, as follows:
E= hv = hc/λ.
Thus, by using the appropriate UV lamps and the appropriate UV filters, the photohalogenation might be convenient. (Take care on the influence of the solvent to the absorbed wavelength.)
Besides, activated hydrogenation cannot be considered as safe and hypobromites are oxidation agents.

[Enthalpy]

I too had hoped at a relationship between the excited state's energy and the wavelength, largely misled by textbooks... Alas, molecules need photons far more energetic than the transition would suggest, as a dire reality. Nor are absorption spectra as simple as in books.

Halogenations are generally conducted in gas phase, and this was my intent - I could have emphasized it. One reason if using an efficient wavelength is that a gas is opaque enough, a liquid wouldn't renew the reactants quickly; an other reason is that solvents may well be opaque or get activated; and still an other reason, that solvents (except perfluoroalkanes) probably participate in reactions where radical halogens are present.

[pgk]

Please, take a look in a UV spctroscopy textbook (an older edition, in preference) and read about the influence of the solvent on the absorbed wavelength.This will help you, a lot.

[Enthalpy]

No solvent here, it's all gas phase, as usual in photohalogenations.

[pgk]

OK. Sorry, for the misunderstanding.

[Enthalpy]

Could hydrogen peroxide make alcohols from alkanes, similarly to the free-radical halogenation? Figures suggest yes, but the compounds will do what they want, as usual.

Permanganates, chromic acid, singlet oxygen, ozone are known for that goal, as well as Fenton's reaction
https://en.wikipedia.org/wiki/Fenton%27s_reagent
which all operate in liquid phase, while here gasses shall react.

To the previous table of bond dissociation energies, I've added -OH bonds taken gratefully from
www.nist.gov/data/nsrds/NSRDS-NBS31.pdf
and the new appended table suggests that

• Hydroxyl is more reactive than chlorine, halfway towards fluorine. No selectivity to expect.
  
• OH can abstract H from an alkane without any help from heat. Consistently, concentrated peroxide is known to ignite kerosene by contact.
• The peroxide should better be dilute and the reactor cold.

No step in the free radical chain reaction is limited by a need for heat, so it should diverge as it does with fluorine. Potential answers:

• Introduce slowly the pre-dilute peroxide in a kind of torch;
• I hope to extinguish the chain reaction using a bit of Br₂ or HOBr, maybe iodine compounds.

Some carbyls hit a brominated compound, and the resulting bromine atom can't abstract a hydrogen from the alkane at room temperature so this chain stops. Then, the proportion of brominated compound and the light intensity shall give some control over the reaction rate - maybe.

Being so reactive, hydroxyls can put functions at difficult positions, maybe on cyclopropane. An alcohol can then be converted to a bromide for instance, but other reactions are known for that goal, for instance using hypobromides or Cl₂+Br₂.

Gasses permit to evacuate mono-alcohols as they form, because these condense much more easily than alkanes do, and the reaction with peroxide needs no heat. So while no selectivity is expected among isomers, at least the degree of oxidation could be better controlled than through other reactions.

-----

I've put in the appended figure absorption spectra downloaded from MPI in Mainz
http://satellite.mpic.de/spectral_atlas (thank you!)
and it suggests that:

• Xe₂ lamps at 172nm would destroy the produced alcohols. Better KrCl or low-pressure Hg lamps.
  
• Water vapour is acceptable in the mixture. Good news, it doesn't need concentrated peroxide.
• 20mbar of H₂O₂ at 300K absorb 254nm light from low-pressure Hg over 0.3m.
  
• HOBr absorbs longer wavelengths and might serve both as an initiator and a quencher, or maybe bromine if it reacts with the peroxide.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Cyclopropane, spiropentane and some more are difficult to halogenate, even with chlorine, both because their C-H bonds are too strong and because they open C-C bonds. Here is an alternative - prospective - attempt where light from an Ar₂ lamp at 126nm shall break the C-H bonds instead.

Ar₂ excimer lamps are commercially available but less powerful than Hg or Xe2, and 5 to 15% power-efficient.

Does 126nm light really break C-H bonds? I've no data about it... Methane speaks in favour with 0.4x the absorption cross-section of propane. Similar sections for n-propane and cyclopropane, too. Though, absorption results from molecular orbitals, not from bonds, and is long finished when the atoms separate or rearrange.

If H° abstraction succeeds, the next step can be an H° capture or a dimerization - fine for me. Or the reactor can contain some halogen X₂, which both R° and H° break efficiently to make RX, HX, X°. Neither H° nor X° would abstract an °H from the candidate RH here.

