zephler
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NBS usage and solubility?
I was wondering who has experience with NBS? I was talking to someone, who was trying to get the bromohydrin product from allylbenzene
(2-bromo-1-phenylpropan-1-ol) using allylbenzne, NBS, and the solvent acetonitrile and around 1% of the vol water (maybe even 0.5%). The NBS did
nothing except for sit at the bottom, looking exactly like it did coming out of the bottle/ So I am wondering, should I just add more water (is NBS
soluble in water??), is NBS soluble in Acetonitrile? I think a viable route is to add more water, if in fact NBS is soluble in water - does anyone
have any experience with NBS to get bromohydrin (br and OH addition to double bonds)?
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smuv
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You need more water, like +20%. Also, it will never work without stirring. There are a lot of lit procedures for this kind of thing, why are you not
following them?
Edit: this reaction works because NBS is INSOLUBLE in the solvent system!
[Edited on 12-4-2010 by smuv]
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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zephler
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thanks! I heard that adding a tad of DMSO and a pinch of C2H4Cl2 worked wonderfully 
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zephler
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and the pinch of the non polar is important that it's only a pinch - to much would cause layer separation and poor yields
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smuv
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I am not really seeing the point of adding the ethylene chloride. I guess the dmso could form the dimethylbromosulfonium ion (and the N-oxide of
succinimide) and act as a Br+ carrier. It should work with aqueous acetonitrile, without any tricks. Maybe foil wrap the flask to avoid radical
pathways. The rxn should be easy to track by TLC. Stirring is important though, as this is a heterogeneous reaction.
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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zephler
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I read in a reference that DCE is sometimes used a solvent - just experimenting
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smuv
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Ok. Do a small scale reaction in 20% water/acetonitrile using say 1.2 eq of NBS with a concentration of 15 mmol alkene to 50 mL acetonitrile. If by
TLC you find a spot corresponding to the dibromide, reduce the alkene concentration. Also if you find the alkene seperating from the acetonitrile,
add just enough acetonitrile to keep the liquid phases homogonous. Foil wrap the flask, stir it like hell.
Also, if you find that you get a lot of dibromide formation and your NBS has a strong orange color to it, you can let the NBS stand under an aspirator
vacuum (in a foil wrapped flask) until it lightens up to give a nearly colorless powder.
It is of utmost importance though that you have only one liquid layer. Formation of an aqueous/organic partition will lead to more dibromide
formation.
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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zephler
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one more thing I am speculating on, I have reason to suspect that the DMSO that I used had trace amounts of Na2So4 -- I suppose that some NaBr got
formed during the desired reaction?
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zephler
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Thanks for the info - upon adding DMSO to the flask (that already had acetonitrile in it and water) the solution turned REALLY FREAKING ORANGE - all
the way to red almost - is this ok, or is this what you are describing?
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smuv
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First of all, where are you getting info for DMSO being used for bromohydrin formation? If you saw it used for an aromatic bromination with NBS,
don't think for a second that it would be a good idea for this reaction.
About your first question, in a high yielding bromohydin synthesis, there should be no bromide present, so NaBr should not form. Also, it is silly to
dry DMSO when you are purposefully adding water to the reaction mixture.
About the RXN mix turning really orange, well this is a bad thing. In an ideal situation the reaction mix should stay colorless. In practice, you
might see a tinge of yellow. Dark orange, means there is bromide present in the reaction mixture which will lead to Br2 addition.
My final words are, follow a lit procedure (without deviation), or do the reaction as I specified. If the reaction does not work well, give me the
results of a TLC, HPLC or NMR analysis. While I am glad to help, without concrete data, any attempt for me to help you would be a crap shoot and
ultimately a waste of my time.
