Metacelsus
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Improved Synthesis of Benzyl Bromide by Free Radical Bromination
The purpose of this experiment was to determine if sodium bromate could regenerate bromine from hydrobromic acid produced during the bromination of
toluene.
Overall Reaction Equation: 5 BnH + 2 Br2 + BrO3- + H+ -> 5 BnBr + 3 H2O
Bromination: BnH + Br2 -> BnBr + H+ + Br-
Regeneration: 5 Br- + BrO3- + 6 H+ -> 3 H2O + 3 Br2
In a 500 mL Erlenmeyer flask, sodium bromate (15.09 g, 100 mmol) was dissolved in 50 milliliters of distilled water. Then, toluene (60 mL, 564 mmol)
was added. The flask was taken outside and placed in direct sunlight. A pressure-equalizing addition funnel was fitted to the top of the flask and
bromine (10.3 mL, 200 mmol) was added slowly with vigorous stirring. When the addition of the bromine was complete, 96% sulfuric acid (2.8 mL, 50
mmol) was added dropwise with vigorous stirring. The flask became noticeably warm and the bromine color gradually faded over the next 30 minutes. When
the bromine color was gone, the bottom layer of the reaction mixture (containing benzyl bromide) was removed in a separatory funnel and dried over
anhydrous magnesium sulfate. The benzyl bromide was collected by fractional distillation under vacuum (water aspirator, approx. 50 torr, b.p. 113-119
oC). Yield: 61.92 g, 43.0 mL, 362 mmol (72.4%).
A warning: Benzyl bromide is an extremely powerful and persistent lachrymator (worse than chloroacetone!). I learned this the hard
way while cleaning the glassware.
[Edited on 11-5-2014 by Cheddite Cheese]
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AvBaeyer
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Nice work and good write-up.
In a similar manner, I have manged to get good yields of benzyl bromide from toluene using NaBr and oxone in bright sunlight. Bromine is produced
instantly during the dropwise addition of an oxone solution to aqueous acidified NaBr and toluene with good mixing. The bromine is immediately
extracted into the toluene and is rapidly consumed in full sun. An excess of toluene was used to ensure mono-bromination. The reaction is very
exothermic so careful moderation is needed on any sizeable scale.
I have also used this method (NaBr/oxone) to brominate double bonds using methylene chloride as solvent. It really beats dealing with elemental
bromine.
AvB
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Metacelsus
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For me, it's easier to make sodium bromate and bromine (from sodium bromide via electrolysis) than it is to get oxone. Oxone is certainly a more
pleasant reagent, however.
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Nicodem
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Cheddite Cheese, thank you very much for you useful contribution.
Do you have any means of checking the product purity and most importantly reaction selectivity. I'm particularly interested in the ratio between the
radical and electrophilic bromination, which is highly sensitive to reaction conditions. It would be nice to see the impurity profile and compare it
to the classical procedure using elemental bromine (e.g., the procedure in Organikum uses Br2, 70% yield). Do you have access to HPLC, GC or
NMR?
I never used elemental bromine for any radical bromination (I always use NBS or dibromodimethylhydantoin - DBDMH). For most substrates bromine is
totally useless, as it gives electrophilic bromination products much faster than the radical bromination occurs. Toluene is a good exception to this.
I found that with NBS or DBDMH a normal CFL light bulb is most effective (when you can't use sunlight). Generally, Vis light is more effective than UV
for Wohl-Ziegler reactions. In my experience, UV at 256 nm is too slow at the same power and various mercury light bulbs require more power for the
same effect while giving more side products (though this is not important on simple substrates like toluene).
AvBaeyer, hydrogen peroxide instead of Oxone works for oxidizing bromide as well. I used it in biphasic reactions for electrophilic brominations.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Metacelsus
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I have limited access to NMR (it requires a trip to a local university). I will try to run a sample of the product by the end of the week.
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Electra
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Quote: Originally posted by AvBaeyer  | Nice work and good write-up.
In a similar manner, I have manged to get good yields of benzyl bromide from toluene using NaBr and oxone in bright sunlight. Bromine is produced
instantly during the dropwise addition of an oxone solution to aqueous acidified NaBr and toluene with good mixing. The bromine is immediately
extracted into the toluene and is rapidly consumed in full sun. An excess of toluene was used to ensure mono-bromination. The reaction is very
exothermic so careful moderation is needed on any sizeable scale.
