flakestoday
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Choosing an acid catalyst
Sorry if there are already posts about this, I searched but couldn't find any so I thought I'd make a new one.
So in my organic I lab we just did a phosphoric acid catalyzed dehydration of cyclohexanol, and my TA told us that the reason we are using phosphoric
acid is that sulfuric leads to charring and other side reactions, and this got me thinking more about the effect the acid can have on the reaction.
Would pTSA have the same charring problems as sulfuric acid? I would think probably just based on the structure, but it would let you do the reaction
in an (initially) water free environment, potentially improving yield (my yield was pretty terrible, I think in part due to the small scale, but I
think also because of the water presence in our phosphoric acid). You could even go so far as to add phosphorus pentoxide to the pot to destroy water
as it forms.
This got a bit rambley, but my main question is how are acid catalysts generally chosen? Are different acids ideal for different reactions? My organic
text just lists common acids but doesn't really give differences aside from the obvious like nucleophilicity.
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Sigmatropic
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I think sulfuric acid causes this charring because it is slightly oxidizing, so yes I would think TsOH or MsOH would work without the charring. IIRC
I've used TsOH in refluxing toluene to eliminate a cyclopentanol in the past.
As for the P205, you'd essentially get polyphosphoric acid which is a typical reagent for dehydrations. P2O5 can also be mixed with other acids which
mixtures are not a viscous goo and thus easier to manipulate, Eaton's reagent for example.
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Triflic Acid
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pTSA isn't oxidizing, and I don't think it dehydrates either. But, I might be wrong.
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Cou
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This isn't about dehydration of alcohols, but acid catalyst is often a choice you have to make in fischer esterification.
Vogel's Practical Organic Chemistry 5th ed says that using sulfuric acid as the catalyst to make cyclohexyl esters will result in a "Very impure
ester" and instead you should saturate the alcohol with anhydrous HCl, which you can generate by dripping hydrochloric acid into anhydrous CaCl2 and
leading a tube into a beaker of the alcohol. The same thing is done for all alcohols that dehydrate very easily, e.g. a secondary alcohol that would
result in a stable alkene.
I've found that while sulfuric acid causes benzyl alcohol to polymerize, p-TsOH doesn't
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flakestoday
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That makes sense about the oxidizing properties of sulfuric acid.
I was able to find a source talking about using pTSA adsorbed on silica gel as a dehydrating agent for alcohol -> olefin (https://www.thieme-connect.com/products/ejournals/abstract/1...), but I think it calls for some experimentation, thanks for the info everyone.
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Texium
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Thread Moved
17-11-2020 at 15:58 |
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