Jackson
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Questions about theroretical synth
I have some questions for a theroritical synthesis.
The first is, the synthesis of 1,2 dibromo butane. The method I saw for synthesis is dehydration of 1-butanol over alumina to form 1-butene. Then it
is reacted with bromine in the presence of an acid catalyst to for the 1,2 dibromobutane. My question is, Does the butanol need to be converted into
butene or can the butanol be reacted with HBr and H2SO4 like you would to make bromobutane but with and excess of HBr? If the dehydration of butanol
to butene is needed, what temperature would the reaction need to take place? 300-500C?
My second question assumes that the butanol does not need to be converted into butene. The question is would 4-amino-butanol survive the reaction to
4-amino-1,2-dibromobutanol using sulfuric acid and hydrobromic acid or would it need to be converted to something like a nitrite.
My third question is about 2-bromobut-1-ene.
I found a document about the synthesis with multiple routes, but I cant tell if some are more viable. It seems like some of the routes are just
incomplete equations. The reaction of 1,2 dibromobutane with acetic acid looks the easiest to carry out but im not sure if it is a useable route. I
will attach the photo with all the routes on the webpage i found.
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Texium
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Quote: Originally posted by Jackson  | | My question is, Does the butanol need to be converted into butene or can the butanol be reacted with HBr and H2SO4 like you would to make bromobutane
but with and excess of HBr? If the dehydration of butanol to butene is needed, what temperature would the reaction need to take place? 300-500C?
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The conversion to butene first is necessary. You can't just slap bromines onto a saturated hydrocarbon
chain. Butene has a boiling point of -6ºC, so you should be able to carry out the dehydration under a reflux condenser and pass the gas that gets
past the condenser into bromine. You shouldn't need such high temperatures. Heating a mixture of butanol and conc. sulfuric acid to full heat on a
hotplate should produce plenty of butene.
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JJay
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HBr can brominate butyl alcohol without any H2SO4, but the reaction is faster with some H2SO4 present. The reaction proceeds forward at room
temperature but is much faster with heating.
Nurdrage has a video on making alkyl bromides: https://www.youtube.com/watch?v=Ydn1D4FSqkc
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Jackson
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Will it be dibromated or just bromated once with sulfuric acid/HBr?
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12thealchemist
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Jackson is trying to make 1,2-dibromobutane, so the reaction of HBr and butanol would be a dead end.
You'd want to use a solvent like DCM to perform the bromination in, but that's common practical chemical sense.
To the best of my knowledge, the amine would not be substantially affected by the concentrated sulphuric acid necessary to effect the dehydration, but
any black carbon crud you form in that reaction would remain in the flask with the protonated 4N-butenylammonium sulphate. Acid/base extraction would
separate the product. As for the subsequent reaction with bromine, I'm afraid I have no idea
The picture you attached looks remarkably like the result of a Reaxys search. As I have access, I shall fill in the missing information.
Rxn 1: With tetraethylammonium bromide; hydrogen bromide In dichloromethane for 2h Ambient temperature
With boron tribromide; acetic acid1.) -78 deg C, 0.5 h, 2.) 20-25 deg C, 24 h Yield given. (70%) Multistep reaction
Rxn 2: No more details regarding conditions, and the paper was irretrievable (Lepingle - Bulletin de la Societe Chimique de France, 1926, vol.
<4>39, p. 757) Also produces cis&trans 1-bromo-1-butene
Rxn 3: With alcoholic potash, well known procedure-type
Rxn 4: Also with anhydrous potassium acetate, and produces α,β-diacetoxybutane. Unfortunately, the paper is in French, but here you are:
Attachment: Montmollin_et_al-1924-Helvetica_Chimica_Acta.pdf (325kB)
This file has been downloaded 316 times
Rxn 5: No additional information; fairly well known procedure-type
Rxn 6: Also comes from the above paper
Rxn 7: Either with potassium carbonate or sodium hydroxide; well known procedure-type
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JJay
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Why can't you dehydrohalogenate the bromobutane and then dibrominate?
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12thealchemist
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That hadn't occurred to me... Though I think you'd get an overall lower yield of dibromide.
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Jackson
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So would it be 2-bromobutane + base —> butene
Butene + 2HBr —> dibromobutene?
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JJay
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No, butene + Br2 -> dibromobutene
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Jackson
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Alright. Would it be a mixture of 1,2 dibromobutane, 1,3 dibromobutane, and 1,4 dibromobutane? Could these be seperated by fractional distilation? I
think they have different boiling points.
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JJay
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It depends on whether you start with a mix of different butene isomers. If you make 1-butene by reacting butyl bromide with a strong base, no, you
will have only a single isomer.
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Sigmatropic
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Pick up a book already, this is getting old fast.
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JJay
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Oh, and actually 1,3- and 1,4-dibromobutane can't be produced directly from any butene isomer.
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