I have also used this method (NaBr/oxone) to brominate double bonds using methylene chloride as solvent. It really beats dealing with elemental
bromine.
AvB |
I was wondering if anyone was going to post this  . Why do you use acidified NaBr?
I have never had to use any acid in the above reaction for excellent yields.
Technically the intermediate is hypobromous acid. What happens is Oxone/NaBr brominate the ring on toluene in the para position. The sunlight
initiates a radical SRN1 reaction. The aryl radical / bromine radical reacts with the methyl group on toluene, swapping out a hydrogen for the bromine. (see
step 7 in the above reaction scheme)
I've never had to use excess reagents to get very results (95%> . The ring
bromination step goes to near 100% yield in under 20 minutes at room temperature. The photostimulation takes a similar time period, and also goes to
near quantitative yield in a very short time, though with a very small amount of dibrominated toluene.
I'm not sure which part you are referring to being exothermic. I've only ever done this reaction on a relatively small scale, and I know the
Oxone/NaBr part by itself isn't too terribly exothermic. I wouldn't really know since most of my reactions that aren't heated at run in a temperature
regulating vessel. Oxone is one of the least heat releasing oxidizing agents, in comparison to some others, such as H2O2. I know it is very important
to use plenty of water to dissolve the oxone since it's solubility is very poor.
Edit:
Your yields could GREATLY improve if you didn't combine the steps. Benzyl Bromide decomposes in water. The key is to do the Oxone/NaBr step with the
toluene, then drain off the water, and then expose the mixture of para-bromotoluene to sunlight. This is how you get a near
quantitative yield by ensuring no benzyl bromide decomposition when it is formed.
[Edited on 13-5-2014 by Electra]
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Metacelsus
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If you look at the overall equation, acid is consumed. That's why I added the sulfuric acid.
The intermediacy of para-bromotoluene seems improbable to me. Do you have any references to support your claim?
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Electra
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| Quote: Originally posted by Cheddite Cheese | If you look at the overall equation, acid is consumed. That's why I added the sulfuric acid.
The intermediacy of para-bromotoluene seems improbable to me. Do you have any references to support your claim? |
I was referring to Ay's post, and yes I have a paper.
The para-bromotoluene intermediate is indeed what occurs. I have separated the reaction steps and verified each reaction product through various
methods. The literature on both of these steps is well documented.
Oxone/NaBr is a very famous and simple combination to brominate rings (or double bonds). Electron donating groups (such as methyl) always direct to
the para position. Such donating groups allow the ring reactions to finish very quickly, in comparison to aromatics with no EDG's. Attached below is a
paper "Direct halogenation of organic compounds with halides using oxone in water", which contains many examples.
Aromatic halides undergo very easy radical reactions in the presence of UV light. The reaction is called an SRN1 reaction. http://en.wikipedia.org/wiki/Radical-nucleophilic_aromatic_s...
This reaction is also very well documented using UV light. The radical intermediate either reacts with a nucleophile or it can abstract a hydrogen
from an alkane (step 7). This is the equivalent of light irradiation of other halogens in the presence of toluene except there is 0% chance of any
electrophilic reaction, and only completely efficient radical reactions take place.
I actually made this specific reaction process the focus of my most recent class project, to which I got a 100 on.
Attachment: 9c960520e39ed777ac (1).pdf (173kB)
This file has been downloaded 910 times
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AvBaeyer
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C. Cheese: Oxone is pretty easy to find out in here California given all the pools and hot tubs. Even the local ACE has it. Otherwise, it can be
easily and relatively cheaply had from online pool chemical suppliers.
Electra:
First, I need to clarify/correct my procedure as I made a mistake in my comments. I add a solution of sodium bromide to a mixture of toluene and
aqueous, acidified oxone while exposed to strong sunlight and with good stirring. I apologize for any confusion I caused. Depending on the
concentration and addition rate of the NaBr, the reaction can get very warm fast.
