Question concerning the radical bromination of an alkyl halide
Hello folks,
Albeit with limited chemistry knowledge, my interest lies in finding a more convenient process (reagent wise); where the side product produced from
the cleavage of an aryl alkyl ether with HBr is recycled to form the methylene bridge of the previously demethylated catechol.
With that out of the way, could the slow addition of elemental bromine at a temperature which never warms up higher than -20C to a flask containing
liquid methyl bromide which sits in a dry ice/acetone cooling bath result in the appreciable formation of Dibromomethane and HBr if equimolar amount
of reagents are used ?
The reaction initiated with uv light to promote homolytic fission of Br2 which would then propagate subsequent free radical halogenations.
Notes:
1. The UV light would be emitted by a clear ~10w mercury-vapor 'UVC Germicidal' lamp placed into a quartz test tube which protrudes through the 45/50
center neck of a 3L 3-neck round bottom flask and is wrapped at the end with ptfe tape in order to create a short tight seal on the vial when fully
inserted.
Furthermore, a clamp may provide additional support, perhaps to a custom made test tube, nevertheless different diameters could be accommodated with
smaller joint adapters if available (34/35, 40/38). Do note that the narrow end of a 45/50 neck is 41mm wide as explained here.
Dimensions of items as observed online:
UV light: ~28mm outer diameter
Quartz test tube: 31, 35, 40mm outer diameter
2. From the documents attached below, it would appear that the reaction would most idealy proceed by the uv radiation of wavelenghts around 265nm
although as some other member has previously pointed out...
Quote: Originally posted by not_important  | | Chlorine radical reactions are triggered by light in the 300 to 380 nm range, Pyrex and similar borosilicates transmit down 280 nm or so; there's no
need for quartz unless the reaction requires that extra energy in the radicals formed. BTW, bromine absorbs in the range 360 to 510 nm.
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3. Due to the nature of the experiment which takes place under a fumehood, a welding helmet set on the 12-13 shade is worn throughout the procedure
while additionally covering any and all exposed skin with thick fabric like leather. Having some sort of human-size separator out of anything opaque
enough could also prove useful.
4. In spite of the freezing temperatures mentionned, I've attached an article describing the hydrolysis of acetone to acetic acid at 30-40C under uv
radiation of 313nm. Insulating the reaction vessel with aluminium foil could be more than sufficient in preventing this and may help a little as well.
5. After an unknown number of hours following the complete addition through a pressure equalizing addition funnel, the light is turned off and
neutralization with NaHCO3 begins. The closed system is then brought back to room temp while connected to a 'funnel-and-beaker' trap with a 25% NaOH
solution just in case any MeBr was left unreacted.
A flow of inert gas allows for the displacement of the heavy yet volatile alkyl halide. The dihalide product is fractionally distilled and dried over
anhydrous magnesium sulfate after a vacuum filtration with a Büchner funnel to separate it from the bromide salt.
6. Could the formation of Bromate anion occur if ozone or moisture were to enter into solution, resulting in lower yields ? Due to time limitations, I
have yet to find the answer but if it were the case, I would dry the reactants, bake the glassware for 20min in an oven at 120C and work under a
stream of argon until the uv light is turned off.
7. The gaseous methyl bromide used was previously recovered by allowing the reaction vessel (in the demethylation step) to warm up to 10-15C, CH3Br
separated from the solution already under inert atmosphere and began condensation through a distillation apparatus leading into a collection flask via
a cold trap (dry ice/acetone).
Myths or fiction ?
Could someone explain to me why I've been reading about risks of power shortage when I presume Chlorine gas is exposed to your power bar/outlet or
explosions if bromine/NBS are exposed to direct sunlight, isn't that the whole point ?
I'm asking for safety reasons as I was unable to find more info on these topics as of now. So many things to research and all..
Attachment: Photodissociation of bromine molecule near 265 nm.pdf (263kB)
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Attachment: Studies on photodissociation of alkyl bromides at 234 and 267 nm.pdf (204kB)
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Attachment: Hydrolysis of Acetone in Ultra-Violet Light (1931).pdf (217kB)
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