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kmno4
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Quote: Originally posted by Nicodem  | | It is quite possible that with the more concentrated H2SO4 there is a delay in water accumulation (due to the extreme hygroscopicity of H2SO4) and
that the peak of conversion to diphenylmethane is actually achieved relatively fast, but further reflux to achieve the theoretical amount of water
only leads to crap formation due to Friedel-Crafts disproportionation / oligomerisation (thus reduced yields). |
I have different impressions. Seems that first , small amounts of H2SO4 gives only (mono)benzyl sulfate + corresponding, small amount of water and
nothing more. At some critical benzyl sulfate concentration, something happens and benzylation starts, with releasing H2SO4. It is just speculation.
Some papers claim isolation of similar benzyl tosylate if TsOH is used instead of H2SO4. However, it does not explain this "critical" amount of H2SO4.
Beside, rate of water formation at initial stage (few drops of acid added) does not depend on concentration (at least in 75-96% range) of this acid.
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Nicodem
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I will confide to your subjective impressions, due to lack of chromatographic evidence.
Benzyl tosylate should indeed form from benzyl alcohol and TsOH under such reactions conditions via the same SN1 mechanism that gives also
diphenylmethane, but that ester is not the most active benzylating species. The benzylating reagent is the protonated alcohol and the transient
carbocation it can form, as is generally the case with Friedel-Crafts alkylations using alcohols. This is why only benzylic, allylic and t-alcohols
can be used (exceptionally s-alcohols work as well, as is the case in the synthesis of cumene from benzene and isopropanol). Just refluxing benzyl
tosylate in absolutely dry benzene is unlikely to give you much diphenylmethane in the same reaction time as would benzyl alcohol with TsOH catalysis.
All this is said just in case you believe that benzylation necessarily occurs via a sulfate or sulfonate ester. This is why your experiment with
n-butanol gave no desired product, even though under the applied reaction conditions the formation of butyl sulfates is possible (but the formation of
a carbocation under these conditions is not possible).
How about posting a detailed report about your synthesis of benzophenone, complete with references, also in the Prepublication section?
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kmno4
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Oxidation of DPM to benzophenone will be posted later, because I still cannot find good procedure. I have just made some SciFinder researches but
among 100 references I do not see any good one 
Good = conversion not less than 90%, selectivity for benzophenone not less than 95%, cheap and fast.
The simplest methode - 10-20% HNO3 under long lasting reflux gives yellow mass. It is said that such oxidation gives only benzophenone and
alpha-nitro-DPM (reference unavailable and in Russian...)
CrO3 in AcOH seems to be OK, but rather large amounts of acetic acid are used for relatively small amounts of DPM (ex. 50 ml AcOH per 5 g DPM). I
would try KMnO4 in AcOH, but I am affraid to get BUM instead of oxidation. In boiling acetone KMnO4 gives selectivity for benzophenone near 100%, but
conversion DPM is only 60% (...who knows why ?). Besides such oxidation has unknown (for me) stoichiometry.
Back to benzylation.
I did not say that that with n-butanol I got nothing.
It was just a trial run, to see what happens with such a primary alcohol. I would choose EtOH, but obviously it is not recomended (beacuse of
experimental reasons).
H2SO4 (conc.) was adding in portions, one portion was about 10 drops. Some acid was added, some water collected and nothing more. More acid - more
water.. Finally I stopped counting amount of acid.... I got some high boiling residue with toluene odour. I do not know what was that, I would have to
repeat this experiment with known amount acid added.
As I said, recovered benzene had b.p. 79 C. To remove unknown impurity, I tried few "tricks". The only thing that worked was aqueous CrO3 + H2SO4
(with mag.stirring). Impure benzene became slightly turbid, then yellow, orange and... colourless and clear 
Interesting - there was no butyric acid smell, so this impurity was not butanol or its ether.
Redistilling of this purified benzene gave normal b.p. 80 C.
I will try this one more time ( or two to collect more eventual products) with 20g of alcohol per 100g of benzene and slightly more than 1 molar
equivalent of H2SO4.
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Satan
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I have an idea about oxidation of DPM. Exhaustive chlorination on -CH<sub>2</sub>-, and then hydrolysis to ketone with aqueous base. Slow
addition of TCCA in CCl<sub>4</sub> (or DCM etc.) might work - just an extrapolation of other chlorinations involving benzylic carbon 
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kmno4
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If my experiments with direct oxidation will fail, then I will try this or similar methode. In this way benzophenone is prepared in OS procedure.
But now I have a few strong oxidants for testing......
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woelen
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Thread Pruned
17-11-2011 at 05:28 |
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