Polverone - 8-5-2004 at 18:42
I hesitate to post this, since it shows just how unsystematic I am when enjoying myself "puttering around the lab," but it was interesting
and I'd hate to forget it.
Lately I've been experimenting with the conversion of benzyl alcohol to the aldehyde via chemical oxidation. It is easy to oxidize benzyl alcohol
to benzaldehyde. Every reagent I've tried so far (iron-catalyzed acidic H2O2, acidic permanganate, neutral permanganate, neutral dichromate) has
worked, as evidenced by the delightful cherry odor of benzaldehyde coming from the reaction mixture. The problem is keeping the aldehyde from going to
the acid, and isolating the formed aldehyde (doubly complicated by my lack of distillation equipment).
Today I had a half-formed idea of how to make and isolate the benzaldehyde, so I ran with it.
I mixed approximately a 5:1 molar ratio of 95% ethanol and benzyl alcohol and added a stirbar[1]. I placed the mixture of alcohols in a jar, in turn
placed in a glass pot of boiling water with foil around the top to keep the heat in. I wanted the alcohol mixture to be pretty warm but my
hotplate's stirrer doesn't work and my dedicated stirrer doesn't heat.
To the hot stirred mixture of alcohols, I slowly dropped in a saturated solution of KMnO4. The (perhaps shaky) reasoning on my part was that if I
added the oxidizing agent slowly to a large excess of alcohol, I could produce aldehydes without going to the acids too much, since the oxidizing
agent would be likely to contact an alcohol before an aldehyde. Some reference or other of mine said that benzaldehyde was less easily oxidized by
chemical reagents (other than molecular oxygen) than aliphatic aldehydes, so I hoped that any formed acetaldehyde might sacrificially protect formed
benzaldehyde. The large amount of ethanol also helped keep everything in a homogeneous solution.
I smelled acetaldehyde fairly early, and smelled it exclusively throughout most of the procedure. I was starting to worry that all of the ethanol
would react before the benzyl alcohol was touched at all. I eventually started smelling the cherry odor of benzaldehyde though, and after a bit more
KMnO4 solution was added I decided that I shouldn't push my luck: it was time to quench the oxidizer and try to isolate the aldehyde(s).
I was planning on trying to isolate the aldehyde(s) as bisulfite adducts. Sodium bisulfite reacts with manganese oxides to form manganous dithionate
(IIRC); in any case it decolorizes the suspension of manganese oxides left by KMnO4 oxidations and gives a white precipitate. Since I wanted the
aldehyde adducts to be as pure as possible, I wanted to separate the manganese by other means. I decided that ascorbic acid was worth a try. I added
it bit by bit and the solution went from purple-brown to brown to light brown. I added a bit more and suddenly there were flocculent particles in the
solution, which had gone almost clear. A few seconds later, the stirbar stopped; it was welded in place by two little orange "islands" of
sticky precipitate. The liquid had only a bit of cloudiness and a light yellow tint. I was able to simply decant the liquid. What a pleasant surprise
that was!
The decanted liquid was not substantially acidic (gave no fizzing with a bit of sodium bicarbonate), so I added sodium bisulfite, shook well, and
stuck the jar in the fridge. I've not yet had good luck separating acetone (my only available test material) as the bisulfite adduct. A conc.
solution of sodium bisulfite reacts very exothermically with acetone to almost instantly give a thick slush of crystals, but these crystals are very
water-soluble. Is it possible to use sodium bisulfite to isolate minor amounts of ketones/aldehydes from other materials? If so, how? These are
rhetorical questions since I will be reading up on the procedures myself.
The most interesting parts of this experiment, I think, were the oxidation of the mixed alcohols and subsequent reduction/removal of manganese with
ascorbic acid. Even if the isolation fails those bits are worth remembering.
[1]A short piece of a thick roofing nail.
Marvin - 11-5-2004 at 06:26
Few things Ive dug up previosly.
The bisulphite addition compound with benzeladehyde should be sparingly soluable and well crystalised. Which is nice because the usual alternative
with ammonia doesnt work.
Chlorinating toluene to benzal chloride and hydrolysing produces benzaldehyde.
One fairly good method is to oxidise toluene with manganese dioxide catalysed by copper sulphate. Its easy to take the reaction too far and produce
benzoic acid though.
Chromyl chloride will oxidise toluene to benzaldehyde and stop.
Copper nitrate will oxidise benzyl alcohol to benzaldehyde and stop. Uncertain is this is anhydrous or hydrated copper nitrate.
Oil of bitter almonds is mostly benzaldehyde with a little HCN, removable by distillation from a ferrous salt.
I dont have full proceidures for these methods, just the information posted.
AngelEyes - 11-5-2004 at 07:24
Does that mean Copper Nitrate could also oxidise Ethanol to Acetaldehyde...and then stop?
frogfot - 11-5-2004 at 14:43
I've seen the MnO2 method mentioned, but they use H2SO4, no copper mentioned. Heres the reference mentioned (didn't typed the names.. to
lazy): J.Chem.Soc., 91, 261 (1907)
Yay, I can check it out tomorrow and scan..
EDIT: Duh, hydrolysis of benzyl chloride? I've just posted a userfriendly benzyl chloride preparation reaction in this thread: http://www.sciencemadness.org/talk/viewthread.php?tid=1997
[Edited on 11-5-2004 by frogfot]
Marvin - 12-5-2004 at 05:01
My understanding is that the only generic reagent that will take an aliphatic alcohol to the aldehyde and stop is pyridinium chlorochromate (PCC).
Its benzal chloride for benzaldehyde, not benzyl chloride - which goes to the alcohol. There is an aligned method from benzene and chloroform but I
dont consider benzene a valid starting material.
Please do scan the reference frogfot.
Organikum - 12-5-2004 at 06:00
Benzylchloride or a mixture of benzyl and benzalchloride are converted to benzaldehyde by addition of hexamine followed by acid hydrolysis (Sommelet
reaction)
I posted this including the references already here:
http://www.sciencemadness.org/talk/viewthread.php?tid=1751
I also read once about a method of converting EtOH to acetaldehyde with 95% yields by using a Pd or Pt catalyst. Liquid phase reaction. Cant remember
the details though.
[Edited on 12-5-2004 by Organikum]
frogfot - 13-5-2004 at 07:12
I checked the reference on toluene --MnO2/H2SO4--> benzaldehyde, but it gave 5% yield!! ahh
However, they used other reagents like CrCl2, and some other thing that gave up to 70% yield.. (darn memory) Anyway, that part of library was in
physics part of the university, so my copy card didn't work there. I gonna get right card and scan/retype the experimental parts of the article..
Btw, anybody heard about oxidation of toluene to benzaldehyde by oxygen above TiO2? This may be tedious but things are easy to get..
Theoretic - 19-5-2004 at 10:37
What becomes of the copper nitrate?
CuNO3?
CuNO2?
Cu(NO2)2?