Sciencemadness Discussion Board

Prep of p-Nitrobenzyl Bromide & Relatives

Sauron - 9-4-2007 at 07:38

Edit: I have changed title of thread because we have meandered away from mononitration of toluene to nitration of benzyl acetate, and finally to nitration of benzyl bromide and benzyl alcohol. All along the final target has remained p-nitrobenzyl bromide so I figured the title ought to reflect this.
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Optimizing Para Isomer in Mononitration of Toluene

You would think this would be an easy point to nail down but, it is surprisingly elusive. Org.Syn is silent and so is Merck. Vogel and Gattermann also, shtun.

The Davis book merely says that o- and p- isomers are 96% of product in this step, m-isomer 4%, and that the ratio of o-and p- can be varied by rxn temperature but gives no details.

Only the trusty old TNT book gives any details and these are scant.

180 parts toluene
325 parts H2SO4 d.1.84
200 parts HNO3 d.1.44

Mixed acids run slowly into well stirred toluene at such a rate as to maintain reaction at <30 C.

Says that three isomers can be sepd by fractionation but if temp went over 30 C at any point then presence of 2,4-dinitrotoluene can lead to explosion.

Elsewhere another ref says even TNT can be distilled in vacuo without even slight decomp.

TNT book has alternate better seperation procedure but scan is badly messed up and unreadable.

I am not interested in DNT or TNT only after best procedure for p-nitrotoluene (other than buying it, which is an option. I have bought o-nitroaniline by the Kg before without any problems.)

I want to optimize yield of p-isomer at expense of o-isomer and I want to know best technique for seperating the two.

This is supporting Klute's experiments as the Org.Syn preps for p-nitrobenzyl bromide and p-nitrobenzaldehyde start with p-nitrotoluene.

I do have PATR 2700 but I am saving that as a last resort since it is a pain to search.



[Edited on 15-4-2007 by Sauron]

garage chemist - 9-4-2007 at 08:10

I have done this nitration on a 0,6 mol scale, like you describe, adding the mixed acids to the stirred and cooled toluene. Only a stochiometric amount of HNO3 is used (in regard to toluene), not more.

After separating spent acid and washing the organic phase, unreacted toluene is distilled off, but this is usually only very little (less than a ml on my scale).

First, an amount of p-isomer is extracted by cooling the (dried) nitrotoluene liquid to -5°C (ice/salt). Some of the p-isomer crystallizes out and is filtered.

The filtrate is distilled over a column (can be done at atmospheric pressure if the column is well isolated, there is no decomposition) and pure o-isomer is obtained as distillate.
When the temperature at the column head rises, indicating that p-isomer starts to come over, the distillation is stopped and the distillation residue cooled again to -5°C and filtered to isolate more of the p-isomer.

This distillation-cooling-cycle is repeated until the isomers have been completely separated.
The p-isomer must then be recrystallized from EtOH/H2O mixture until it has the correct melting point, in order to remove the m-isomer.

O/p separation by only fractional distillation is possible, but the poiling points of the isomers are too close together to make this economic on lab scale. The combined distillation/freezing method is much more effective.


If you do this synthesis, make one large batch in order to make all of your desired isomer at once. It is a lot of work and will have you occupied for several days.

Sauron - 9-4-2007 at 08:28

Thanks. Looks like $37/Kg (Acros) is better way to go.

No info yet on optimizing for p-isomer. David suggests o-isomer "greatly predominates" (my recollection from previous reading is something like 50:45:5 for 0 p m) which just means making a lot of unwanted o-isomer. Can't think of much to do with that but reduce down to o-toluidine but I have no immediate use for that either.

Thanks for help

-----------------

Finally found some quant. data in Hoggett's excellent book, for toluene the ortho isomer predominates to the extent of 55-60% of total, p-isomer 35-40 % so excluding the m-isomer, the ortho is about 2/3 of product, which for my purposes - simply sucks.

