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IudexK2
(Hive Bee) 10-26-01 18:04 No 229058 |
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Sulfuric acid in LAH reductions | Bookmark | |||||
In PiHKAL, Shulgin frequently adds conc. H2SO4 to LAH/THF before adding the nitrostyrene... can somebee explain to me how this works and if it is a good/necessary thing to do or not? Example: A solution of 2.0g LAH in 100mL anhydrous THF under He was cooled to 0°C and vigorously stirred. There was added, dropwise, 1.3mL of 100% H2SO4, followed by the dropwise addition of a solution of 3.1g 3,4-dimethoxy-5-ethoxy-beta-nitrostyrene Many thankx, IudexK |
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Rhodium (Chief Bee) 10-26-01 18:23 No 229064 |
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Re: Sulfuric acid in LAH reductions | Bookmark | |||||
2 LiAlH4 + H2SO4 => 2 AlH3 + Li2SO4 + 2 H2 AlH3 is a reducing agent with milder reducing properties than LiAlH4. |
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IudexK2 (Hive Bee) 10-26-01 18:40 No 229068 |
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Re: Sulfuric acid in LAH reductions | Bookmark | |||||
Ahhhhhhhhhhhhh I see. Thankx. Is AlH3 generally a better reducing agent for nitrostyrenes, or is it dependant on the nitrostyrene in particular? |
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Rhodium (Chief Bee) 10-26-01 18:45 No 229070 |
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Re: Sulfuric acid in LAH reductions | Bookmark | |||||
I don't know really. Tabulate the Pihkal yields for all Shulgin's AlH3 reductions and see if the yields are generally better than the ones using LiAlH4. |
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megamole (Hive Bee) 10-26-01 21:22 No 229105 |
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Re: Sulfuric acid in LAH reductions | Bookmark | |||||
The reason for treating LAH with sulfuric acid is that it generates alane, AlH3, which in the presence of THF or ether, is not as strong of a Lewis acid. While LAH can remove aromatic halogens and a great deal more functional groups, alane will not. In cases where a greater degree of a certain selectivity is needed, alane is far superior. Speaking of which, are there any people here with practical experience working with in-situ generated alane and borane complexes? There are so damn many facinating practical applications of these materials, but there has been such little discussion on the subject (and even fewer experimental results) here at the Hive. We need to resolve this! |
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yellium (Hive Bee) 10-27-01 01:22 No 229161 |
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Re: Sulfuric acid in LAH reductions | Bookmark | |||||
I've been informed that alane reductions work. And you don't need 100% H2SO4, 96--98% also works. |
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IudexK2 (Hive Bee) 10-27-01 02:15 No 229165 |
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Re: Sulfuric acid in LAH reductions | Bookmark | |||||
Or anhydrous AlCl3. |
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