Why do I use acidified NaBr? The redox reactions involved are as follows:
HSO5- + 2H+ + 2e- => HSO4- + H2O
2Br- => Br2 + 2e-
Overall
HSO5- + 2H+ + 2Br- => HSO4- + H2O + Br2
(Lost my formatting here, hope you can see what I mean)
Acid is required for the oxidation of the halide. Even though oxone contains potassium bisulfate which contributes to the acidity of the reaction, I
prefer to add 2 moles of “protons” (eg 1 mole sulfuric acid) for each mole of halide to be oxidized. Things just seem to work better that way.
I use excess toluene to ensure that there is little if any dibrominated product. The separation of product from toluene is a trivial matter by
distillation, assuming GOOD ventilation. The benzyl bromide does not decompose in this reaction as it is virtually insoluble in the aqueous phase.
The above reaction is also great for a quick preparation of iodine from iodide on a useful scale.
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Electra
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Edit: The addition of acid causes this reaction to take an alternate pathway, both reach the same product, but the one without acid is far
less exothermic. They take relatively the same amount of time
Links I will refer to in this post
Direct halogenation of organic compounds with halides using oxone in water
Bromodecarboxylation of Arylpropiolic Acids with Oxone® and Sodium Bromide
Oxidations of Halide salts by oxone result in an electrophilic intermediate. Oxone causes a two electron oxidation. It cannot magically take one
electron from each bromide ion.
The electrophilic hypohalous intermediate is documented in both documents above. Oxone reacts with NaBr to form an electrophilic bromine intermediate
which brominates the ring. UV irradiation causes homolytic cleavage of the Aryl Halide forming an Aryl Radical and Bromine Radical in equilibrium. The
equilbrium under UV light lies far to the right for benzyl bromide. This is because the methyl group hydrogens are the most favored hydrogens in the
radical abstraction on toluene.
Here are the reactions:
* First reaction is in the second paper above
* OH- + Br+ is also written as HBrO
1 [2e- Oxidation] ) HSO5- + Br- ----> OH- + Br+ + SO4-
2 [Electrophilic Halogenation] ) Ph-CH3 + OH- + Br+ -> Br-Ph-CH3 + H2O
3 [SRN1 reaction] ) Br-Ph-CH3 + hv -> Br Radical + Ph-CH3 Radical
4 [Radical Exchange] ) Ph-CH3 Radical + Br Radical + Ph-CH3 -> Ph-CH3 + Ph- CH2Br
This specific synthesis reaction is what my final class project was about, to which I spent well over 15-20 hours of research.
Edit:
You don't need to add any acid to get the benzyl bromide. Acids (like sulfuric) are in equilibrium with Hypobromous Acid to form Sodium Hypobromite.
Which is in equilibrium with chlorine.
2 H+ + BrO− + Br− <----> Br2 + H2O
This reaction is driven to the right by the presence of acid, forming bromine. UV irradiation forms the bromide radicals. This is a waste of sulfuric
acid, which is nasty to handle anyways. There's not really a reason to use it because when you dont use it, the above reaction that I have layed out
takes place, in nearly the same time period, with the same, if not better yields. All that is required is 1/2 Mol Oxone, 1 Mol NaBr, and 1 Mol
Toluene.
[Edited on 13-5-2014 by Electra]
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Dr.Bob
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Here are some other preps, maybe some will be helpful. Interesting set of posts.
But I don't see any mention in the ones that I could open of the reaction going through the para-bromotoluene intermediate. Do you have any
references to that specific intermediate? I agree that radical conditions favors benzylic bromination, but don't understand the theory of the
bromine attacking at the para position first then transferring the bromine. if that were the case then 4-bromotoluene would form benzyl bromide on
exposure to sunlight, which I don't see any case of in the literature.
Also,
"The addition of acid causes this reaction to take an alternate pathway, both reach the same product, but the one without acid is far less exothermic.
They take relatively the same amount of time" -
Catalysis by acid or any reagent can not change the overall exothermic nature of a reaction. If the rate is the same, and the SM and product are the
same, ANY path must provide the same amount of heat produced. I don't understand your statement.
