Sciencemadness Discussion Board - Improved Synthesis of Benzyl Bromide by Free Radical Bromination

Improved Synthesis of Benzyl Bromide by Free Radical Bromination

Metacelsus - 11-5-2014 at 12:41

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]

AvBaeyer - 11-5-2014 at 18:42

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

Metacelsus - 12-5-2014 at 04:58

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.

Nicodem - 12-5-2014 at 07:27

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.

Metacelsus - 12-5-2014 at 18:12

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.

Electra - 12-5-2014 at 18:23

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]

Metacelsus - 12-5-2014 at 18:44

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?

Electra - 12-5-2014 at 19:02

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 909 times

AvBaeyer - 12-5-2014 at 19:54

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.

Electra - 12-5-2014 at 20:42

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]

Dr.Bob - 13-5-2014 at 09:12

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]

Electra - 13-5-2014 at 11:27

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.

AvBaeyer - 13-5-2014 at 13:49

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.

Electra - 13-5-2014 at 14:23

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.

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]

Dr.Bob - 13-5-2014 at 18:46

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.

Electra - 13-5-2014 at 19:00

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]

Dr.Bob - 14-5-2014 at 07:10

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.

Nicodem - 15-5-2014 at 10:02

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 S_RN1 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.

Quote:

Here's some supporting papers.

http://www.sciencedirect.com/science/article/pii/S0040403901...
http://pubs.acs.org/doi/abs/10.1021/jo00947a036?journalCode=...

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:

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.

NMR analysis of product

Metacelsus - 15-5-2014 at 12:06

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)
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Attachment: 13C.pdf (18kB)
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Nicodem - 16-5-2014 at 06:15

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.

AvBaeyer - 16-5-2014 at 17:57

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

Metacelsus - 17-5-2014 at 12:46

In my above post, I forgot to include benzal bromide in the purity calculation. Nicodem's numbers are correct.