Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: HP on August 26, 2005, 01:23:13 PM
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Hi everyone...
I would like to know if possible to conjugate hydroxy compounds to acrylic acid ??? and the reaction conditions and catalyzators used this. I know the OH hydrogen is not very mobile but may be its possible do this reaction...
Waiting your suggestions friends :P
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I think that in general the reverse reaction is too fast to allow the Michael product to be isolated.
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I red this i am searching for is called "oxa-Michael addition" of alcohols to unsaturated compounds. I red its catalized by acids and some other.. Do you think this reaction will work with some alcohols and acrylic acid in presense of conc. H2SO4 and if could what is the mechanizm this addition?
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With a carboxylic acid you're probably more likely to just get a Fisher esterification reaction. After that you might get some of the beta-alkoxy ester from a subsequent Michael addition.
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Thanks for reply.
You are right for the esterification reaction with free carboxylic group of acrylic acid. If i first protect it as ester with 1mol equiv alcohol then probably the second mole alcohol will react by Michael addition with the unsaturated acrylic ester..Also i am thinking the possible addition reaction of H2SO4 to unsaturated bond of acrylic acid forming its sulfate - do you think its possible as intermediate if first sulfuric acid with acrylic one and how it will influence its reaction with the alcohol? Sorry for the many questions but its interesting for me :)
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The sulfate anion is a very poor nucleophile, so I don't think that trying to use that as Michael nucleophile would work.
Esterification of the acid might help, but then you have to worry about trans-esterification in the next reaction.
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Transesterification is possible but if i use the same alcohol in excess first for esterification the carboxylic group and eventually next step for oxa-Michael addition to acrylic ester if that work- thats what i am thinking about...I know alcohols react with conc. H2SO4 first forming corresponding sulfate ester which is stable if not heating to dehydrating alkene product. May be this sulfate OH will be much reactive than alcoholic OH for Michael type addition reaction with activated double bond of acrylic ester but then to form the desired product: R-O-CH2CH2COOR i imagine SO3 should be eliminated or something other happens ??? I am not very familiar with these mechanisms just try my imagination :)
Greetings HP
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Yes, using the same alcohol would solve the trans-esterification problem.
As to making sulfate esters, I think that you need pretty strong dehydrating conditions to form things like dimethyl sulfate from sulfuric acid and methanol.
For a Michael addition you are probably better off with an alcohol than with a sulfate since the alcohol electrons on O aren't delocalized onto the rest of the molecule, and therefore are more nucleophilic. Plus, RO- is a worse leaving group that a sulfate ion so that might slow down the reverse reaction too.
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Thank you for the comprehensive expl. The picture become clear to me i think. It's something similar the synthesis of diethyl ether from 2 mol ethanol and conc. H2SO4. But when we have activated double bond in the acrylic ester then next portion of alcohol in the present of acid immediately will form alkoxonium ion R-OH2+ which probably mediate alcoxy addition to activated 2bond. On the other hand i am little confused because remember the hydration reaction of ethylene with steam in presense of H3PO4 forming ethanol which further probably don't react with ethene molecule to form diethyl ether and i dont know why so :)
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Yeah, I had thought about the ether side reaction too. I'm not exactly sure why that doesn't happen with phosphoric acid, but I do know that the reaction with ethanol and sulfuric acid is actually quite inefficient as is only used because all the ingredients are so cheap.
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Aham i think it doesnt happen becase kinetic control: the reaction mixture pass true heterogenic supported catalyst eg SiO2/H3PO4 for a very short time in the reactor and the formed ethanol flow out before it could convert to diethyl ether by bimolecule dehydration reaction with the acid(2moles alcohol loss 1mol water) or the possible side oxa-Michael reaction with 1 mol ethene by other mechanism. The reverse reactions of hydration ethene and its dehydration are in equilibrium. Heres some links i found:
http://www.chemguide.co.uk/physical/equilibria/ethanol.html
http://www.chemguide.co.uk/physical/catalysis/hydrate.html
http://www.chemguide.co.uk/mechanisms/elim/dhethanol.html#top
Interesting discussion i continue thinking ::)
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That is interesting. I don't know very much about the various processes used for mass production of commodity chemicals. I wish I new more about them though. The amount of engineering that goes into making those processes so efficient and cost effective is really remarkable.
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Yes, efficiency's essence of chemistry enginering. Theres some new facts i lerned this current topic. Theres many artcles for addition of alcohols to acrylonitrile and the process is called cyanoethylation. It's catalyzed most often by bases.
R-OH + H2C=CH-CN --> R-O-CH2CH2-CN which nitrile could easily hydrolyzed to corresponding eter-acid. This i am asking for is if the nitrile group is much better activating the double bond group than the carboxylic or ester-caboxylic one and how to explai it with induction- mesomeric effects or polarity bonds?
I know that cyanacrylates are extreemly reactive compounds which readily react and with water by polimerizing in the well known super glue. In its molecule theres two groups: cyano and ester-carboxylic one and they both interact with the double bond to "super activate" it. Theres superposition of 2 +I effects i think.
2HC=C(CN)COOR
H2O + 2HC=C(CN)COOR--> HO-CH2CH(CN)COOR + 2HC=C(CN)COOR--> quick polymerization by ion mechanism but i still dont look the carboion(cation or anion) :) ? Dont try between your fingers or eyelids!!
If i am not alredy boring discussion may to continue thanks.
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Well, after the first Michael addition of OH- into the acrylate you have a very acidic proton (pKa ~ 12) between the two withdrawing groups. The anion is then capable of performing a second Michael addition into another acrylate and so on.
