[magician4]

I think that toluene is a by-product in styrene manufacture. This may be the source of the benzaldehyde, if you have any toluene in your styrene?

Styrene industrial grade typically is like 99,5% +
mostly it is made from ethylbenzene via cat. dehydration, and the ethylbenzene is made from benzene and ethylene
  no major source for typically up to 5% benzaldehyde (from toluene) here

a typical analysis of industrial grade styrene will show like 0.02% benzaldehyde
http://www.lookchem.com/Product_1967917/CasNo_100-42-5/Styrene.html#.UdLpSdhGQgk


  there should be a real pathway from styrene to benzaldehyde: this is not coming from impurities



@ curiouscat :
70°C  with 30% H₂O₂ looks pretty harsh to me on the H₂O₂

maybe this is enough to provoke (minor) thermal decomposition already: H₂O₂  H₂O + O^.
with molecular oxygen around, this might rearrange to form ozone, which then will split the styrene

  to test for this theory, you might wish to either lower temperature or work with protective gas, and see if reduction of temperature (less oxygen-radicals) or reduction of elementary oxygen will reduce the typical amount of benzaldehyde thereby


regards

Ingo

[curiouscat]

@ curiouscat :
70°C  with 30% H₂O₂ looks pretty harsh to me on the H₂O₂

maybe this is enough to provoke (minor) thermal decomposition already: H₂O₂  H₂O + O^.

Probably. I see some bubbling in the H2O2 as it is in the dosing funnel and especially where it meets the reaction mix.

with molecular oxygen around, this might rearrange to form ozone, which then will split the styrene

  to test for this theory, you might wish to either lower temperature or work with protective gas, and see if reduction of temperature (less oxygen-radicals) or reduction of elementary oxygen will reduce the typical amount of benzaldehyde thereby

Great idea. I will try that.

[curiouscat]

Could you make your product by chemical reduction (NaBH₄) of phenacyl chlroide?

Probably could if it were a lab synthesis.

Commercially it won't make sense (I think). NaBH4 and phenacyl chloride are both more expensive I suspect than HCl + H2O2. Even Styrene is a bulk commodity chemical. Even with perfect yields versus the imperfections of my status quo, I doubt it's gonna work.

I doubt the economics would work out. But I could be wrong.

[curiouscat]

@ curiouscat :
70°C  with 30% H₂O₂ looks pretty harsh to me on the H₂O₂

maybe this is enough to provoke (minor) thermal decomposition already: H₂O₂  H₂O + O^.
with molecular oxygen around, this might rearrange to form ozone, which then will split the styrene

  to test for this theory, you might wish to either lower temperature or work with protective gas, and see if reduction of temperature (less oxygen-radicals) or reduction of elementary oxygen will reduce the typical amount of benzaldehyde thereby

Just thought I'd post an update regarding my follow up on one of @magician's ideas:

Tried reducing the reaction temperature. This screws up the selectivity. Apparently as T reduces I make more and more of the dichloride. (As an aside, is this something people would expect, even in hindsight, from the underlying chemistry?)

So, one productive result was that I'm now working at slightly higher T and this gives a better halohydrin : dichloride (i.e. product:byproduct) ratio.

Thanks @magician!