Interestingly as the alkyl group increases
Me<ethy,2-propyl,t-Bu the o:p ratio drops consistently till in last case the p-isomer forms to extent of 80% of total. Too bad I have no need for p-nitro-tert-butylbenzene.


[Edited on 10-4-2007 by Sauron]

vulture - 9-4-2007 at 09:44

Not much you can do there. Separation by column chromatography could be a possibility.

If you really want to do something extraordinary, you could try exploiting steric hindrance. Mononitrate isopropyl benzene (I'm not even sure if the steric hindrance would suffice to reduce ortho nitration) then oxidize that with KMnO4 to p-nitrobenzoic acid. Ofcourse that leaves you with the acid which is rather difficult to reduce back to the methyl...depends on the intended purpose really.

Quote:

Interestingly as the alkyl group increases Me


Not entirely sure what you mean, but alkyl substituents are para and ortho directing.

not_important - 9-4-2007 at 10:13

Temperature doesn't seem to have much o/p effect, contrary to early reports, but low temperatures do reduce the amount of meta isomer.

Using traces of Hg(II) salts, switching acids and nitrating agents, and using molecular sieves, all can affect the o/p ratio.

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Sauron - 9-4-2007 at 12:41

Sorry @vulture, that paragraph is supposed to read

Interestingly as the alkyl group increases Me ethy,2-propyl,t-Bu the o:p ratio drops consistently till in last case the p-isomer forms to extent of 80% of total. Too bad I have no need for p-nitro-tert-butylbenzene.

My use of a certain character apparently caused the system to dump the text

Yes it looks steric, yes cumene (isopropylbenzene) does shift the o-p ratio but not as much as t-Bu-benzene.

Your proposed scheme occured to me but, I think I'd just as soon do an end run and oxidize p-toluidine to p-nitrotoluene instead, if I don't just give up and buy the stuff.

The target is p-nitrobenzyl bromide.

You can get there three ways

Bromination of nitrotoluene w/Br2
Bromination of p-nitrobenzyl alcohol w/ HBr conc or NaBr/H2SO4
Nitration of benzyl bromide

The nitrobenzyl alcohol could be had by Cannizzaro rxn of nitrobenzaldehyde, itself available from the nitrotoluene.

As usual these aromatic transformations are pretty trivial five finger exercises like Chopsticks on the piano. A student can work them out, a good student can conduct them.

The point of the p-nitrobenzyl bromide is that @Klute wants to oxidize mixed xylenes and then seperate the three di-acids, but the acids themselves won't distill (some decompose, some sublime, some have bps too proximate) and similar hassle with the simple alkyl esters.

The p-nitrobenzyl esters (prepared from the bromide and Na salt of acid) have very divergent bp's. and ought to be volatile enough to distill at a good vacuum under 10 torr. Normally these esters are only used for qualitative analysis but I proposed using them preparatively. p-bnitrobenzyl bromide is about $2/g so better to make it.

Just another 5 finger exercise. The stuff is a bit lachrymatory but not nearly as bad as the other reagent: p-bromophenacyl bromide! which is one Br away from a very nasty old military tear gas, one of the worst of them. Bromoacetophenone. And the extra bromine certainly will not reduce its effects.

vulture - 9-4-2007 at 13:08

Quote:

Yes it looks steric, yes cumene (isopropylbenzene) does shift the o-p ratio but not as much as t-Bu-benzene.


t-butylbenzene would be the best ofcourse, but you can't get rid of that by oxidation as it has no alpha hydrogen.

Sauron - 9-4-2007 at 13:12

And that's why I am detouring to p-toluidine.

If there's no convenient and selective oxidation then I'll just buy the nitrotoluene. It's $37/Kg, which means by the time it gets here it will likely cost me $75/Kg, not a big deal.

Hell, if the p-toluidine is more than that I'll drop it like a hot rock too.

Klute - 9-4-2007 at 14:38

Hum, I didn't consider the problem the o-isomer in nitrating toluene... from what I remenbered, the p-isomer was the main product, but that is apparently not the case. garage chimist way of seperating the isomers looks like agood idea, although time consuming... I'm sure o-nitrotoluene can have some use somewhere, I'll leave it in the back of the closet until I found one. Could just as well make o-nitrobenzaldehyde and try a few condensations products...

