Magpie
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pinacolone
Using a school procedure (Pavia et al) I recently made pinacol hexahydrate. I confirmed its identity via melting point. Then I converted this to
pinacolone (aka 3,3-dimethyl 2-butanone) using H2SO4 followed by a simple distillation. Noting that this is a methyl ketone I assumed I could use the
iodoform test for confirmation of identity.
My results, however, were negative. (I verified my iodoform test via acetone and MEK with very positive results, getting the pale yellow precipitate
of CHI3.)
My "pinacolone" is an alkane smelling clear liquid that is only slightly miscible with water. I don't really have enough to do a bp.
I would like to know if anyone else has made pinacolone and if they had positive results with the iodoform test.
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Klute
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I remember doing this reaction in school. As I recall it, the rearrangement was simply a hour's reflux with 10% H2SO4 and steam distillation of the
pinacolone, which was a nice smelling yellow or green oil.
You could try a bisulfite (or DNPH, or whatever you have at hand) test to back up the iodoform, if that one would also be negative, maybe your
pinacol wasn't what you thought it was.
Did you use the trivial Mg/Hg in toluene method? The hydrate has a relatively low melting point IIRC, no?
I would love to retry this reaction at home, if only the amalgam didn't need so much HgCl2.. What ratio Mg/Hg did you use? I remember using a
outrageously large amount of HgCl2 at school...
It's hard to see where problems could occur during the rearrangement, if you used dilute acid. How long did you reflux? Did you then exract, wash
and distill? Maybe you formed some kind of condensation products during distn if traces of acid or abse where still present. But it is strange that
there isn't even a little bit of methylketones in there.
What makes you say what you have is an alkane? What was it's boiling point?
[Edited on 28-2-2008 by Klute]
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Magpie
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Klute: to make the pinacol the ratios were: 28mL acetone, 1.65g Mg, and 1.95g HgCl2. So, yes a lot of HgCl2 is required. Melting point of the
hexahydrate was ~45.4C in agreement with my handbook.
My procedure for the pinacolone is somewhat different than yours: Mix 3g of the pinacol hexahydrate with the hot solution of 4.8mL of con H2SO4 mixed
with 7.2 mL H2O. Then distill until the condensate becomes clear. I assume this is, in effect, a steam distillation. No refulx period was
specified.
The organic is then separated from the water and dried with Na2SO4. It was a perfectly clear liquid which smelled more like an alkane (say kerosene)
than the peppermint mentioned in Wikipedia.
So it seems that I most likely don't really have pinacolone? I don't have enough for a boiling point determination.
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bfesser
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| Quote: | Originally posted by Magpie
My "pinacolone" is an alkane smelling clear liquid that is only slightly miscible with water. I don't really have enough to do a bp.
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You probably have way more than enough to do determine a boiling point. There's a microscale technique in one of the many technique books I own that
describes how to take a boiling point with only a few microliters of the liquid (below). It also describes pretty much the same method, but on a
slightly larger scale, using a cut length of 5 mm glass tubing sealed at one end to hold the liquid (or small test tube), rubber banded to a
thermometer, with an inverted melting-point capillary tube inside the 5 mm tube.
| Quote: |
<strong>Microscale Method.</strong> In microscale experiments, there is often too little product available to use the semimicroscale
method described above. However, the method can be scaled down in the following manner. The liquid is placed in a 1-mm melting-point capillary tube
to a depth of about 4-6 mm. Use a syringe or a Pasteur pipet that has had its tip drawn thinner to transfer the liquid into the capillary tube. It
may be necessary to use a centrifuge to transfer the liquid to the bottom of the tube. Next, prepare an appropriately-sized inverted capillary, or
<strong>bell.</strong>
The easiest way to prepare a bell is to use a commercial micropipet, such as a 10-μL
Drummond "microcap." These are available in vials of 50 or 100 microcaps and are very inexpensive. To prepare the bell, cut the microcap in half
with a file, or scorer and then seal one end by inserting it a small distance into a flame, turning it on its axis until the opening closes.
