AndersHoveland
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substitution on hydrophosphites
I have a question about nucleophilic substitution on hydrophosphite groups.
The first thing that should be addressed is the misconception some of you may have about the structure of hydrogen phosphite derivitives, including
phosphorous acid. The structure of phosphorous acid is not P(OH)3. Actually, one of those hydrogens is on the phosphorous atom itself. P(OH)3
tautomerizes to the more preferable form HP(=O)(OH)2. In terms of bonding, the hydrogen atom is electron-donating to the oxygen atom, or it can be
thought of this way: one of the hydrogen atoms ionizes off and complexes to the phosphorous center. (probably the real valence of the P is somewhere
between 3 and 5, bond orders are not always integers)
So take something like dimethyl hydrogen phosphite ester, (CH3)2POH. How would this react with bromomethane? Would it simply form the trimethyl ester,
or could the phosphorous atom be alkylated? Do you understand what I mean?
When reacting methyl alcohol with PCl3, normally we do not have to deal with these effects because the methyl chloride that forms is not really an
alkylating agent, the rate of reaction is so slow as to be negligible. In case you were not aware, basically the reaction is:
PCl3 + 6 CH3OH -->(CH3O)3P + 3 CH3Cl + 3 H2O
(it is more complicated than that, but this equation sums up the reaction)
If a lower ratio of alcohol is used, (CH3)2POH can be formed. Note that the 'OH' is not a hydroxyl group here.
But what if we reacted PBr3 with methyl alcohol? Then the bromomethane formed could readily participate of nucelophilic substitution, further reacting
with the intermediate product.
What would the reaction be between (CH3O)2POH and CH3Br ? When the bromine atom accepts an electron, which part of the phosphite group is the new
methyl group going to bond to?
CH3P(=O)(OCH3)2
I realize this question could potentially have application in nerve gas synthesis, which is not where I am trying to go with this at all. Trimethyl
phosphite can easily be isomerized to dimethoxymethylphosphorous oxide at elevated temperatures (though the product in this case is heavily
contaminated with the dimer), so it is not as if this discussion would be very helpful to those who might have such aims. Just curious about the
chemistry here.
I suppose in some ways this might be analogous to the substitution reaction between silver nitrite and bromomethane (forms mostly nitromethane if you
were wondering).
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AndersHoveland
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rather interesting article related to the subject: http://www.fluoride-history.de/p-mfp.htm
monoflourophosphate is used, in small quantities, in most brand name toothpastes. yet monofluorophosphate esters are cholinesterase inhibitors,
basically like a milder form of nerve gas. diisopropyl fluorophosphate has been used as a parasympathomimetic drug in ophthalmology, as a miotic agent
in treatment of chronic glaucoma, and as a miotic in veterinary medicine.
Are the chemical properties of an ester really so different from its parent ion? Likely the only reason for the much increased toxicity of the ester
in this case is that it is lipid soluble. I think I will stick to sodium fluoride in my toothpaste, thanks. Perhaps there may be truth to some of what
fluoride alarmists have been trying to warn us about.
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softbeard
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Sodium monofluorophosphate is an ionic species, that has absolutely no anti-cholinesterase activity. It does not penetrate to the central nervous
system. You need the alkyl double esters for that. I never got constricted pupils from brushing my teeth with MFP-containing toothpaste. I think this
argument is a none-starter.
Probably nothing wrong NaF either, but it is more toxic.
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AndersHoveland
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It may be that the anti-cholinesterase activity of the monofluorophosphate ion is just several orders of magnitude less, below detectable limits. That
still does not necessarily rule out possible cumulative negative health effects.
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softbeard
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Quote: Originally posted by AndersHoveland  | | It may be that the anti-cholinesterase activity of the monofluorophosphate ion is just several orders of magnitude less, below detectable limits. That
still does not necessarily rule out possible cumulative negative health effects. |
That is pushing it. "A poison is too much of anything" It just does not penetrate the CNS (central nervous system). It really is as simple as that.
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