-------------
Preparation of benzyl bromide - Beska, Emanuel; Zuziova, Jozefina; Konecny, Vaclav; Sutoris, Viktor; Halgas, JanFrom Czech. (1987), CS 240391 B1 Feb
13, 1986. | Language: Slovak
PhCH2Br is prepd. by bromination of PhMe with Br and an oxidizing agent (such as Cl or SO2Cl2) catalyzed by sunlight or artificial light, or
eventually by radical-reaction catalysts (such as dibenzoyl peroxide, 1,1'-azobisformamide, or 1,1'-azobisbutyronitrile). Thus, 180 g PhMe was treated
with 2 drops of Br at 60° with stirring, the reaction mixt. thus was slowly added with 79 g Br in 1.5 h under simultaneous addn. of Cl at 0.3 g/min,
the resultant mixt. was heated to 100° to give 149.3 g PhCH2Br fraction contg. 3.8% PhCH2Cl.
-------------
Free-radical side-chain bromination of alkylaromatics in supercritical carbon dioxide (SC-CO2)
Tanko, James M.; Blackert, Joseph F.
Science (Washington, DC, United States) (1994), 263(5144), 203-5.
Direct bromination of toluene and ethylbenzene form the corresponding benzyl bromides in high yield. The obsd. selectivity in SC-CO2 is
similar to that obsd. in conventional org. solvents. Also, SC-CO2 is an effective alternative to carbon tetrachloride for use in the classical
Ziegler bromination with N-bromosuccinimide. Reaction yields are high, side products are minimized, and bromine-atom selectivities are
obsd. Thus, SC-CO2 must be useful as a viable, environmentally benign substitute for many of the solvents typically used for
free-radical reactions.
----------------
Side chain brominations by N-bromosuccinimide; IV. Novel and improved preparations by selective choice of solvents
Offermann, Werner; Voegtle, Fritz
Synthesis (1977), (4), 272-3.
Side-chain bromination of PhMe, m-C6H4Me2, and 1,3,5-Me3C6H3 with N-bromosuccinimide was improved by using
HCO2Me as a solvent in place of conventional CCl4 solvent, e.g., bromination of PhMe in HCO2Me gave PhCH2Br in 89% yield vs 67% yield for CCl4.
2,6-Lutidine was similarly brominated in C6H6 as solvent to give 20% 2,6-bis(bromomethyl)pyridine.
-----------------
A sustainable two-phase procedure for V-catalyzed toluene oxidative bromination with H2O2-KBr
Galloni, Pierluca; Mancini, Marco; Floris, Barbara; Conte, Valeria
Dalton Transactions (2013), 42(33), 11963-11970.
A sustainable V(v) and Mo(vi) catalyzed two-phase procedure for bromination of toluene under quite mild conditions is proposed; H2O2 is the
primary oxidant and KBr is the bromine source; metal precursors are com. available salts. The reaction is efficient without any addnl. solvent. By
using PhCH3 as a solvent/substrate good yields, together with interesting selectivity toward the formation of PhCH2Br, are obtained with both metal
ions. Recycling of the catalytic phase is also possible. Useful information on the V-peroxido chem. was obtained.
-------------
High selectively oxidative bromination of toluene derivatives by the H2O2-HBr system
Ju, Jie; Li, Yu Jin; Gao, Jian Rong; Jia, Jian Hong; Han, Liang; Sheng, Wei Jian; Jia, Yi Xia
Chinese Chemical Letters (2011), 22(4), 382-384. | Language: English
An aq. soln. of hydrogen peroxide and hydrogen bromide illuminated by a 60 W incandescent light bulb serves as a source of bromine radicals. Various
substituted toluenes (NO2, Cl, Br, H, CH3) were high selectively brominated at the benzyl position for monobromination in CH2Cl2 at ice water with
catalyst-free. This simple but effective bromination of toluene derivs. with an aq. H2O2-HBr system is characterized with the use of
inexpensive reagents and a lower impact on the environment, which make it a good alternative to the existing bromination methods.
[Edited on 13-5-2014 by Dr.Bob]
[Edited on 13-5-2014 by Dr.Bob]
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Electra
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| Quote: Originally posted by Dr.Bob | Here are some other preps, maybe some will be helpful. Interesting set of posts.
But I don't see any mention in the ones that I could open of the reaction going through the para-bromotoluene intermediate. Do you have any
references to that specific intermediate? I agree that radical conditions favors benzylic bromination, but don't understand the theory of the
bromine attacking at the para position first then transferring the bromine. if that were the case then 4-bromotoluene would form benzyl bromide on
exposure to sunlight, which I don't see any case of in the literature.
