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Author: Subject: Allyl Bromide Preparation
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[*] posted on 10-12-2019 at 13:49
Allyl Bromide Preparation

Allyl alcohol was prepared from glycerol and formic acid under inert atmosphere, hydrolysed with NaOH and fractionally distilled to yield the 73 % allyl alcohol water azeotrope. This was then reacted with 48 % hydrobromic acid and sulfuric acid and the allyl bromide distilled as per the conventional method. It was then redistilled with 3A molecular sieves drying agent to yield the final product which is stored over additional 3A molecular sieves. Allyl bromide I’ve prepared in this way has been used as is, without further drying nor refinement, for the preparation of Grignard reagent and in Grignard reactions, so has been proven suitable for such application(s).

The allyl alcohol preparation is based on patent: WO2008092115A1
“Conversion of glycerol from biodiesel production to allyl alcohol”

Allyl alcohol is an irritant and potent lachrymator, therefore suitable overhead ventilation was used.

This is effectively a rudimentary distilling exercise in inert atmosphere, with reaction temperature measurement in addition to the usual still head temperature measurement for distilling.
Basic equipment was used as depicted in the images, whilst I used a 500 ml 3 neck flask for convenience, an ordinary 500 ml RBF could easily have sufficed with the appropriate fittings. Reaction mixture temperature was measured with a thermocouple probe in a sealed glass tube mounted using a thermometer adapter.

Allyl Alcohol Preparation (2 runs in total)

Reagents per run:
165 g glycerol
144 g 85 % formic acid (3 distillation cycles, 48 g per cycle)

(Therefore a combined total of 330 g glycerol, and 288 g 85 % formic acid were used in this experiment.)

Argon was used for the inert atmosphere, though only really necessary at the start before the vapour displaces the air.

1st cycle
165 g glycerol was added to the 500 ml RBF, along with 48 g 85 % formic acid. Stir bar also added.
Setup for distillation with condenser, vigreux column, addition funnel, and still head thermometer. During the distillation, reflux/air cooling from the column helps to keep the formic acid from distilling, increasing yields by about 15 % or so compared to when I’ve used without.

Start : Air is displaced with argon. Mantle heat is set at about 65 – 75 % of maximum, and maintained at this setting. As the solution heats, the viscosity decreases to allow the stir bar to stir, stirring was started as soon as it was possible to do so.
Target reaction temperature I’ve used is about 235 – 245 C, attained after water distils off.

T + 15 minutes: Reaction temperature = 130 C, still head temperature = 100 C, water distilling.
T + 45 minutes: Reaction temperature = 175 C, still head temperature = 100 C, water distilling.
T + 52 minutes: Reaction temperature = 216 C, still head temperature = 99 C, water distilling, approx. 15 ml collected.
T + 55 minutes: Reaction temperature = 224 C, still head temperature unstable, varying between 91 – 97 C, CO2 evolution, though no significant frothing.
T + 68 minutes: Reaction temperature = 240 C, still head temperature at 91 C, heat setting reduced to 60 %. Approx. 25 ml collected. Product distilling at about 1.5 drips per second.
T + 73 minutes: Reaction temperature = 240 C, still head temperature at 86 C, approximately 1.5 drips per second of distillate.
T + 90 minutes: Reaction temperature = 246 C, approximately 55 ml collected. Heat is turned off and allowed to cool down to below 200 C. Thus concluding the first of three cycles. (The water distillate wasn’t discarded, all the distillate was kept together. Water will be needed for the hydrolysis later.)

90 minutes per cycle.

Another 48 g of 85 % formic acid is added to the addition funnel, and then added to the reaction mixture over about 1 minute. The heating is then turned on as before, and the cycle is repeated, and then repeated again with the remaining 48 g 85 % formic acid.
At the end of the 3rd cycle, there was just under 200 ml distillate, and about 20 ml or so of dark orange liquid remaining in the distilling flask.
Total elapsed time was about 5 hours.

The above was repeated the next day, with another 165 g glycerol, and 144 g 85 % formic acid. Total combined distillate was about 400 ml.

*No further inert atmosphere required*

Distillate hydrolysis / allyl formate decomposition
1 hr reflux with 80 g NaOH

The RBF used for the distillation was cooled, cleaned out, and the clear distillate (~400 ml) added. The RBF was placed in a water bath, and 80 g NaOH slowly added in 3 portions.

Caution: about half of the NaOH will boil the mixture without cooling, so external cooling is essential, or a prolonged delay between additions.

The glassware was setup for reflux, and refluxed for just over 1 hr.

By the end of the reflux/hydrolysis, the solution was dark red-burgundy coloured.
The allyl alcohol – water azeotrope was then fractionally distilled, slowly, from the dark alkaline solution, collecting the fraction 87 C – 90 C.

(The pure 72.9 % azeotrope boils at 88.2 C, and has a density of 0.905 g/cc).

Distillation required about 2.5 hours, with 200 g collected. This was then redistilled, to collect the constant boiling azeotrope, this required about 1 hour (in previous runs, this re-distillation was omitted without problems, so it isn’t essential for allyl bromide preparation).

Total yield = 192 g. Density = 0.90 g/cc.

It was then transferred to an amber storage bottle, and the internal air displaced with argon to attenuate the formation of explosive peroxide(s).

No titration was carried out to determine precise purity of the allyl alcohol, however previous experience has shown that it is sufficiently pure for the preparation of allyl bromide in the following step.