BTW, Sauron, the OrgSyn procedure looks a little tedious for o-nitrobenzaldehyde, using pyridine and all, wouldn't a persulfate oxidation leave the nitro group alone? Or even using electrolysis as Cycloknight did with toluene... I wanted to give it a try one day or another, this could be the occasion of building a cell

I'm glad you mention nitration of benzylbromide, I hadn't read that thread before answering in the xylene one, this route seemed more convinient for me, the only problem could be the benzylbromide, IIRC it's not a particluary nice compoud to manipulate, worse than benzylchloride, but I could be wrong. I'll check. If this is the case, I'll give the toluene nitration a try, using garage chemist way of seperating the isomers, but i must admit spending a few days on simply seperating the products doesn't attract me, as I don't have much free time, and this more of an secondary interest project. That discussion on the xylenes oxidation made me want to try a few of those "school synths". Founding that old protocol on the permanganate oxidation brought me a few years back :)

pantone159 - 9-4-2007 at 15:30

From o-toluidine, p-toluidine and aniline, you can make mauve.

Sauron - 9-4-2007 at 20:34

In this country benzyl bromide and benzyl chloride are both restricted (but benzyl cyanide isn't, go figure!) Yes benzyl bromide is nasty unplesant stinky lachrymatory stuff but, so are lots of useful things. If you want nice smells, go into fragrance chemistry. (And definitely avoid sulfur and phosphorus chemistry!)

I have not looked up prica and availability of p-nitrobenzyl alcohol but if available and cheap that would surely obviate need for bromine (use HBr instead) and also eliminate need for the aldehyde (no need to do the Cannizarro.)

Otherwise just brominate benzyl alcohol w/HBr or NaBr/conc H2SO4 or TCT/KBr/DMF then nitrate that.

At least part of garage chemist's workup was fractionation of o-nitrotoluene. If you skip that you will have a bit less yield of p-isomer, just the part that came out in first chilling. But you would save a lot of time and meanwhile just bottle the crude o-isomer and defer the distillation till later.

Assuming 35% of yield is p-isomer, on a scale that can be conducted in a 1L pot, starting with 180 g toluene you will be making not less than 90-95 g p-isomer and close to 200 g of o-isomer every time. If you recover 1/3 to 1/2 of the p-isoner from first chilling, you still have a goodly amount. If you scale up to a 3L or 5L flask, then depending on your needs you may only have to do a single nitration, and you'd have maybe 100-150 g p-isomer from first chill, and a similar amount to be worked up from the crude mostly o-isomer mother liquor.

Just remember to control addn rate of mixed acids so the pot temp NEVER goes over 30 C, and restrict total HNO3 to stoichiometric amount.

@g_c mentioned external cooling. Ideally I'd like to do this in a jacketed vessel with a temp controlled recirc. chiller. I have the latter but not the former. Personally I would not use an ice bath, I'd use a water bath at RT and equilibrate bath and pot (by stirring both) because IMO while you don't want rxn to go over 30 you also probably don't want it too cold. Addition rate of the mixed acids is the key. Also, mix the acids the day before so they cool down to RT before you add them. A digital thermometer with a big display is very useful.

not_important - 9-4-2007 at 21:05

How about HNO3, toluene, and Clayfen or Zeolite beta-I, with water removed as toluene-water-HNO3 azeotrope, with claims of 95% acid utilization + recovery, about 2/3 of the toluene converted to the mononitrate, o/p ratios in the rane of 1:2 in favour of para.

Might be worth a try, nothing really exotic used.

Or has this already been examined?

[Edited on 10-4-2007 by not_important]

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evil_lurker - 9-4-2007 at 22:05

I was reading the DMSO bulletin the other day and it stated that some halongen molecules can be removed from certain aryl compounds and replaced with an NO2 group by simply reacting in DMSO saturated with NH3 gas.... not sure if it applies to all isomers, but perhaps by chlorinating toluene and reacting one could get a higher percentage of the desired isomer.