Also,
"The addition of acid causes this reaction to take an alternate pathway, both reach the same product, but the one without acid is far less exothermic.
They take relatively the same amount of time" -
Catalysis by acid or any reagent can not change the overall exothermic nature of a reaction. If the rate is the same, and the SM and product are the
same, ANY path must provide the same amount of heat produced. I don't understand your statement.
|
For references for toluene being brominated in the para position, please see my above post, and/or this link:
http://www.sciencemadness.org/talk/files.php?pid=331126&...
It is a very simple electrophilic bromination. Electron Donating Groups such as methyl groups are strongly activating in this reaction and always
para-directing. While Electron withdrawing groups are meta/ortho directing, and strongly deactivating, so much so that they hardly undergo a reation.
There are plenty of examples in the above document to demonstrate this.
The path with the acid is different because it includes wasteful steps. Part of which include the base-catalyzed elimination of sulfuric acid, which
is always a very exothermic process. This occurs at step two when electrophilic bromine (hypobromous acid OH- Br+) is generated)
You are right, as far as I know this particular intermediate has never been mentioned in literature, but the method which it happens by is very well
documented. Not every intermediate to every reaction has been performed before. This is part of the reason I did this for my Org Chem 1 class final
project.
Now I will point this out for the third or fourth time in this topic.
http://en.wikipedia.org/wiki/Radical-nucleophilic_aromatic_s...
Please see step #7 in the reaction mechanism.
| Quote: | Alternatively the phenyl radical can abstract any loose proton from 7 forming the arene 8 in a chain termination reaction.[./quote]
7 = R-H.
This is a simple radical exchange. It is extremely similar to the homolytic clevage of two bromine atoms.
Br2 + hv + R-H -> 2 Br* + R-H
2 Br* + R-H -> R-Br + HBr
Only in this exchange there is no generated HBr
PhBr + hv + R-H -> Ph* + Br* + R-H
Ph* + Br* + R-H -> Ph + R-Br
This it not a new reaction mechanism in any way. The only difference is that the Bromine radical is generated from the aryl halide. The equilibrium
lies very far to the right of Benzyl Bromide because the benzyclic hydrogen is the most easy to be abstracted. Even if di-brominated toluene was
generated temporarily, the equilibrium would eventually balance out to be mostly single brominated toluene, assuming constant hv irradiation.
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AvBaeyer
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Electra,
This reaction sequence:
PhBr + hv + R-H -> Ph* + Br* + R-H
Ph* + Br* + R-H -> Ph + R-Br
will not occur under any normal laboratory conditions. The homolytic photo-dissociation of an aryl halide is a very high energy process and occurs
only at very short wavelengths that require special equipment to carry out. Therefore, the intermediacy of a ring halogenated toluene in the reaction
of bromine with toluene under normal lab conditions to give benzyl bromide makes no sense whatsoever. Frankly, I think you should go to the lab and do
some experiments. Find or make some ring brominated toluene and see if there is any way under normal lab conditions that you can convert this to
benzyl bromide.
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Electra
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AvBaeyer,
I suggest you do freshen up on your photocatalysis knowledge. Photocatalysis is such an efficient process because the light is absorbed specifically
by the halide bond. Extreme temperatures are not required.
I have done these experiments, with photochemical reactors, direct solar irradiation, and with metal halide uv lamps. I have also
analyzed the reaction intermediates between each step. Density tests, BP tests, and other available processes. These procedures are not new and have
been tested multitudes of times. According to your profile you are a retired chemist? This isn't something I should have to explain to you.
Here's some supporting papers.
http://www.sciencedirect.com/science/article/pii/S0040403901...
http://pubs.acs.org/doi/abs/10.1021/jo00947a036?journalCode=...
The latter paper dictates :
* I have changed words in [ ] to deter "cooks" from searching the forum for them. If you look at the paper you will see why I did
this.
| Quote: | However, under irradiation, bromobenzene and [propanone] enolate ion react rapidly to form [arylated propanone] in high yield. Even a 150-W
tungsten light bulb external to a Pyrex flask causes reaction to occur.
Several other substituted benzenes, with various nucleofugic groups, also undergo this photochemical reaction, as reported in Table I.
