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Old 05-27-2009, 11:40 AM   #1
Jmz123
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Chemistry help?

The odds of anyone here knowing this exact topic is unlikely, but it's worth a shot.

I have my chemistry exam coming up in about a week, and I just came across this in the list of thing's we're meant to know:

carry out the preparation of an halogenoalkane from an alcohol and explain why a metal halide and concentrated sulfuric acid should not be used when making a bromoalkane or an
iodoalkane

Now to make it more complicated, the example that the book gives uses that method to make bromobutane, so it's kinda contradictory.

Ignoring that though, does anyone know the answer to why they shouldn't be used?
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Old 05-27-2009, 11:42 AM   #2
EndTheRapture51
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Concentrated sulfuric acid is an oxidising agent?
It'd like oxodise the alcohol into a Aldehyde then a Carboxylibic Acid, so the substiutation reaction doesnt take place.

Or something.

I dunno, I need to read my advanced notice paper.
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Old 05-27-2009, 11:42 AM   #3
National_Anthem
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I actually answered that very question. I've got it in my notes somewhere, let me just find it.
EDIT: Sulfuric acid is a strong enough oxidising agent to oxidise iodide and bromide ions into hydrogen, but is too weak to oxidise chloride ions.
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Old 05-27-2009, 11:48 AM   #4
Jmz123
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Quote:
Originally Posted by EndTheRapture51
Concentrated sulfuric acid is an oxidising agent?
It'd like oxodise the alcohol into a Aldehyde then a Carboxylibic Acid, so the substiutation reaction doesnt take place.


Hmm no the book says it takes place as two reactions. The first is the metal halide with sulfuric acid to produce a sulphate and a hydrogen halide. The hydrogen halide then reacts with the alcohol to produce the halogenoalkane

I don't understand what the problem could be :S
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Old 05-27-2009, 11:59 AM   #5
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You can easily make a halogenalkane by simple acid catalyzed SN2 chemistry using conc. sulfuric acid as a catalyst/solvent and using the corresponding hydro halide (the acid of the halogen).

Example - convert ethanol into bromoethane by dissolving ethanol into a mixture of sulfuric acid and hydrobromic acid. In the acidic solution, the alcohol gets protonated, giving a good leaving group. There should be an equilibrium of Br- present in solution, and Br- can act as a nucleophile to perform SN2 with water formed as the byproduct.

i'm a PhD chemistry student, and I'm not sure about the not using a metal halide.... It may well be what the person was saying about how sulfuric acid could oxidize the left over metal to a metal hydride and a sulfate, and metal hydrides are strong bases... so in the presence of sulfuric acid, that could form gaseous hydrogen... which is never good.
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Old 05-27-2009, 12:03 PM   #6
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Quote:
Originally Posted by Musicman48858
that could form gaseous hydrogen... which is never good.


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