Sauron - 10-4-2007 at 00:12

@evil, what is link to DMSO bulletin or can you provide the particular article?

p-chlorotoluene ought to be readily available, or preparable.

I have found that oxidation of primary arylamines to nitro cmpds is known, using Dupont OXONE in acetone, I do not know yet what the cost of reagent might be and am still hunting the reference which is

The oxidation of aromatic amines in the presence of “electron-rich” aromatic systems Tet.Lett, 29, 4501 (1988)


Remember, my target is cheap so a costly prep just pushes me to buy it instead. I am unfamiliar with this Dupont product, and will have to look it up.

Aha! OXONE is 2KHSO5+ KHSO4 + K2SO4. Cheap, avail by Kg. Still need to make sure it leaves methyl group alone. p-nitrobenzaldehyde would be acceptable; p-nitrobenzoic acid, not so, as I would then have to selectively reduce it back to the aldehyde or alcohol.

Apparently this rxn is done in the cold in acetone and the actual oxidizer is dimethyldioxirane formed in situ from the acetone and the monoperoxysulfate.



[Edited on 10-4-2007 by Sauron]

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vulture - 10-4-2007 at 01:39

Quote:

Aha! OXONE is 2KHSO5+ KHSO4 + K2SO4. Cheap, avail by Kg. Still need to make sure it leaves methyl group alone. p-nitrobenzaldehyde would be acceptable; p-nitrobenzoic acid, not so, as I would then have to selectively reduce it back to the aldehyde or alcohol.


I've tried making benzaldehyde from toluene with persulfates. It needs alot of heat and the yields are LOW, so I don't think oxidation of the methyl is going to be a big problem.

From nitrobenzoic acid you could probably make the methyl ester and reduce that to the aldehyde/alcohol. Unless somebody knows a method to reduce aromatic acids?

S.C. Wack - 10-4-2007 at 02:06

One would think that if much optimization was possible and not a secret, Urbanski would know. Toluene nitration was certainly something he was into.

Sauron - 10-4-2007 at 02:17

Great! this looks both promising and interesting.

I found that article on Science Direct, which unfortunately does not give away the DOI. Elsevier has rendered the Tet Lett website useless, now.

Originally I obtained the reference (sans title) from THE NITRO GROUP IN ORGANIC SYNTHESIS. The first author's name was badly misspelled (Zbrowksi, when it is really Zabrowski) which helps not at all when searching! And Science Direct graphical abstract does not work. Oh well. Maybe the better angels of References will come through as usual.

p-toluidine is <$30/Kg. OXONE is $23/Kg.

Here's an abstract:

An aqueous Oxone-acetone combination has been developed for the transformation of certain anilines into the corresponding nitrobenzene derivatives. This process involves sequential oxidation steps proceedings by way of an intermediate nitrase compound.

Example given is 2-aminoanisole. Aq.oxone/acetone (i.e., DMDO) and DCM with a phosphate buffer are employed. 3.2 eq oxone used. Another source says 0 C. Don't know scale or yield yet.


]





[Edited on 11-4-2007 by Sauron]

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evil_lurker - 10-4-2007 at 06:07

Ok here is the article to which I was referring...

Its in Part IV section 5(b).

It appars that I was wrong.. that replaces a halogen with a amine group, not a nitro. :D

Would be great for going from chlorotoluene to x-toluidines without the messy metal reduction and steam distillation.

Sauron - 10-4-2007 at 12:41

Thanks, something to keep in mind. But sorry, link did not show up in your post.

I am not surprised that DMSO satd with NH3 replaces Cl with -NH2

-NO2 was rather startling after all.

But it's a useful transformation and one usually done via diazotization.

What is solubility of ammonia in DMSO?

[Edited on 11-4-2007 by Sauron]

evil_lurker - 10-4-2007 at 14:49

http://www.gaylordchemical.com/bulletins/Bulletin105B/index....