.....
"Most of the reactions of Table I were conducted in Pyrex flasks irradiated by 350-nm ultraviolet lamps in a Rayonet photochemical reactor.
....
Iodo- and bromobenzene were totally consumed within 5 min, the first time of first observation"
.......
"When irradiation was provided by an external 150-w tungsten lamp, 1 hr was required for complete reaction of bromobenzene, in contrast to
less than 5 min in the photochemical reactor. At the lower intensity of illumination, study of the effects of other substances on reaction
rate was convenient. As shown in figure 2, reaction was exceedingly slow in the presence of 4.3 mol % of di-tert-butyl nitroxide, a radical scavenger.
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A 150-w Tungsten light bulb is able to initiate this reaction through a [non quartz] flask and a tungsten light bulb does not even output the correct
frequency to break the bond. This is simply further proof of how sensitive aryl halides are to UV light. They absorb light on a very large UV
frequency range, with the most optimal being 254 nm.
[Edited on 13-5-2014 by Electra]
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Dr.Bob
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Thank you for the specific references, I will look at them and try to make sense of the proposed mechanism. I am just trying to make sense of your
schemes.
I didn't doubt toluene being brominated in the para position, w/o light, I just have never seen evidence of the conversion of the para-bromotoluene to
benzyl bromide. I will look over the papers shown to see if that particular detail is documented there. Good science must stand up to careful
examination. If the data shows the correct evidence, then it supports the theory. That is all I am looking for. Thanks for posting the
information for our examination.
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Electra
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| Quote: Originally posted by Dr.Bob | Thank you for the specific references, I will look at them and try to make sense of the proposed mechanism. I am just trying to make sense of your
schemes.
I didn't doubt toluene being brominated in the para position, w/o light, I just have never seen evidence of the conversion of the para-bromotoluene to
benzyl bromide. I will look over the papers shown to see if that particular detail is documented there. Good science must stand up to careful
examination. If the data shows the correct evidence, then it supports the theory. That is all I am looking for. Thanks for posting the
information for our examination. |
Excellent point. It is important to note though that neither of those papers make any specific reference to the light irradiation para-bromotoluene
nor with regard to benzyl bromide. Though, studying the reaction mechanisms that are at place, and the famously known SRN1, it is easy to see how that
particular reaction could take place. Just because nobody has done it yet doesn't mean it can't take place. Well, besides the fact that I did it. At
some point every chemist realizes that before any specific reaction was documented in a research study, there had to be somebody who thought there was
potential for it to work.
[Edited on 14-5-2014 by Electra]
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Dr.Bob
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| Quote: Originally posted by Electra | | It is important to note though that neither of those papers make any specific reference to the light irradiation para-bromotoluene nor with regard to
benzyl bromide. Though, studying the reaction mechanisms that are at place, and the famously known SRN1, it is easy to see how that particular
reaction could take place. Just because nobody has done it yet doesn't mean it can't take place. Well, besides the fact that I did it. At some point
every chemist realizes that before any specific reaction was documented in a research study, there had to be somebody who thought there was potential
for it to work. |
I agree that these papers support SN1 substitution of bromine on aromatics with light. But as you say, none of these papers support the reaction
mechanism of
toluene --> 4-Br-toluene ---> benzyl bromide
and I still don't see any of the experimental proof of that. If you have done the simple experiment of
4-Br-toluene ---> benzyl bromide using sunlight or other hv, then please do post it.
Just because nobody has done it yet doesn't mean it can or has taken place either. So while I can't proof it can;t happen, I don't see proof that it
has taken place.
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Nicodem
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Cheddite Cheese, there are a few examples of benzylic brominations with the BrO3-/Br-/acid/H2O systems in
the literature:
DOI: 10.1021/es4019282
DOI: 10.1039/B713829F
Bulletin of the Korean Chemical Society, 1995, 16, 371-374.
