MeSynth
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Alternatives to LAH (Lithium Aluminum Hydride)
Synthetic reduction note: replacing lithium aluminum hydride Page 076
A: Tin and hydrochloric acid technique Page 076
B: Hydrogenation using nickel, palladium, or platinum with or
without charcoal carrier Page 077
C. Reduction of the nitro intermediates with sodium borohydride Page 078
This came from the lab history of narcotics vol 1 amphetamines and is pretty interesting since LAH is so expencive and watched. The synthesis of
things not amphetamine often call for LAH since it is so easy to use it is the ideal reactant. The purpose of this thread is to act as a starting
point for the discussion of alternatives to LAH. Keep in mind that not every reaction requiring LAH can use one of these alternatives in its place.
Here is some info I have found not contained in this book..
--http://www.chemguide.co.uk/inorganic/group4/oxstates.html--
In organic chemistry, tin and concentrated hydrochloric acid are traditionally used to reduce nitrobenzene to phenylamine (aniline). This reaction
involves the tin first being oxidised to tin(II) ions and then further to the preferred tin(IV) ions.
--http://www.docbrown.info/page07/redox3.htm--
^^^^^^^^^^^^^
8.5(c) The reduction of nitro-aromatics with tin and concentrated hydrochloric acid.
In the laboratory, reacting a nitro-aromatic with a mixture of tin and conc. hydrochloric acid by heating under reflux will reduce it to a primary
aromatic amine (-NH2 directly attached to benzene ring). In industry a cheap metal like iron powder and acid are used or a direct reduction in the gas
phase with hydrogen/transition metal catalyst (8.5(d).
In the 'laboratory' preparation, the mixture may need heating to complete the reaction (can be refluxed?) and formation of phenylamine (aniline) from
nitrobenzene can be summarised as
C6H5NO2 + 6[H] ==> C6H5NH2 + 2H2O
but the 'real' equations are rather more complicated, the simplest redox equation I can come up with is
2C6H5NO2(aq) + 14H+(aq) + 3Sn(s) ==> 2C6H5NH3+(aq) + 3Sn4+(aq) + 4H2O(l)
which shows the formation of the phenylammonium cation because the amine is a base and formed in an acid medium. The tin(IV) ion is actually a
chlorocomplex ion of tin, SnCl62-, the hexachlorostannate(IV) ion, so the full ionic-redox equation is more correctly written as ... if you really
must!
2C6H5NO2(aq) + 14H+(aq) + 18Cl-(aq) + 3Sn(s) ==> 2C6H5NH3+(aq) + 3[SnCl6]2-(aq) + 4H2O(l)
Oxidation state changes: 3Sn (0) inc. to (+4) balanced by 2N decreasing from (+3) to (-3).
and then solid or conc. aqueous sodium hydroxide is added to free the amine (immiscible) from its arylammonium cation
C6H5NH3+(aq) + OH-(aq) ==> C6H5NH2(l) + H2O(l)
The primary aromatic amine is then be extracted by steam distillation.
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By hydrogenation with nickel I think he means Raney nickel which is nickel aluminum alloy with the aluminium leached out by treatment with sodium
hydroxide. You see this one being used a whole lot when reading through synthesis.
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Anyone have any knowledge to share about swaping out alternatives for LAH?
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Nicodem
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The reductions of nitroaromatic compounds was practically completely covered in several threads on the forum. Just about all known reducing reagents
and methods were discussed. The least you could do, would be to UTFSE in order to post review the up to date discussions. This way at least we would
have a starting point for new discussions instead of rehashing old stuff.
PS: The synthesis of amphetamine does not involve the reduction of nitroaromatics! It involves the reduction of a nitroalkene which is something quite
different and has a more limited choice of reagents when compared to nitroaromatics. Also, invoking such sources even when irrelevant to the topic
makes you appear kind of unfit for this forum, particularly at your 4th post.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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MeSynth
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Quote: Originally posted by Nicodem  | The reductions of nitroaromatic compounds was practically completely covered in several threads on the forum. Just about all known reducing reagents
and methods were discussed. The least you could do, would be to UTFSE in order to post review the up to date discussions. This way at least we would
have a starting point for new discussions instead of rehashing old stuff.
PS: The synthesis of amphetamine does not involve the reduction of nitroaromatics! It involves the reduction of a nitroalkene which is something quite
different and has a more limited choice of reagents when compared to nitroaromatics. Also, invoking such sources even when irrelevant to the topic
makes you appear kind of unfit for this forum, particularly at your 4th post. |
it was just an example of how tin could be used as a reducing agent. No need to get your panties in a bundle...
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SHADYCHASE54
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God I so love the elitest attidudes attached to advice and the constant claim that ignorence on a particular topic should somehow preclude ones
participation on this forum. As far as I know this is a site advertised upon the www. which is acessable by most and open to all willing to join. As
far as I am concerened even if a person is interested in the manufacture of something potentially shady as long as his question is relatively
pertinent to his topic and he is at least more educated than a mere cook book reader (which is always obvious) then his question has some merit, if
to no one else then to him or herself. To expect all out there to be fully versed upon every past topic before posting is absurd; or that the only
posts that should appear is one that is going to offer something new or debaitable for the most learned of us is also ubsurd. If someone so disagrees
with a question because its been covered sometime ago is to kill all discourse and not allow the newer of us to banter to and fro. If someone
disagrees with a post because of content or repetativeness then don't respond to it at all, most likely if the member posting is new and is ignored
then they will quickley seem to disapear back to Rhodium. This is merely an opinion directed not only at this posting but to many I have seen I am
personally not looking for drug talk but it is sometimes funny to read it. I like new blood on the system and thusly watching the weeding process
unfold, long live sciencemadness.
[Edited on 31-7-2011 by SHADYCHASE54]
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peach
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But you just did.
Thiourea Dioxide --> better than borohydride?
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Melgar
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Or my personal favorite: aluminum/galliium alloy. drop in a chunk, and stand back! gallium can be obtained from "mercury-free" thermometers that
look like they have mercury in them. And you don't need much gallium, just enough to activate the surface of the aluminum. And pure aluminum can be
obtained from aluminum wires or the rotors of old ac induction motors. In this case, you probably should use isopropyl alcohol as a solvent/hydrogen
donor because water is way too reactive.
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Random
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Does that mean carboxylic acids can be reduced with iron/hcl, activated aluminium and similar things like LAH reduces them? Or does iron/hcl reduce
ketones to alcohols or alkanes?
[Edited on 5-8-2011 by Random]
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not_important
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| Quote: Originally posted by Random | Does that mean carboxylic acids can be reduced with iron/hcl, activated aluminium and similar things like LAH reduces them? Or does iron/hcl reduce
ketones to alcohols or alkanes?
[Edited on 5-8-2011 by Random] |
No, they will not. Generally takes metal or boron-containing hydrides, or H2 + catalyst under strenuous conditions.
There are some other trick ways, heating the mixed calcium or other metal salts of the acid and formic acid to produce the related aldehyde (in fairly
low yields generally), then reducing the aldehyde with milder reducing agents.
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