If microcaps are not available, a piece of 1-mm open-end capillary tubing (same size as a
melting-point capillary) can be rotated along its axis in a flame while being held horizontally. Use your index fingers and thumbs to rotate the
tube; do not change the distance between your two hands while rotating. When the tubing is soft, it is removed from the flame and pulled to a thinner
diameter. When pulling, keep the tube straight by <em>moving both your hands and your elbows outward</em> by about 4 inches. Hold the
pulled tube in place a few moments until it cools. Using the edge of a file or your fingernail, break out the thin center section. Seal one end of
the thin section in the flame; then break it to a length that is a bout one and one-half times the height of your sample liquid (6-9 mm). Be sure the
break is done squarely. Invert the bell (open end down), and place it in the capillary tube containing the sample liquid. Push the bell to the
bottom with a fine copper wire if it adheres to the side of the capillary tube. A centrifuge may be used if you prefer. Figure 13.4 [sorry, I don't
know where my scanner is right now...] shows the construction method for the bell and the final assembly.
Place the microscale assembly in a standard melting-point apparatus (or a Thiele tube if
an electrical apparatus is not available) to determine the boiling point. Heating is continued until a rapid and continuous stream of bubbles emerges
from the inverted capillary. At this point, stop heating. Soon, the stream of bubbles slows down and stops. When the bubbles stop, the liquid
enters the capillary tube. The moment at which the liquid enters the capillary tube corresponds to the boiling point of the liquid, and the
temperature is recorded.
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Pavia/Lampman/Kriz/Engel. <em>Microscale and Macroscale Techniques in the Organic Laboratory.</em> Thomson Brooks/Cole: 2006; pp 192-195.
ISBN-13: 978-0-495-29247-0
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Magpie
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Thanks, bfesser, for the microscale bp technique. I have that in my text by Pavia et al also, but had forgotten it is there as I have never
used it.
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Klute
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Maybe a too concentrated acid caused some kind of dehydratation? At the same time it would be pretty strange that such a procedure would be faulted..
Indeed that is a very large amount of mercury needed... It's unfortunate no subsitutute/diminution has been found in all these years (well, I jhave
stumbled on any yet).
The thick noxious cake of MgO and Hg takes a few toluene extractions to get back most of the product IIRC. BTW, no other solvant than acetone is used
in your procedure?
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PHILOU Zrealone
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There is a big chance if boiling was too long and H2SO4 too concentrated that you ended up with condensation...owing to the bulky and unreactive
tertbutyl moiety, there is a good chance you end up with 1.3.5 tritertbutylbenzene... like propanon wich on H2SO4 boiling provide a good deal of 1.3.5
trimethylbenzene (mesitylene)..here the effect must be more pronounced in favor of full aromatic cyclisation...this would explain the lack of iodoform
reaction...no more methyl alfa to the CO ready to react. This would also account for the kerosene like smell...
CH3-CO-C(CH3)3 -H+-> C6H3(-C(CH3)3)3 + 3 H2O
following http://caligula.bcs.deakin.edu.au/bcs_admin/msds/ChemicalSum...
MP=67°C
BP= 121-122°C/12mmHg
[Edited on 28-2-2008 by PHILOU Zrealone]
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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Magpie
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Klute asks:
| Quote: |
BTW, no other solvant than acetone is used in your procedure?
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Yes, 19mL of toluene was also used. And I had the acetone amount wrong. It was actually only 11.2 mL. I'll correct the earlier post if possible.
BTW this procedure has been called trivial, but the use of Mg, HgCl2, and the strict need for dry materials and exclusion of moisture during the
formation of pinacol make it not trivial IMO.
I used the semi-micro scale boiling point method (per Pavia) today with a result of 107C. This is consistent with my CRC 49th ed handbook value of
106C for pinacolone. This is a neat and easy method for determining a boiling point, BTW. This result, however, further increases the intrique as to
whether or not I actually have made pinacolone.
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Klute
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Yes, I agree the reaction isn't the easiest, trivial wasn't the best choice of words, should have said "conventionel".
IIRC, strong mechanical stirring is needed to efficiently stir the thick lumps/sludge of Mg/Hg....
Do you plan on trying the reaction again?
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Magpie
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Klute says:
| Quote: |
IIRC, strong mechanical stirring is needed to efficiently stir the thick lumps/sludge of Mg/Hg....
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Yes the reaction mix did solidify when making the pinacol hexahydrate. However, I had no agitation, nor was it called out in the procedure.
I don't know if I will ever try to make the pinacol again as I would have to first make (or buy) HgCl2. Making it seems risky and buying would be
expensive with the hazmat charge, etc.
But I do have a little pinacol hexahydrate left so could try to make a little more pinacolone.