Try that... also there is a ton more publications on their site.

I tried uploading it this morning but I think that the board server didn't like it too much, plus the fact that I'm on dial-up.

[Edited on 10-4-2007 by evil_lurker]

Klute - 10-4-2007 at 15:11

Vulture- I too tried making benzaldehyde from toluene using Oxone, which was designated as persulfate. In that case, very low yields, even with benzyl alcohol... BUT, when looking into more depth, I realised a common mistake is made wheh refering to persulfates: both peroxodisulfates S2O8 2- and peroxomonopersulfates HSO5- are often simlly called "persulfates". I tried the same reacions with (NH4)2S2O8, and obtain acceptable yields from toluene (40-75%) and good to very good with benzyl alcohol (50-95%) depending on time, temperatures and amount/type of catalysts. Followed US4146582 The best combinations where 2-3 hours at 70°C (water bath) with small quantity of FeSO4/Cu(OAc)2 salts. Changing the specific amount of both salts didn't seem to have much of an influence, so as the ratio, but not much care was taken to experiment on that. The Fe2+ are to catalyse homolytic rupture of the O-O bound so free radical formation, and the Cu salts have something to do with protecting the aldehyde from further oxidation and avoiding oxidation of the benzene nucleus IIRC. Nothing is mentionned in the patent about the pH, but I think ir has to be at least neutral/slightlyt basic to avoids H2S2O8,H2SO5 and H2O2 formation, the 2 first will surely oxidize the aldehyde into the acid very easily. With benzyl alcohol no cosolvant was used, just violent stirring with water, and with toluene a small amount of methanol was used (ethanol will react with persulfates, GAA will cause side reactions (Benzylacetate) as opposed to what is mentionned in the patent) and acetone should work as it is not attacked by the peroxodisulfate, although it was not tried.. Not too much recations were tried with toluene, mainly benzylalcohol.

Works pretty well, although the results weren't always regular, the reaction must be very sensitive to a few parameters not always easy to understand. Using only Ag+ as catalyst lead to having benzoic acid as major product, although using it in combination with Cu2+ could change the result. Using oxone gave lousy results, I think that this comes from the fact that peroxodisulfate is a symetrical peroxide which favors free radical formation much more that the K salt of H2SO5 which acts more as a buffered peracid..

Give it another try

Sauron - 11-4-2007 at 02:04

The Oxone oxidation of p-toluidine just is uneconomical in the extreme.

While a Kg of Oxone is not expensive, it is only 1.5 mols while a Kg of p-toluidine is 9 mols. 3.2 mols Oxone is required to treat 1 mol of the amine. That means almost 2 Kg Oxone to oxidize 107 g p-toluidine. They cost about the same, weight for weight, but, damn!

Twenty Kg Oxone to oxidize 1 Kg p-toluidine.

So, I have two choices. I can just buy the p-nitrotoluene, or I can nitrate toluene and do the tedious workup of the minor product p-isomer. The only advantage to the latter course is accumulating a bunch of o-nitrotoluene which I can convert to o-toluidine at my leisure.

Klute - 11-4-2007 at 04:04

kilo of ammonium persulfate is about 4.5mol... It can be used to oxidize aniline to nitrobenzene by using it in acidic medium (H2SO4) to produce Caro's acid H2SO5, pat US4049786 treats of a very efficient way of hydrolysing S2O8 2- by conc. H2SO4, tried personally with excellent results... That's how Caro discovered his "nitrobenzene producing substance" as he called it at the time "100 years of Bayer Villiger Oxidations" M.Renz & B.Meunier

That could be more econimical... using peroxodisulfate as it is, in neutral or basic medium causes polymerisation to ployaniline...

Hum, after all, it's still at least 1kilo of peroxodisulfate for 107gr of methylaniline.. Maybe electrochemistry route?

Sauron - 11-4-2007 at 06:53

Nine Kg is better than 20 Kg, depending on price. But still.

