Being no cheap industrial compound, the intermediate di(chloromethyl)ethene must be produced
The reference synthesis
takes pentarythritol, converts three hydroxyls to chlorides, the fourth to an acid, then eliminates by heat 1*C, 2*O, 1*H, 1*Clhttp://www.orgsyn.org/demo.aspx?prep=V75P0089http://www.dtic.mil/dtic/tr/fulltext/u2/a267508.pdf
Orgsyn calls direct halogenation
the "best alternative": 34% yield (starting from the monochloro), mixed products difficult to separate. Though, I feel it interesting for mass production
- Isobutene and chlorine are really cheap, even for a bad yield. Bromine less so (4k$/t), but it can be recovered from the waste.
- The reaction must be swift and low-tech. Excellent GaN 405nm Leds dissociate Br2, excellent AlGaN 385nm Leds and mp-Hg lamps dissociate Cl2.
- Accurate distillation is a small worry at big scale, and freezing instead must separate the dihalo isomers better.
Random substitutions at 6 allylic sites would give 20% desired distant dihalo, 13% vicinal dihalo, 36% less halogenated and 31% more halogenated products at optimum 33% per site, so the 34% yield reported with elemental chlorine is already better than random.
- Addition to the double bond is hence limited. Little halogen and much light are said to help. This is easily done in a continuous process. Cl2 molecules absorb 365nm light with 4*10-20cm2, or over 10mm at 1kPa and 300K, while Br2 absorbs 15 times better at 405nm and other species nothing.
- Allylic substitution is very selective over vinylic, and other reactions are visibly under control.
- Precipitation of the dihalo may already limit the number of halogenations in the reported yield. This improves if the reactor distills the species (drawing appended), so that only dihalo exits.
- Steric hindrance may already hamper the gem-dichloro, offsetting the bond dissociation energy that favours it. Bromine would improve both the hindrance and the bond dissociation energy.
Taking ethane as a model available in "Bond dissociation energies" from Yu-ran Luohttp://staff.ustc.edu.cn/~luo971/2010-91-CRC-BDEs-Tables.pdf
we see that a first chlorine makes a second hydrogen abstraction easier locally, while a first bromine brings no clear preference.BDE +-
420,5 1,3 CH3CH2-H
423,1 2,4 CH2ClCH2-H
406,6 1,5 CH3CHCl-H
397,9 5,0 CH3CHCl2-H
415,1 8,4 CH2BrCH2-H
415,0 2,7 CH3CHBr-H
397,1 5,0 CH3CBr2-H
Would you see better tricks? NBS and NCS are known for allylic halogenation, but as they allegedly create X2
first, steric hindrance should stay the same. Hypohalite acids? X-O-tBut? HX elimination bringing the double bond?
Comments, remarks, suggestions, even objections maybe
? Thank you!
Marc Schaefer, aka Enthalpy