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Author: Subject: Obtaining Benzaldehyde
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[*] posted on 14-12-2018 at 02:47
Obtaining Benzaldehyde

Benzaldehyde is used for many applications in organic syntheses (Cannizzaro reaction, condensation with acetone, making anti-seizure medications etc.) that seem amateur friendly. Obtaining benzaldehyde is however a concern.

Benzaldehyde is available as bitter almond oil (approx. $30 for 100mL online - Australia), and not available OTC. The main concern is the cyanide present in the sample (it is even advertised to contain cyanide as a toxin). Certainly, performing organic reactions with cyanide is unacceptable due to the risk of HCN leakage.

Benzaldehyde can also be obtained from retro-aldol reaction (of cinnamaldehyde) or from partial oxidation of benzyl alcohol, both of which are available at around the same price as benzaldehyde in pure form and without issue. These procedures are however very cumbersome from what I have read on SM and involve steam distillation procedures (requiring glassware which I do not have).

How significant is this cyanide contamination, and otherwise what routes are viable to benzaldehyde?
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[*] posted on 14-12-2018 at 03:26

There is a thread here on toluene to benzaldehyde. Good luck. I thhink it is possible using KMnO4 and a catalyst. But tricky. I have had a go and obtained an odour but nothing more.
Oxidation of benzyl alcohol using chromyl chloride seems to be reliable. Coincidentally I was watching NileRed on this today. Chromal chloride is not without its own issues though.

There are probably other routes.
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21-12-2018 at 21:02
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[*] posted on 22-12-2018 at 16:28

I don't think, that there is HCN present, I think it would react with the benzaldehyde to form the corresponding cyanohydrin. Even if it's present there should just be trace amounts in it, for some applications the benzoic acid, that's formed from the Benzaldehyde due to oxidation would be more of a concern, especially if the stuff is a bit older it can be quite a lot. So I would advise you to distill the Bitter Almond oil before using.
In terms of the HCN: Bitter almonds don't contain the HCN directly, but around 5% amygdaline, which only contains around 0,3% HCN, also it normal is just slowly released enzymatic, so that shouldn't be that dangerous. Do the destillation outside or in a well ventilated area and you should be safe. If you want to be sure maybe test for cyanides using a mix of Fe²⁺ and Fe³⁺ Ions.
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[*] posted on 22-12-2018 at 19:09

If you can get a hold of benzyl alcohol, then the Anelli oxidation would give you benzaldehyde. You need TEMPO as a catalyst and bleach as the oxidant. TEMPO can be hard to get a hold of, but Melgar is currently offering it in the Reagents and Apparatus Acquisition forum.

Once, out of curiosity, I ran GCMS on an almond fragrance for making candles and it turned out that it was something like 70% benzaldehyde and 30% benzyl benzoate. If I can find the brand I'll let you know. But that seems like an easy distillation and that product definitely doesn't contain HCN (not that I think your product does either. If it's for general consumers I can't imagine HCN being present).

Edit: this is the product that I can confirm the composition of

[Edited on 23-12-2018 by Justin Blaise]
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