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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Nicodem
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Quote: Originally posted by zephler  | | I was talking to someone, who was trying to get the bromohydrin product from allylbenzene (2-bromo-1-phenylpropan-1-ol) using allylbenzne, NBS, and
the solvent acetonitrile and around 1% of the vol water (maybe even 0.5%). |
First of all, all this is nonsense already in its essence. This is because the bromohydrine derived from allylbenzene is not
2-bromo-1-phenylpropan-1-ol! How about writing down the reaction on paper so that you at least have a concept of what you talk about? In an idealy
regioselctive reaction you get 1-bromo-2-hydroxy-3-phenylpropane from allylbenzene. Secondly, a reaction mixture containing only 1% of water is not
going to give you much of the bromohydrine. Mixtures of acetonitrile/water or acetone/water of composition from 1:1 to 4:1 are generaly used. Thirdly,
what exactly is the point of this thread, if all you do is provide nonsense gueswork, while at the same time refusing on checking the literature about
how it is actually done? You are not being helpful to yourself at all!
| Quote: | | The NBS did nothing except for sit at the bottom, looking exactly like it did coming out of the bottle |
Then what you have is not NBS. NBS is well soluble in acetonitrile and somewhat soluble in water.
Use the phrase "I heard..." without providing any reference just a few more times and your popularity on this forum will drop to extreme sub zero
levels. Besides, adding "a pinch of C2H4Cl2", whatever that is, would cause phase separation. And you don't want phase separation when you are doing a
bromohydrin.
…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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UnintentionalChaos
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| Quote: Originally posted by smuv | | First of all, where are you getting info for DMSO being used for bromohydrin formation? If you saw it used for an aromatic bromination with NBS,
don't think for a second that it would be a good idea for this reaction. |
Water/DMSO is fairly standard for bromohydrin formation with NBS, as far as I am aware. So much so, that it was one of the examples in my undergrad
organic chem class when teaching about halohydrin synthesis.
I don't have access, but this is referenced in several places and the abstract is fairly telling:
DOI: 10.1039/J29710000085
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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Nicodem
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That is true, but using DMSO in brominations with NBS is not popular as NBS slowly oxidises DMSO. It works as a solvent for the formation of
bromohydrines from well nucleophilic alkenes, but it fails on less reactive ones. I would guess its nongenerality makes it not the first option to
chose. I would guess that it would fail on less reactive ones like allylbenzene, though it surely works on reactive ones like their isomers
propenylbenzenes.
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zephler
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OK - to end the confusion - I was looking at this synth, and I simply substituted acetonitrile for DMSO:
Place in 1 L RB flask 37 ml (35 g) propenylbenzene (bp 69°C at 13mmHg), then 250 ml of DMSO, add 15 ml H2O, and with stirring, add 106 g of
N-bromosuccinimide* in small portions during 15 min. A yellow color appears when NBS is added, and by the time the addition is complete, the color is
bright-orange. During the addition temp rose up to 50°C. Stir another 15 min, then pour into 1 L of cold water and 3-5 times extract with 100-ml
portions of ether. The combined etheral extracts washed water, then NaCl soln, extract dried over MgSO4 and ether was distilled off. Obtained 70 g of
the bromohydrin as a yellow oil.
trans-Propenylbenzene yields (S*,S*)-erythro-1-phenyl-2-bromo-1-propanol (92%), and cis-propenylbenzene yields
(R*,S*)-threo-1-phenyl-2-bromo-1-propanol (95%)
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zephler
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Thank you for pointing out the grievous error that I made with respect to the product! Oh well, it was a learning process! It's junk now 
| Quote: Originally posted by UnintentionalChaos | | Quote: Originally posted by smuv | | First of all, where are you getting info for DMSO being used for bromohydrin formation? If you saw it used for an aromatic bromination with NBS,
don't think for a second that it would be a good idea for this reaction. |
Water/DMSO is fairly standard for bromohydrin formation with NBS, as far as I am aware. So much so, that it was one of the examples in my undergrad
organic chem class when teaching about halohydrin synthesis.
I don't have access, but this is referenced in several places and the abstract is fairly telling:
DOI: 10.1039/J29710000085 |
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