Bronstein
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allyl mercaptan
A few days ago I attempted to make some allyl mercaptan. I used the method from Vogel, only modified to used allyl bromide instead. If you don't
remember this method it is simply refluxing an alkylbromide with thiourea in water, and then later hydrolysis with NaOH.
It worked, however I got a very poor yield. I started with 13,9g allylbromide (10ml), and at the end of hydrolysis only a very thin layer of mercaptan
was on the surface. Combined with the DCM extract of the aqueous phase the final yield was perhaps 1 ml (not weighed).
It there some reason for allyl bromide to give such low yields in this reaction?
Vogel says the first part of the reaction with butyl bromide will become homogenous in 30 minutes, but in this reaction it only took 5-10 minutes. I
though this was because it was more reactive, but perhaps it's because of some sidereaction?
The mercaptan doesn't stink as terrible as I had thought, it actually smells very similar to garlic. Throughout the second part of the reaction I
could detect the smell escaping the condensor, filling most of the apartment. I could even still smell it the next day. When smelling the pure liquid
directly however, it has a somewhat more unpleasant smell.
However, after rinsing out the beaker I used for evaporating the DCM extract, I could smell a faint flowery smell totally unlike the mercaptan. Is
this the smell of some sideproduct perhaps? Or is it what the mercaptan smells like in low concentrations?
[Edited on 5-1-2008 by Bronstein]
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Nicodem
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It is normal that the reaction took much less than with butyl bromide since allyl bromide is several magnitudes more electrophilic.
Judging from your workup it is also unlikely that the liquid you got is allyl mercaptan since its bp is 67-68°C. You would have to distill the CH2Cl2
extract through an efficient column to separate allyl mercaptan from CH2Cl2. It is more likely that what you obtained is mainly diallyl disulfide (bp
180-195°C). Mercaptans easily oxidize to disulfides unless all solvents used are deareated and if its solutions are left exposed to air for too long.
Coincidently, diallyl disulfide is the main "aroma" of garlic.
…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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not_important
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Seconding what Nicodem said.
As a side, here is a bit of antique chemistry that produces allyl mercaptan along with H2S and several other products; messy but the starting
materials are cheap.
see "Action of Sulphur on Glycerol" pp 577
http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleFo...
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JohnWW
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Have the neighbors complained about the smell yet, Bronstein?
BTW I wonder if you are related to the late Leon Trotsky a.k.a. Lev Bronstein.
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Sauron
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The Bronstein's in Louisiana are first cousins of Trotsky. Major proprty owners in the French Quarter of New Orleans, I went to school with one of
them.
They own a lot of art galleries and are notorious exploiters of starving artists.
Al Rose, the music historian whose book was made into the film STORYVILLE, was one of Trotsky's bodyguards in Mexico. He was off duty the night that
Ramon Mercador buried an ice-axe in Trotsky's skull at behest of Lavrenti Beria.
Al was aghast when he found out that I knew the "secret" of the Bronstein connection to Trotsky, he was quite paranoid about the KGB exacting
reprisals. But the Bronstein kids were quite open about it and when my classmate visited the USSR upon her return to New Orleans she complained
because no one in Russia seemed to know who her cousin (father of the Red Army, Lenin's greatest friend) even was. In the Soviet Union, 20 years after
the death of Stalin, Trotsky was still an unperson.
Sic gorgeamus a los subjectatus nunc.
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Bronstein
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Now that I re-read Vogel again, I notice I made two errors. 
First error: I mixed up in my head the procedures for hexyl mercaptan and butyl mercaptan, I had actually followed the procedure for hexyl mercaptan.
That's why I had expected most of the mercaptan to separate from the aqueous layer directly after the hydrolysis, without an extraction.
Second error: As my mind was all set on not requiring an extraction, I forgot that before the extraction you were supposed to acidify the aqueous
layer. I suppose this is because the lower members of the mercaptans are too soluble in the alkaline water that they do not separate as readily as a
layer already after the hydrolysis. (Are they present as the mercaptide salt?)
I think I have to try this a second time.
| Quote: | | Judging from your workup it is also unlikely that the liquid you got is allyl mercaptan since its bp is 67-68°C. You would have to distill the CH2Cl2
extract through an efficient column to separate allyl mercaptan from CH2Cl2. It is more likely that what you obtained is mainly diallyl disulfide (bp
180-195°C). Mercaptans easily oxidize to disulfides unless all solvents used are deareated and if its solutions are left exposed to air for too long.
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Should not at least the thin layer which separated from the reaction mixture be the mercaptan? This is what vogel says:
| Quote: | | [...] during [the hydrolysis] the thiol separates since it is largely insoluble in the alkaline medium. Allow to cool and separate the upper layer of
almoste pure hexane-1-thiol |
I had planned to distill it, but since I got such a small amount I skipped it.
| Quote: | | BTW I wonder if you are related to the late Leon Trotsky a.k.a. Lev Bronstein. |
Yes, I took my alias from him, but there is no relationship.
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Nicodem
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Mistakes like that are normal when you don't have the insight on what you are doing. The pKa of allyl mercaptan is about 10 (mercaptans are
approximately as acidic as phenols). You have to acidify or else only little product is in its non-deprotonated state (depending on the pH and
hydrophobicity of the mercaptane, but you can be sure that at pH above 12 most of allyl mercaptane is deprotonated thus remains in the water phase).
But like I already said, you can not avoid the oxidation of mercaptans to disulfides unless you use all necessary precautions. All solvents used need
to be deareated (either by boiling them for a while or vacuuming them or purging them with argon). R-SH compounds oxidize rapidly in polar solvents
and especially rapidly at basic pH. Using inert atmosphere during hydrolysis until acidification would also be preferred (alternatively you can add
some Na2SO3 before hydrolysis to inhibit the oxidation). After acidification you can steam distill your product, and since there should be no more
allyl bromide, the distillate should be fairly pure provided you add some Na2SO3 in the receiving flask to inhibit the oxidation to the disulfide.
Then separate the organic layer, dry it over Na2SO4 and store it air tight under inert atmosphere. I hope you don't do this in any populated area or
your neighbors will surely call the landlord, firefighters or the police, in any case making you sorry for doing such experiments. I also hope you
have an efficient fume hood since mercaptans are not just terribly stinky but also toxic.
Allyl mercaptan is not a particularly stabile compound. If you need it for some nucleophilic substitution, you better consider if you can do it
one-pot rather than isolating it.
…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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