Only 100 g of the 73 % allyl alcohol was used for the following allyl bromide preparation, the remaining 92 g was kept aside for future experiments.

Allyl Bromide Preparation

321 g 48 % hydrobromic acid
86 g >95 % sulfuric acid (x 2)
100 g 73 % allyl alcohol
3A molecular sieve was used as drying agent, though other drying agents could be used instead.

Glassware/equipment is setup for distillation, with addition funnel. Vigreux column was used, but probably not necessary.
With magnetic stirrer on, 321 g 48 % hydrobromic acid, 86 g sulfuric acid and 100g 73 % allyl alcohol were added to the RBF, in that order. After they are added the solution colour was orange.
Note: sulfuric acid will readily polymerise allyl alcohol to red tar, so it’s important to get the order of addition correct!

A further 86 g sulfuric acid (purple drain unblocker) was added to the addition funnel.

With the equipment setup for distillation (including still head thermometer), with vigorous magnetic stirring the additional 86 g sulfuric acid was slowly added over about 3 minutes. This caused the solution to heat up and distil slightly, the solution colour changed to a darker orange. After a few more minutes when the distillation subsided, the heating was started on medium heat. Care was taken to ensure the vapour didn’t overwhelm the condenser.
This required about 30 minutes, still head thermometer reading gradually increasing to about 65 C or so. When water starts to come over and sits on the allyl bromide, it’s about time to stop.

(The small amount of tar that remains in the distilling flask readily dissolves in and was cleaned using white spirits.)

All of the distillate was added to a 125 ml separating funnel, and washed with 20 ml of 5 % K2CO3 solution.

After settling, the lower allyl bromide layer was drawn off and added to a dry 500 ml RBF in preparation for the final distillation.

Note: Pure allyl bromide bp is 70-71 C (ref: chemspider)

Equipment is setup for fractional distillation (w/vigreux column) and distillation started. After the lower boiling fraction, up to around 68-69 C or so, has come over, some water will have also and can be seen floating as droplets on the surface of the distillate. At this point the distillation was stopped, cooled and then 2 teaspoons of 3A molecular sieve added to the RBF. Dried by allowing to stir for 15 minutes. The lower boiling fraction is discarded, the receiver cleaned and dried.
The distillation was then continued, slowly (about 1 drip/second). Collecting the pure allyl bromide fraction.

Final yield was 111 g, density measured to be 1.4 g/cc. This was then transferred to a dry jar containing 8 g 3A molecular sieve, then placed in a freezer for storage. No inert atmosphere has been used for storage thus far.
I’ve stored water clear allyl bromide in this way for 2 months and counting, and with no discernible discolouration nor decomposition.

Attachment: Allyl Alcohol from Glycerol Patent.pdf (124kB)
This file has been downloaded 8 times

[Edited on 11-12-2019 by CycloKnight]
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[*] posted on 10-12-2019 at 14:05

Nice write up CycloKnight! I have often contemplated carrying out this preparation but was put off by my perceived risk of ending up with a flask full of black c**p. But you seem to end up with only a small amount of dark residue. Is it water soluble?
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[*] posted on 10-12-2019 at 19:58

epic x))))
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[*] posted on 11-12-2019 at 02:41

A very nice preparation, thanks for sharing!

I would have thought that allyl-alcohol and sulfuric acid would produce tar even if diluted by HBr but it seems this is not the case. Probably the water in 48% HBr is enough to inhibit polymerization.

Why is substitution of the OH with Br is preferred over say HBr addition to the double bond? Water in the system?
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[*] posted on 11-12-2019 at 05:26

Quote: Originally posted by Pumukli  

Why is substitution of the OH with Br is preferred over say HBr addition to the double bond? Water in the system?

The mechanism for allyl bromide formation is loss of H2O from the protonated alcohol to form an allyl cation, followed by addition of the bromide nucleophile. SN1 mechanisms are favored for allylic alcohols due to the delocalization (and thus stabilization) of the cation.

HBr addition to the double bond could possibly take place, but this would be relatively disfavored. The alkene, instead of the alcohol, would have to be protonated, and this is much less likely to happen.

As below, so above.
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[*] posted on 11-12-2019 at 14:26

CycloKnight beautiful and thank you for sharing. Allylalcohol synthesis from glycerol and formic acid (or dehydrated oxalic acid) is something I would like to try in the future though I have 500 ml of allyl alcohol and 1 l of allylbromide. My allylalcohol is in big sealed glass ampule and I do not have hearth to break it. But I would like to synthesize e.g. allylpentanoate (pineapple scent not found in nature unlike natural ethylbutyrate, ethylhexanoate).
One step interested me - using inert gas for expelling oxygen (in the patent attached they used nitrogen) - do you think it is really necessary and does it increase yield significantly? You did it only once at the beginning, never at repeated steps. When your glassware cooled down and vapors condensed, by my opinion some air entered you flask. If there would be 500 ml of air inside at the beginning, that is approximately 0,1 g of O2. Does it hurt or not? Perhaps it could induce acrolein formation?
I always try to reduce unnecessary steps, but perhaps it could be wrong in this synthesis and I should follow every step carefully and use argon or nitrogen too.
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[*] posted on 11-12-2019 at 16:41

Nice work. Plan on doing a write up on the impending Grignard reaction? :)
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