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Klute
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| Quote: | Originally posted by Magpie
[...] However, I had no agitation, nor was it called out in the procedure. |
I'm quite surprised about this. You just added the HgCl2 in acetone to the Mg under toluene? Did the amalgamation proceed well even without
stirring?
I'm pretty sure good stirring would promote better contact with the amalgam, and boost up yields though. I'm surprised a school procedure doesn't
advise stirring, maybe for simplicity if the reaction proceeds well enough without. How old is the text book?
The large amount of HgCl2 needed is a real bummer, as this reaction is pretty interesting by the theory and in practic.
I will have a closer look to see if any improved procedure are available, needing less mercury.
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Nicodem
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I remember when I was much younger I tried to recycle the excess Mg turnings from a Grignard reaction (didn't have enough Mg to just trough it away).
After washing the Mg with diluted HCl to remove the hydroxide slurry, I washed them with acetone so they would dry more rapidly. Well, the acetone
started heating and boiling in contact with Mg turnings really scaring me. I found out about the pinacol formation only later. The Mg was apparently
still activated from the Grignard reaction even though I washed it with water and all. I think other methods than amalgamation could therefore also
work. One could try to activate Mg turnings with I2/toluene until iodine discoloration and then add acetone. Or whatever other variation.
Synthesis of pinacol hydrate:
http://www.orgsynth.com/orgsyn/prep.asp?prep=cv1p0459
Rearrangement to pinacolone:
http://www.orgsynth.com/orgsyn/prep.asp?prep=cv1p0462
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scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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Magpie
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Klute, I'm sorry, the procedure did specify occaisionally stirring with a stirring rod, crushing the Mg, but keeping this brief to avoid the
introduction of atmospheric moisture. My notes say I also occaisionally shook the apparatus. From my notes: "voluminous grey matter formed." After
an hour of reflux 4.5mL of H2O was added and the mix reluxed for an additional 1/2 hr. "There was an immediate reaction w/the water. Bulky grey
material disappeared."
The lab text I used is "Organic Laboratory Techniques," 1998, by Pavia et al, apparently poplular in the US.
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Klute
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Thanks for the details Magpie.
Nicodem, how did you notice the formation of pinacol with the acetone? DSid you obtain it as a hydrate?
I'm curious about why this recation requires so much mercury. Conventional amalgam only require a relative small amount to obtain a rather large
activated surface.
Could be be because the formed magnesium oxide is much more "obstructing" than Al2O3 for example? Or does the mercury have another role than just
transfering electrons?
It's nice to see Al/Hg can be used for this reaction, as mentionned in the org syn procedure; would a equally large ratio of mercury be needed there
too? The fact that they claim that there is organic mercury compounds in the product could suggets mercury also plays a role during the coupling,
with some kind of transitive organo-mercury compoud being formed, or could it just be some side reactions?
The electrolysis method looks very interesting too, as the reagants bseide the Mg/Hg are cheap and availble. US3984294 claims pinacol can easily be
made in a dual chamber cell with a PbO2-coated lead anode and a copper or graphite cathode, using a quat salt as supporting electrolyte. Actually,
this really make me want to try it out. Don't they use a rather large current density though: 10A/cm2?
I would be using a 2x100mL cell, with a glass frit membrane. I guess lead and copper will be used as I could make suitably sized electrodes. This
would be a nice reaction for a first home-electrolysis 
US3984294: Electrochemical manufacture of pinacol
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Nicodem
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No, no, I was not making the pinacol. I just observed a very exothermic reaction in the filter cake of Mg turnings (still activated from the Grignard)
which I was stupidly trying to wash with acetone. I know of no other reaction between Mg and acetone so I can only assume what happened is the pinacol
coupling due to the Mg surface being still activated/oxide free. I never isolated anything from the filtrate, but there was this strange strong smell
given off (I don't know if pinacol has any smell – it certainly does not even look volatile at all).
Aluminium requires only the tiniest amount of Hg to break the oxide surface, so I don't know if it is really comparable with Mg. Pinacol couplings can
be affected by metals other than Mg, even by alkali metals (Li, Na, K) and these do not need any Hg present. The interesting thing is that a different
diastereoselectivity, often opposite, is observed when using different metals (in cases where diastereomeric glycols can form). The reaction proceeds
by a SET transfer of an electron resulting in the formation of ketyl anion radicals which couple to give the corresponding deprotonated glycols – so
the Hg should not play any other role other than Mg surface activation.
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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)
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