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You are absolutely right for the acidic proton after addition H2O to cyanacrylate!Can you please figure the anion and how exatly it's forming and the next developing the polymerization process as you viw it?
Thanks!
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You mean something like this?
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Perfect example for anionic polymerization movies! But as i see this wonderful scheme you show the Nuc attack of HO:- as i understand but then what happen with the H from the water or where it goes? If happens this reaction H-OH + H2C=C(CN)COOR --> HO-CH2CH(CN)COOR do you mean the forming CH is very acidic and if so may be next reaction with H2O qould lead to H3O+ and the obtain the anion? Also do you think its possible and "migration"polyaddition mechanism of polymerization of cyanacrylate as just this CH proton migrate from monomer-to-monome to polymer :) In fact from my studies i know the mechanism of polymerization is exactly as you show by anionic polymerization and thats why so quick and efficient but who knows;)
Thanks again!
HP
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You probably need some catalytic amount of base to initiate the reaction. The proton between the two withdrawing groups is less acidic than hydronium (H3O+).
You might get some product just base on the equillibrium between the protonated and deprotonated forms, but base would definitely be a good idea.
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Well hope still in the topic :)
How really to form carb anion of cyanacrylate monomer only with very small amount of water absorbed by air moisture on the surface we wont to stick. I am hinking about these hypotheses:
First addition of H2O to cyanacrylate to intermediate HO-CH2CH(CN)COOR
As you see there's 2 mobile protons of this molecule CH acidic and OH - i dont know ??? Do you think its possible these quick post reactions:
HO-CH2CH(CN)COOR + H2C=CH(CN)COOR--> HOCH2C:-(CN)COOR(methylol substituated carb anion 1) + H3C-C:-(CN)COOR (2nd methy substituated)
Also if HO-CH2CH(CN)COOR - H2O--> H2=C:-(CN)COOR monomolecule disproportion but then i am not specialist if this resonanse structure is possible or how stable?
And last i can imagine now for really in situ Michael addition of intermediates:
HO-CH2CH(CN)COOR + H2=C(CN)COOR--> HC(CN)(COOR)-O-CH2CH(CN)COOR which somehow to form bi-anion which should be very active and would make the polymerization twice faster ???
Now i am searching for some research works onto the cyanacrylates polymerization mechanism problem and if find something will share you.
Ops i think i have mistake in water addition to cyanacrylate :P
H-OH + H2=C(CN)COOR--> H3C-C-OH(CN,COOR) Is this by Marcovnikov rule?
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The addition of OH will be anti-Markovnikov in this case because of the withdrawing groups. The beta carbon is much more electrophilic than the alpha carbon (draw out the resonance structures and you will see why).
To initiate the react you could add some aqueous NaOH. You could also use a different basic, nucleophilic activator like sodium methoxide or sodium acetylacetonate.
The proton of the alcohol in the OH + acrylate aduct will be a lot less acidic than the proton between the two withdrawing groups (pKa of about 16 and 12, respectively), so addition of the oxygen instead of the carbon would be unlikely.
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You are right for anti-Markovnikov addition. But at the literature no such water addition described( i mean to Michael product HOCH2CH(CN)COOR) but its always given aduct as on your scheme HO:-(base from water) + H2C=C(CN)COOR--> Base-CH2C:- and so on anionic polymerization of cyano acrylate monomer by classic mechanism...To virtue the fortune of H+ from water which is surrounded by very big excess of cyanoacrylate molecules than water theres may many possibilities by my view: H+ H2)--> H3O+ or may be H+ + CN --> ?
Also no to forget the last ethap of every not living polymerization : termination of the polymerization process..I suppose one of the end-groups of the "died" polymer chain should be HOCH2C(CN)COOR-- and on the other end --CH(CN)COOR :)
In the literature write the polymerization cyano acrylates is initiated also by nucleophiles like akcohols and amines. Well in this view i think its hard in situ to obtain RO:- or RNH:- and only Michael addition possible what do you think ??? R-O-CH2CH(CN)COOR and then polymerization mechanism may be much interestin :)
Continue thinking...
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Sorry for the stupids i wrote with water as base HO:- actually the basic is unshared el pair at O-athom so H2O:(base) + H2C=C(CN)COOR--> +H2O:-CH2C:- :
But the possible addition of H from water by Michael is also possible reaction what do you think ???
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Yes, it could be water, perhaps.
As to the termination of the polymerization, it probably occurs via alkylation with some terminating species (e.g. MeI) or, as you suggest, by protonation of the carbanion.
I don't understand what you mean by saying that things like MeO- are hard to obtain. NaOMe can be made easily with methanol and NaH.
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Hi
And now if the oxa-Michael adition is possible with strong electrophils like alkoxide then i am thinking what happen with the alkali athom in conjugation with acrylic compounds for example:
R-O:-Na+ + CH2=CH-COO:-Na+--> R-O-CH2CH:-(Na+)COO:-Na+<-->
R-O-CH2-CH=C(O:-Na+)2
What do you think about the possible resonance structure of CH-COO segment is it possible ???
Thanks
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Well, once you have a carboxylate anion for the acrylic acid the Michael addition will become very difficult because the C-C double bond won't really be activated anymore. It would be like trying to add RO-Na to 1-butene. Really hard to do!
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Hah the theory is grey but green the tree of life :)
My first experiments this reaction show very good yelds of oxa-Michael product
and i am encouraged that...
Very good discussion and pleasure for me! Hope was interesting for you and useful for all...
Regards