01-28-02 04:52
No 261670
Hi all!!!!!
This is from Rh's page...
1500 ml of concentrated H2SO4 was added to 900 ml H2O, then cooled. Then 1.5 l of absolute ethanol was slowly dripped in, carefully to avoid temperature rise. Cool the mixture and add 1100-1200 g KBr (or equimolar amount of NaBr). Heat the mixture on a sand-bath at the lowest temperature at which the ethylbromide distills over, collect it in water. Wash the ethyl bromide again with water and dry it with CaCl2. Yield 90-96%.
A 90-96% yield of etBr from 28.5 mole etOH is impossible based on the amount of KBr (10mole) used... Right? OR-- is the 90-96% yield based on the amount of KBr used? SWIM say's "sorry" in advance if her calculations are off.
SWIM is looking for an easy etBr synthesis that does not involve a reaction that spews HBr gas.
Thanks!!!!
xoxo-Julia
(Stoni's sexual toy)
01-28-02 15:51
No 261818
> A 90-96% yield of etBr from 28.5 mole etOH is impossible
> based on the amount of KBr (10mole) used... Right? OR--
> is the 90-96% yield based on the amount of KBr used?
Yields are always based on the limiting reagent, in this case KBr.
The boiling point of EtBr is very low! If I were you I'd consider ethyl iodide instead. It's more costly to prepare though.
A mixture of 500 g (3.94 mol) of iodine, 800 mL of 84% (w/v) ethanol, and 60 g (2.224 mol) of aluminum foil in small pieces is warmed gently. Once started, the reaction increases progressively in vigor, but can be controlled by cooling; it subsides in about 10 min, and is complete in an hour. The product is distilled until deep red fumes appear. A cold mixture of 700 mL of 84% ethanol and 400 mL of 85% sulfuric acid is added to the cooled residue. After 15 min, distillation is begun, and continued until no more oily drops form in the water in the receiver. The yield of crude ethyl iodide is 260 mL (504 g; approx. 80% yield).
Taken from ../rhodium /nitroal
(Hive Bee)
01-28-02 18:26
No 261864
Thanks, Osmium...
Now that SWIM thought about it over-night.. She thinks making the KBr the limiting reagent is a pretty good thing since etOH is slightly easier to come by than KBr.
xoxo
Julia
(Ubiquitous Precursor Medal Winner)
01-29-02 22:14
No 262392
Drone 342's version at ../rhodium /alkylbr
is similar, but uses a 1-hour reflux before distillation. That sneaky old ethyl bromide just creeps out into the air, and makes you feel jolly but sleepy. Just as if it sort of evaporates away... One source suggests collecting it under water, and that's helped me. Using ice water in the condenser, and chilling the receiver flask is also good for the yield.
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