I would worry about Caro's acid chewing up the Me group under those rxn conditions, this is toluidine not aniline.

Still, it is worth a look at the lit.

I wonder why it did not merit a mention in THE NITRO GROUP IN ORGANIC SYNTHESIS?

Which only talked about mCPBA and Oxone. (no not together!) I was not attracted to mCPBA.

--------

Got the Tet Lett article and through its refs, found a peroxuacetic acid oxidation of p-touidine to p-nitrotoluene in 72% yld. However this was done at Redstone Arsenal (Army rocketry lab) with 90% H2O2/Ac2O so no, I do not think I will be doing that one. Similar article with peroxytrifluoroacetic acid, also used 90% peroxide. Oh well.

The Tet Lett article employs a simpler dimethyldioxirane oxidation with the DMD generated in situ from Oxone, acetone and water under buffered PTC conditions with DCM at 0 C for 15 minutes. All that is admirable, but for the need for 3.2 mols (2 Kg) Oxone per mol substrate (107 g in my case) which means not only huge expense but also huge apparatus, a 50 L RB flask to process one mol. This is pretty absurd. In fact I have 72 L flasks but, I'm not likely to press them into service this way. It's silly.

As a last resort I will look at the m-chloroperbenzoic acid references But I have the feeling it's going to be: buy the target or resign myself to the furshlugginah niration and workup.

Well, the mCPBA thing was a general procedure for aliphatic amines to nitroalkanes as it proceeds via azodioxy intermediates. Those are tautomeric with nitroso compounds, so can be trapped by mCPBA. Just not applicable to aryl amines. Oh well.



[Edited on 12-4-2007 by Sauron]

[Edited on 12-4-2007 by Sauron]

Klute - 11-4-2007 at 14:19

Most of time, when possible, pur peracetic is prepared from max conc.H2O2 and AcO2, or GAA. But in most procedures where peracetic is needed, a solution of peracetic acid in AcOH works also, just less efficient (diluted), and in industrial setups generates alot of waste, "wet" carboxylic acid which is very costly to recycle... If the presence of water is REALLY detrimental, generating peracetic from perborate and GAA helps minimizing the amount. I'm also working on a way of producing peracetic with peroxodisulfates in GAA, also minimizing the water content.... In "OXIDATION OF PSEUDOCUMENE IN ACETIC ACID". O. A. Kholdeeva, I. V. Kozhevnikov,. V. N. Sidel'nikov, and V. A. Utkin There suggest that peroxodisulfates react in glacial acetic acid in this way:
Na2S2O8 + 2 HOAc = 2NaOAc + H2S2O8
H2S2O8 + 4HOAc = H2O2 + H2SO4 + Ac2O

The H2O2 and acetic anhydride would react to form peracetic acid, catalysed by the sulfuric acid. Indeed, it would be the equivalent of mixing anhydrous H2O2 (!!) and Ac2O in GAA in presence of sulfuric acid... But how quickly/completly this reaction takes places is hard to say. Alot of other document relate to the use of caro's acid to catalyse and increase peracetic acid formation from H2SO4 and H2O2. The persulfates take along time to dissolve in GAA, addition of stoechiometric amount of H2SO4 to an "wet" solution of peroxodisulfate in ACOH causes immediate hydrolysis to Caro's acid, and surely a certain amount of peracetic acid. I'm trying to see if similar results can be obtained in anhydrous conditions, of course it wouldn't be a hydrolysis anymore, and hopefully peracetic formation will be favored.. My only (and biggest) problem is an efficient way of titrating the different peroxo species (H2SO5, AcOOH, H2O2) seperatly with available reagents... Anyway, I think this could be a very practical way of preparing a versatil oxidant solution comprimizing of peracetic and persulfuric acid, possibly usefull, for example, in the oxidation of amines to nitro compouds.

I hadn't thought of the problem from the methyl group. Basic/neutral peroxodisulfate would surely oxidize it to the alcohol and aldehyde, and possibly to the carboxylic acid, but I'm not sure this would happen with Caro's acid. Don't remmeber seeing any applications where it was used to oxidize aromatic methyls, but it is a possibility. Perhaps bufferening the acidic conditions could minimize this.

