Cloner
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dehydrating higher alcohols with bisulfate
If a primary alcohol is heated with sodium bisulfate, a sulfate adduct results. It is supposedly possible to form a terminal alkene by heating this
adduct.
Using a high boiling alcohol, a high boiling terminal alkene results. If a mixture of alcohol and bisulfite is heated, there is oxidation (smell of
SO2, black coloration) and a liquid distills over containing water and oil. The original water present in the bisulfate and formed in a reaction runs
out eventually and nothing comes over anymore. Steam distilling is the only way to get the organics out, since further heating only results in
destruction, glassware full of tar etcetera. Yield is not shockingly great in any case.
A possible side reaction is also ether formation.
I acted on a suggestion to 'dehydrate over bisulfate'. No more detailed description.
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entropy51
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| Quote: | | I acted on a suggestion to 'dehydrate over bisulfate'. |
This may not be what was meant, but I have prepared styrenes in fairly good yield by slowly dripping the phenylmethyl carbinol into a flask containing
a thin layer of KHSO4 spread over the bottom of the flask while distilling out the styrene under aspirator vacuum. The flask is heated to 250 C or so
and you need inhibitor in both the flask and the receiver. If the carbinol is added too rapidly it distills over into the product.
I can't recall the reference for this procedure, but it certainly works well for preparing certain styrenes.
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DJF90
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Entropy - I would expect this to work well for the formation of styrenes as the alkene formed is conjugated with the phenyl ring - a rather stable
product thermodynamically (hence why polystyrene depolymerises on heating...), although not kinetically, which is why old bottles of styrene are
partially polymerised (at least this is my understanding). Other alkenes may not be formed so easily, and although you do not claim they would, I
thought I would point this out. It also helps if the alkene can be distilled easier than the alcohol, allowing a shift in the equilibrium between
alcohol and alkene. If one has the reagents, a better method may be E2 elimination of the corresponding tosylate using t-BuOK.
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entropy51
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DJF90, I agree, but "dehydrate over bisulfate" rang that bell. It's the only prep I know of that meets that description.
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Nicodem
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Cloner, please learn how to use references and then use them. Also, how about writing about your experiment in a comprehensible way so that we may
understand what you did? Experimental procedure, substrate, conditions, analysis... you know, the usual stuff.
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Cloner
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Nicodem: I know what you mean but I intended to ask about the procedure itself. This experiment is nothing special enough to post on this forum. It is
an old book I mentioned. This procedure seems to have been standard in the times when chemistry was all about heating stuff. For this reason alone, it
is worth attempting, besides I wanted to have long-chain alkenes.
In the case of ethanol, ether is formed from ethanol and sulfuric acid while formation of ethene is said to start at slightly higher temperature.
Around 160 degrees for ethene formation if the description is right. I did not perform this experiment. I did draw from it, since an aliphatic primary
alcohol is an aliphatic primary alcohol in terms of thermodynamics.
In my case, the alcohol is saturated alkanol (linear n-decanol).
The experiment:
I have heated 50ml of the alcohol with 130g sodium bisulfate getting distillate of water and oil. The temperature was 175 degrees. Oil and water came
over. I added 25 ml water three times then reheating until nothing came over. After extraction and washing there was about 15ml of distillate oil
remaining.
In an earlier experiment I used open flame to heat until destruction (the amount of liquid obtained was about 60% of the amount obtained when heating
with oil at 175 degrees)
In both cases considerable amounts of tar are formed. This is to be expected because there is a SO2 smell.
A few drops of the oil are treated with bromine water leading to decolorization but not quantitative indicating alcohol and alkene.
Heating the same alcohol with sulfuric acid gives nothing but tar in the flask and very little distillate.
Quote: Originally posted by entropy51  | | Quote: | | I acted on a suggestion to 'dehydrate over bisulfate'. |
This may not be what was meant, but I have prepared styrenes in fairly good yield by slowly dripping the phenylmethyl carbinol into a flask containing
a thin layer of KHSO4 spread over the bottom of the flask while distilling out the styrene under aspirator vacuum. The flask is heated to 250 C or so
and you need inhibitor in both the flask and the receiver. If the carbinol is added too rapidly it distills over into the product.
I can't recall the reference for this procedure, but it certainly works well for preparing certain styrenes. |
The temperature of 250 degrees cannot be right, might it be 150 degrees instead? Or do you mean some highly substituted styrene analog?
[Edited on 16-10-2009 by Cloner]
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entropy51
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250 C is the temperature read on a thermometer touching the KHSO4.
The styrenes were singly brominated or chlorinated on the ring.
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Nicodem
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| Quote: Originally posted by entropy51 | This may not be what was meant, but I have prepared styrenes in fairly good yield by slowly dripping the phenylmethyl carbinol into a flask containing
a thin layer of KHSO4 spread over the bottom of the flask while distilling out the styrene under aspirator vacuum. The flask is heated to 250 C or so
and you need inhibitor in both the flask and the receiver. If the carbinol is added too rapidly it distills over into the product.
I can't recall the reference for this procedure, but it certainly works well for preparing certain styrenes. |
That would be Organic Syntheses, Coll. Vol. 3, p.204 (1955).
