cactusmae - 11-3-2012 at 14:29
I tried debromination to get alkene and failed several times. The procedure I carried with the zinc dust and CH3CN reflux at 80 degree C. Somebody
told me the old Znic dust not working anymore for the debromination rxn. I was wondering why? Any other suggestions? Thanks.
BromicAcid - 11-3-2012 at 16:11
You say 'old zinc dust' do you mean that literally or are you just saying it as an expression? Could it be that your zinc is mostly just zinc oxide?
Endimion17 - 11-3-2012 at 16:52
Zinc is usually activated by strongly mixing it with a bit of mercury which disrupts the oxide layer, but that's one way of producing a toxic, fuming
gunk. That mixture can be used for filling some columns used to reduce some inorganic solutions. I'm not quite sure what would happen if it's mixed
with cyanomethane. Smells like one potential way of getting organomercury compounds, so don't even try.
Also, I don't get the "debromination" part. Where's the bromine?
cactusmae - 11-3-2012 at 16:52
Zinc dust was bought at 2001.
cactusmae - 11-3-2012 at 17:03
-CHBr-CH2Br group
solo - 11-3-2012 at 17:36
...did you activate your zinc dust ,
"Commercial zinc dust (16 g., 325 mesh) was activated by stirring for 3-4 minutes
with 100 ml of 2% hydrochloric acid. The zinc was immediately filtered under
suction, washed to neutrality with water, and then washed with 50 ml of ethanol,
100 ml of acetone, and diethyl ether. The resulting powder was dried at 90 deg C
under vacuum (10 minutes) and was used within 10 hours of preparation."
...source,
http://www.erowid.org/archive/rhodium/chemistry/cinnamyl.txt
and usually zinc-copper couple is used in dehalogenation.....see,
Kinetics and mechanism of reductive dehalogenation of carbon tetrachloride using zero-valence metals
Preparation of Zinc-Copper Dust.-To a hot, rapidly stirred solution of 2.0 g. of cupric acetate monohydrate in 50 ml. of glacial
acetic acid was added 35 g. of zinc dust. After about 0.5 min. all of the copper had deposited on the zinc. The couple was allowed to settle for 0.5-1
min., then as much of the acetic acid as possible was decanted taking care not to lose the silt-like couple. The dark reddish gray couple was then
washed with one 50-ml. portion of acetic acid followed by three 100-ml. portions of ether.6 The moist couple was ready for use.
....source,
Cyclopropanes from an Easily Prepared, Highly Active Zinc-Copper Couple, Dibromomethane, and Olefins
Attachment: phpNODFur (403kB)
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[Edited on 12-3-2012 by solo]
[Edited on 12-3-2012 by solo]
Attachment: Kinetics and mechanism of reductive dehalogenation of carbon tetrachloride using zero-valence metals.pdf (514kB)
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PHILOU Zrealone - 15-3-2012 at 07:00
Just a side note because I found it very interesting and the info was not that easy to find ... only one line referenced it in one of my chembooks
among many dozens...
--> The elimination of geminal dibromide with Zn dust is trans-elimination...
This display a convenient way to protect a double link because the addition of Br2 is also a trans-addition... so the initial conformation cis-ene or
trans-ene can be recovered.
Also this gives good ideas as how to go from gem-diols to cis or trans alcenes...
My idea was to get a synthetic way to cis BrCH2-CH=CH-CH2Br from butantetrol (erythritol).
[Edited on 15-3-2012 by PHILOU Zrealone]