Woh, I would like to have a 72L flask! I think i would sleep with it every night :) Want kind of joint do have on such glassware? (dimension-wise)

Sauron - 11-4-2007 at 21:53

I finally found a better route to p-nitrobenzyl bromide that avoids the various problems of the other routes.

The problem with nitration of toluene is tedious workup only. Then for some people elemental bromination might be a problem but not for me.

The better method is nitration of benzyl acetate as described by Hurd et al in J.Org.Chem. 12, 369 (1947) The p-nitrobenzyl acetate is isolated in 36% yld after two recrystallizations. The o-isomer remains in mother liquor and can be recovered.

The ester is then saponified with hot methanolic NaOH by an Org Syn. method (CV 3, 652). The p-nitrobenzyl alcohol (which is costly to purchase, and unavailable by nitration of benzyl alcohol) can then be brominated by conc HBr, or by NaBr/H2SO4, or by TCT/DMF/KBr, PBr3, etc. Cf. Norris, Watt, and Thomas, JACS 38, 1077 (1916) for HBr methos, 3.3 g acid per g alcohol, but quantitative yld.

So, three steps from cheap benzyl acetate rather than two steps from toluene but involving more hassles.

If anyone would like the Org.Syn. and/or JACS articles Ill post them.

This alcohol has also been prepared by electrochemical means (Elbs, Ber. (Ref.), 29, 1122 (1896) and chemical means
(Dieffenbach, Ger. pat. 214,949 [Frdl., 9, 156 (1908–10)]. I admit I have not examined these, because of my poor German skills and skittishness re patents, and I am satisfied with the straightforwardness and simplicity of the route outlines above and the good details of procedure and yield at each stage. There are no expensive or exotice reagents to obtain (or unobtain.)

[Edited on 12-4-2007 by Sauron]

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Klute - 12-4-2007 at 11:29

I beleive Benzylacetate is easily made in high yield with anhydride acetic, but can surely also be prepared by using GAA, H2SO4 in a Dean Stark trap or with other means of deplacing equilibrium...
That considerably simplifies the preparation.. Thanks Sauron!

Sauron - 12-4-2007 at 13:00

True, but it's about same price as benzyl alcohol. The Vogel procedure from the alcohol, GAA and a little sulfuric catalyst is not very high yld (looks like <50%) Clearly AcCl or Ac2O will work better but why waste them on a cheap commoditized ester? I'll just buy it. Also it's pretty high boiling so any distillation to purify it will need to be in vacuo or else you need to interpose an air condenser before the water cooled condenser or risk cracking the Pyrex.

If you really want to prepare the benzyl acetate then there's the available route of making benzyl chloride from benzyl alcohol and TCT (CC). Simply mix 6 mols of the alcohol to 1 mol of the chlorinating reagent (no other solvent, the excess alcohol serves for that) and reflux for 60-120 minutes, yield 72%

If 72% isn't good enough then you prepare the TCT/DMF complex and react that with benzyl alcohol, this gives 98% yield in 15 minutes! Benzyl bromide can also be prepared this way by addition of KBr to the reaction. For details see attachment. (room temp, DCM solvent, 1:1 mol eq)

The lit says benzyl bromide can be nitrated but I am unable to obtain the article in Bull Soc Chim (not sure Fr or Belg) from 1921, so I have no details. I may go experimental and work this out, which would make the target p-nitrobenzyl bromide available in two steps from benzyl alcohol. I have lots of TCT and no problem buying more. However this happy situation does not apply to a lot of our vrethren in Europe.

As I now have the mp of p-nitrobenzyl bromide (mp 99-100 C) maybe the nitration will be facile like it is for benzyl acetate.

To make the benzyl acetate from benzyl chloride you react it with sodium acetate in AcOH, the Org.Syn. procedure is given for the nitrated benzyl chloride, and is 80% yld. This is a lot better than Vogel's sulfuric acid catalyzed esterification of the alcohol with acetic acid (q.v.) which is less than 50%.

