Q's on the allyl bromide synth

[El_Zorro]

I've been reading the allyl alcohol-->allyl bromide synth in TSII.  And, of course I've got some questions.

1)Strike says to add the HBr(aq) to a flask, then add concentrated H₂SO₄.  Wouldn't that give HBr(g)?  Or is the HBr appreciably soluble in an H₂SO₄ solution?  If so, what if I were to substitute HCl to give allyl chloride, would that not definately give HCl(g)?

2)If I were to substitute a molar amount of ethanol for the allyl alcohol, this would give the bromo/chloroethane, just like Strike said, right?  What is the bp of chloroethane?
Do not go gentle into that good night.  Rage, Rage, against the dying of the light.  --Dylan Thomas

[El_Zorro]

somebody throw me a fuckin' bone, here!

Rhodium must be takin' a shit or something.
Do not go gentle into that good night.  Rage, Rage, against the dying of the light.  --Dylan Thomas

[hypo]

> Strike says to add the HBr(aq) to a flask, then add
> concentrated H2SO4.  Wouldn't that give HBr(g)?

don't think so.

btw, you can also start with NaBr.

> 2)If I were to substitute a molar amount of ethanol for
> the allyl alcohol, this would give the bromo/chloroethane,
> just like Strike said, right?

yes, yes. check Org.Syn. vol. I for tons of alkyl bromide
synths. see also rhodiums site for a file on alkyl bromides.

> What is the bp of chloroethane?

chemfinder.cambridgesoft.com - look yourself!  

[El_Zorro]

> Strike says to add the HBr(aq) to a flask, then add
   > concentrated H2SO4.  Wouldn't that give HBr(g)?

   don't think so.

But H₂SO₄ is such a powerful dehydrating agent, I can't see how it wouldn't.  For God's sake, it rips the water out of fucking sugar!  But don't tell me if I used HCl to make chloroethane that it wouldn't give HCl(g).
Do not go gentle into that good night.  Rage, Rage, against the dying of the light.  --Dylan Thomas

[PrimoPyro]

It wont make gaseous HBr because the HBr will stay in solution with the (now aqueous) H2SO4. That is why hypo said no it wont make gaseous HBr. You would have to heat the solution to drive off the HBr.

[El_Zorro]

Alright.   I asked at first if the HBr was soluble enough in an aqueous solution of H₂SO₄ to stay in solution.  All hypo gave me was "don't think so."

Now how about substituting HCl(aq) for the HBr?  That'll prolly give off some gaseous HCl, won't it?
Do not go gentle into that good night.  Rage, Rage, against the dying of the light.  --Dylan Thomas

[PrimoPyro]

Of course it will give off "some" HCl, but how much is very likely to be directly proportional to the temperature of the system. Since both HCl and HBr are gasses, their solubility in the liquid phase medium will decrease with increasing temperature. So, if performed at 5C this reaction might not evolve any gaseous acid at all, but if performed at 80C+ I bet it would spew all over the place.  

[hypo]

i just checked a wise book and it says:

1) for bromides, mix H2SO4 with the alcohol under cooling and then add the HBr. don't use H2SO4 for secondary and tertiary alcohols.
2) for chlorides, use 2 eq. HCl and 2 eq. ZnCl2. better yet, use PCl3 or PCl5

btw: the same book says to make allyl bromide _without_ H2SO4!

[El_Zorro]

Yeah, Primo, but the solubility of HCl and HBr does not follow the set rules of gas solubility.  For instance most gases don't form maximum boiling point concentrations in water, do they?

But anyway, I personally think that some HCl would be given off, so I ould think that modifying the procedure to putting ALL of the H₂SO₄ into the dropping funnel and the Everclear and the HCl into the flask and dripping in the sulfuric.


Do not go gentle into that good night.  Rage, Rage, against the dying of the light.  --Dylan Thomas

[hypo]

> But anyway, I personally think that some HCl would be given off,
> so I ould think that modifying the procedure to putting ALL of
> the H2SO4 into the dropping funnel and the Everclear and the HCl
> into the flask and dripping in the sulfuric.

well, that is the wrong way. this way you _will_ have local excess of
H2SO4, wich will generate heat and give of gases. mix the H2SO4 with
the alcohol, cool and add slowly the HBr/HCl ==> no gas.

btw, if the pros use ZnCl2, there probably is a reason...

[El_Zorro]

damn.  you're a smart motherfucker, hypo.  I'll have to look up that ZnCl₂ shit, too.
Do not go gentle into that good night.  Rage, Rage, against the dying of the light.  --Dylan Thomas

[El_Zorro]

alright, who's ot the ref's on the ZnCl₂?
Do not go gentle into that good night.  Rage, Rage, against the dying of the light.  --Dylan Thomas