There are also examples of Br-/acid/H2O2(aq) systems, for example:
DOI:10.1016/j.tetlet.2006.07.109
| Quote: Originally posted by Electra | | Technically the intermediate is hypobromous acid. What happens is Oxone/NaBr brominate the ring on toluene in the para position. The sunlight
initiates a radical SRN1 reaction. The aryl radical / bromine radical reacts with the methyl group on toluene, swapping out a hydrogen for the bromine. (see
step 7 in the above reaction scheme) |
That is pure nonsense. If there would be any truth in what you say, then how come you are unable to produce a single reference in your support? It's
been long ago proven that all the Wohl-Ziegler reactions, and radical halogenations in general, proceed trough the free-radical chain mechanism (DOI:
10.1021/cr60135a004 and DOI: 10.1021/ja00903a020).
If the reaction would proceed via SRN1 then the reactant would be bromide and the other product would be a hydride ion (reacting with
whatever is there to react with). And why does the benzylic halogenation require a radical initiator or light, if not because of the radical chain
mechanism?
| Quote: | | Your yields could GREATLY improve if you didn't combine the steps. Benzyl Bromide decomposes in water. The key is to do the Oxone/NaBr step with the
toluene, then drain off the water, and then expose the mixture of para-bromotoluene to sunlight. This is how you get a near
quantitative yield by ensuring no benzyl bromide decomposition when it is formed. |
Again, you are making things up. There is not a single reference for any ortho- or para-halogenated toluene to benzyl halide
reaction in the literature.
In fact, toluene does not even significantly electrophilicaly react with Br2 at the conditions used in benzylic brominations. Ring
bromination of toluene with Br2 requires a catalyst to proceed at an acceptable rate. So there is no chance for your proposed two step
pathway to benzyl bromide trough bromotoluene as being part of the mechanism, as benzyl bromide forms faster than bromotoluenes would in the absence
of radical initiators or light.
Those papers are totally irrelevant to the current discussion. What the hell do they have to do with benzylic halogenations?
| Quote: | | Why do you use acidified NaBr? I have never had to use any acid in the above reaction for excellent yields. |
You cannot obtain bromine from bromide with bromates or H2O2 unless in acidic media. Try writing down a redox equation in the
absence of acid. It is not possible. You can only write down a redox equation giving hypobromate in the absence of acids. Generally, if a redox
equation does not make sense, it makes for a good argument that that reaction is impossible. Oxone is an acid itself, so with it you need no
additional acid (which does not mean that there is no acid present).
| Quote: | | A 150-w Tungsten light bulb is able to initiate this reaction through a [non quartz] flask and a tungsten light bulb does not even output the correct
frequency to break the bond. This is simply further proof of how sensitive aryl halides are to UV light. They absorb light on a very large UV
frequency range, with the most optimal being 254 nm. |
I don't know which reaction you were talking about here (and I doubt you know it either), but the thread topic reaction does not work best with UV
light. It works best with Vis light, optimally the blue light around 420 nm (DOI: 10.1021/i300024a606). In fact, sunlight works best. The second best
is a white CFL light bulb, and the least effective is the 254 nm UV light.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Metacelsus
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NMR analysis of product
The 1H and 13C spectra are attached. There are evidently some ring-substituted impurities. The overall purity is 93.2% by
integration.
Attachment: 1H.pdf (23kB)
This file has been downloaded 517 times
Attachment: 13C.pdf (18kB)
This file has been downloaded 504 times
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Nicodem
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That's a relatively good purity. From the benzylic hydrogens I calculated the ratios of the four products to be:
benzyl bromide: 88.9 mol%
benzal bromide: 4.6 mol%
p-bromotoluene: 3.4 mol%
o-bromotoluene: 3.1 mol%
Of course, you must have altered the initial selectivity ratio with the fractionation, which explains the lower proportion of the benzal bromide when
compared to the related published in situ bromide oxidation methods. The bromotoluenes are however unlikely to get removed with a simple
fractionation, hence their ratios are relevant to the reaction selectivity.
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AvBaeyer
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C. Cheese,
Again, a nice piece of work. The NMR work is very informative and gives good insight into the product distribution. I assume that the product ratios
obtained when I use oxone are very similar (sure wish I had NMR access again!) though I do use a large excess of toluene to reduce benzal bromide
formation.
Nicodem,
Thanks for taking Electra to task. I had not gotten around to a reply to him yet but no more needs to be said. His remarks only show that information
is not knowledge.
AvB
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Metacelsus
International Hazard
Posts: 2539
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In my above post, I forgot to include benzal bromide in the purity calculation. Nicodem's numbers are correct.
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