[Edited on 13-4-2007 by Sauron]

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Sauron - 15-4-2007 at 01:21

We have been working on the assumption that benzyl alcohol would not survive the conditions of aromatic mononitration; this turns out to be erroneous.

Benzyl alcohol can be nitrated to a mixture of isomeric, principally o- and p-nitrobenzyl O-nitrates. (nitrophenyl nitroethers)

These in turn can be hydrolyzed to the alcohol, displaced to nitrobenzyl acetates, or the nitro group can be displaced by halogen via NaX/KX in acetone, all of these reactions are high yield.

Given the moderate yield (50-60 %) of the NaOH saponification of p-nitrobenzyl acetate to the corresponding alcohol, the selection of starting material is now an open question.

Benzyl acetate is significantly cheaper than benzyl alcohol but this is offset by the losses in saponification unless one's final target is the nitrobenzyl ether.

If the target is the nitrobenzyl bromise or chloride then there are two available two-step routes:

nitration of benzyl alcohol then displacement of nitro group with alkili halide

Or

Halogenation of benzyl alcohol then nitration

The nitration of benzyl acetate is done with mixed acids. The nitration of benzyl alcohol is done with fuming nitric acid (d.1.5) with NOx removed by urea and chilled to -10--15 C.

Syntheses of organic preparations and intermediate products based on nitrate esters
P. M. Kochergin and L. S. Blinova
Journal: Pharmaceutical Chemistry Journal
Issue: Volume 1, Number 1 / January, 1967
Pages 5-7

also follow-on article in Number 2.



[Edited on 15-4-2007 by Sauron]

[Edited on 15-4-2007 by Sauron]

Attachment: nitrobenzyl nitrate.pdf (234kB)
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Ozone - 15-4-2007 at 07:58

Sauron,

Here's a neat paper for your toilet-reading enjoyment:

Oxidation of Aromatic Amines with Hydrogen Peroxide Catalyzed by
Cetylpyridinium Heteropolyoxometalates

This looks quite tweakable, selective and in most cases mild. Yields on nitrotoluene were quoted ~92%.

Cheers,

O3

Attachment: Sakaue 1992.pdf (805kB)
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Sauron - 15-4-2007 at 09:03

Thanks @O3. I dunno where I can get 35% h202 in Thailand. This is one to file away in case I come into some p-toluidine. Also working up from 3 mmol scale to a Kg of purif product is sometimes a pain.

For immediate purposes I think I will stick to benzyl alcohol nitration, the lit. has this on a 200 g purified product scale.

Call me timid but don't call me late for lunch!

garage chemist - 15-4-2007 at 09:29

The workup of the toluene nitration sounds somewhat more tedious than it is. The freezing will only have to be done 2-3 times to get a good separation (depending on the column), and the recrystallization of p-NT from EtOH/H2O may only have to be done once if done well.

If p-nitrobenzyl bromide is the desired product, I would also choose to go with the toluene nitration. May I ask what the p-nitrobenzyl bromide is needed for? I dont know any uses for this substance right now.

O-nitrotoluene is useful as well, not only for o-toluidine, but also for e.g. anthranilic acid, or 2-halotoluene, or 2-substitued benzoic acids and benzaldehydes.

Sauron - 15-4-2007 at 10:31

@gc, thanks. The target compound and its close relatives

p-nitrobenzyl alcohol
p-nitrobenzyl acetate
p-nitrobenzaldehyde

have some importance in medicinal chemistry including anticancer drugs.. The alcohol and bromide also are used as protecting reagents in peptide synthesis, in which I am much interested.

The bromide is used in qualitative analysis of carboxylic acids

Klute and I are messing around with a process for seperating isomeric phthalic acids. He wants to oxidize mixed toluenes, and these esters are my suggestion for how to seperate the acids produced, by vacuum distillation.

[Edited on 16-4-2007 by Sauron]