A more friendly (milder) method would be heating with pyridinium tosylate (Tetrahedron, 50 (1994) 12159–12184).
…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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entropy51
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Thanks for that reference, Nicodem. I searched Org Syn today, because I thought that was the source. I searched for styrene in title and bisulfate
in the text. But they call it "potassium acid sulfate", so it didn't find it.
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DJF90
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Nicodem - the reference you give for using pyridinium tosylate is incorrect; the title of that paper is
"Stereoselective synthesis of meta- and three-bridged cyclophanes with intramolecular [2 + 2] photocycloaddition by using the steric effect of
methoxyl group"
and I see no mention of alkene formation in the abstract either. Would you mind double checking its the paper you meant to reference?
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Cloner
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Interesting find
Anyhow, this is a secondary alcohol which does not form sulfate esters as readily as primary alcohols.
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Nicodem
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| Quote: Originally posted by DJF90 | Nicodem - the reference you give for using pyridinium tosylate is incorrect; the title of that paper is
"Stereoselective synthesis of meta- and three-bridged cyclophanes with intramolecular [2 + 2] photocycloaddition by using the steric effect of
methoxyl group"
and I see no mention of alkene formation in the abstract either. Would you mind double checking its the paper you meant to reference?
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It is there. I do not have the access to these volumes of Tetrahedron in the digital form so I will just type out the essential information from that
paper:
| Quote: | A solution of 78 g (0.236 mol) of the diol 7a and 11.6 g (46.2 mmol) pyridinium p-toluenesulfonate in 1.8 L benzene was refluxed with a Dean-Stark
apparatus for 5 days. After extraction with 2 L of benzene, drying over Na2SO4, and evaporation, pure olefin 8a was isolated in 90.7% yield by column
chromatography (SiO2, benzene).
[+ analysis data, etc.; 7a: 1,2-bis[3-(1-hydroxyethyl)-4-methoxyphenyl]ethane, 8a: 1,2-bis[3-(1-vinyl)-4-methoxyphenyl]ethane; yields for other
similar substrates ranged from 72 to 92%] |
In the same paper there is also an example using KHSO4 in DMSO:
| Quote: | A solution of 2.10 g (5.20 mmol) of tetrol 26a and 3.10 g (22.8 mmol KHSO4) in 63 mL of DMSO was heated at 180 °C for 3.5 min. The mixture was poured
into ice-water. After extraction with 100 mL of benzene, drying over Na2SO4 and evaporation, tetraolefin 27a was obtained in 43.0% yield by column
chromatography (SiO2, benzene).
[+ analysis data, etc.; 26a: bis(3,5-bis(1-hydroxyethyl)-4-methoxyphenyl)methane; 27a: bis(3,5-divinyl-4-methoxyphenyl)methane; yields for other
similar substrates ranged from 43% to 47%] |
However, alpha-phenylethyl alcohol can be dehydrated even by just heating in DMSO without any acid (Helvetica Chimica Acta, 64 (1981)
389-398), though the yields are poor because of polymerisation even though radical inhibitors were used. beta-Unsubstituted styrenes are always
problematic with any such method because they tend to polymerize. Yields of styrene above 50% are hard to obtain using any of the acid catalysed
methods, so the 90% yield with Py*TsOH is exceptional for a vinyl group formation. I would expect plain alpha-phenylethyl alcohol to give a lower
yield even applying this catalyst, but still higher than KHSO4. All these methods should not work for normal primary alcohols and would work for
utmost few secondary aliphatic alcohols, in my opinion. Using KHSO4 on silicagel was reported to give a 64% yield of styrene (Journal of the
Chemical Society, Perkin Transactions 1 (1989) 707-710). In addition this KHSO4/SiO2 catalyst is claimed to dehydrate secondary alcohols as well
(or so they say in the abstract). They also add that "the dehydration activity of methanesulfonic acid was also enhanced (ca. 14 times at 90 C) by the
presence of silica gel." The same authors claim CuSO4/SiO2 works similarly as well (Tetrahedron Letters, 28 (1987) 4565-4568).
For primary aliphatic alcohols it is most rational to use a two step method (for example, tosylation followed by elimination with a base).
| Quote: Originally posted by Cloner |
Anyhow, this is a secondary alcohol which does not form sulfate esters as readily as primary alcohols. |
The formation of an alkali alkylsulfate is not required for the elimination. The only thing necessary for the elimination to occur is for the acid to
protonate/coordinate with the OH group, the heat then does the rest. If an alkali alkylsulfate would form quantitatively then the yields using NaHSO4
would be much better since these decompose to the olefins by heating. There was a paper posted here a couple of months ago where ammonium
alkylsulfates were formed by heating the alcohol with sulfamic acid. These esters decompose to olefins by heating. So if you need a better yielding
one-pot method, I suggest you to look into it. (edit: The paper I'm talking about was discussed in the Good way to Methylamine HCl?, uploaded here). Sulfamic acid is relatively cheap and also easily available.)
[Edited on 19/10/2009 by Nicodem]
…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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