I've no data about the 126nm absorption by halogens... A bold extrapolation of wavelength ratios from lower energies would put I₂'s 175-190nm peak around Kr₂ light for Br₂ and around Ar₂ light for Cl₂, with sections similar to the alkanes. As halogen splitting only wastes light, maintaining 1/10 the alkane pressure looks reasonable.

126nm absorption by HX and RX needs an extrapolation too. Maintaining the HX and RX pressure at 1/100 to 1/10 the alkane pressure should avoid their destruction.

In case this tentative wants to work, it can directly brominate or iodinate substrates restive even to chlorine.

Marc Schaefer, aka Enthalpy

[Enthalpy]

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
==============
558     H-C≡CH
497     H-OCF₃
---- Tools -----
529     H-C≡N
487     H-C₆F₅
472     H-C₆H₅
464     H-CF=CF₂
464     H-CH=CH₂
446     H-CH₂CF₃
--- Targets ----
452 ?   H-sPen
445     H-cPr
--- Compare ----
497     H-OH
431     H-Cl
366     H-Br
--- Compare ----
422     H-Pr
411     H-iPr
400     H-tBu
=============

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 recombine?

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.

----------

Light absorbed by a halide shall split the halogen from the sought radical. It's a safe assumption for °CH₂CF₃. 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 C₂N₂?

----------

CF₃CH₂Br is one sound example in the spectra appended in two messages. At KrCl's 222nm, its section is 2*10^-19cm², so 10^-2bar partial pressure at 298K attenuate by exp(1) in 0.2m, and CF₃CH₂I would enable lp-Hg lamps. The target hydrocarbon at 1bar and byproduced CF₃CH₃ 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^-1bar. Iodine must be kept a bit under 10^-1bar 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 CF₃CH₂I.

What does light to other hydrogen-greedy radical precursors do? I've no data. For instance C₆H₅-Cl, -Br and -I absorb 222nm light 1000× better than C₆H₆ 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

[Enthalpy]

Here the other spectra.

[Enthalpy]

Light can replace a haloatom or group with an other or suppress it by splitting the reactant. Or at least, the appended diagrams suggest it. Competitor reactions exist without light.

Among halides.

• Br Cl uses 222nm light from a good KrCl lamp to split R-Br. Then R° reacts with ambient Cl₂.
  R-Br shall not react with Cl₂ under the chosen conditions; some Br₂ reduces the risk.
  The example iPr-Br absorbs light with 3*10^-19cm², so 1bar at 298K attenuates by exp(1) in 1.4mm.
• Cl Br would need 193nm light from a less good ArF lamp, and the produced R-Br must be evacuated quickly.
  Cl₂ and Br₂ proportion shall be limited to save light, and R-Br shall not react with them.
• Cl I would be unreasonable. Only over Cl Br I.
• I Cl easily uses a good lp-Hg lamp or some other.
• Br I uses a KrCl lamp, but I₂ must be kept low and R-I evacuated.
• I Br easily uses a good lp-Hg lamp or some other.

A 405nm violet Led can measure the Br₂ concentration, a red Led I₂, and a 365nm Led Cl₂+Br₂.

Nitro group. NO₂ releases O at light hence shall be kept low.

• NO₂ Cl, Br or I uses a KrCl lamp, maybe lp-Hg or XeBr towards Cl and Br.
  Produced R-I must be evacuated quickly, R-I less so.
• I NO₂ easily uses a good lp-Hg lamp.
• Br NO₂ maybe with a KrF or lp-Hg lamp.
• Cl NO₂ only over Cl Br NO₂.

Amino group. But is there a safe source if NH₂, and what wavelength does it absorb?

• NH₂ Cl, Br or I uses a KrCl lamp, provided X2 doesn't react with the amine.
• I NH₂ using lp-Hg.
• Br NH₂ maybe, using KrF or lp-Hg, if evacuating R-NH₂ quickly.
• Cl NH₂ only over Cl Br NH₂.

Removing a halide or a group demands a better H source than H₂. 1,4-cylohexadiene is represented by 1,4-pentadiene on the diagram. Toluene seems less useful. HI and the more caring HBr look interesting.

• Cl H only over Cl Br H.
• Br H with a KrCl lamp if keeping cylohexadiene low.
• I H with a lp-Hg lamp.
• NO₂ H with a KrCl lamp, maybe longer wavelengths also.
• NH₂ H with a KrCl lamp.

Marc Schaefer, aka Enthalpy