posted 01-27-2000 01:36 PM
Since the appearance of Ritter's writeup of this method in Total Synthesis
II, much discussion has taken place about it but, it has sometimes seemed,
little has been clarified. This is due in large measure to the sensitivity
of this reaction to even the most minor changes in its many variables.
With this writeup, I hope to provide a clearer view of the method and to
allow others to benefit from the hard-won experience of someone (not me)
whom we'll call Mr. A. Ritter's original writeup, while inspiring, lacked
details about the many nuances that, once understood, allow the amateur
chemist to really understand this reaction's dynamics. Thus I have tried
with this writeup to help the neophyte who has only physical observations
and scant written material to guide him (although I suspect and hope that
it may even help a few more seasoned cooks as well).
The first thing I'd like you to look at is the array of interrelating
variables in this reaction that make it so delicate. They are as follows:
1) the thickness/type of the aluminum
2) the consistency (i.e. flat,
ground, etc.)
3) the amount of HgCl2 used in relation to the amount of
aluminum
4) the addition rate of the MeNO2/MDP-2-P
5) the size of
the reaction vessel in relation to the scale of the reaction
6) the
ability to effectively stir the reaction
7) the coldness of the water
through the reflux condenser (yes, even that!)
The above factors are sort of submitted in an order of importance (#1
being most important), but in reality they are all inextricably related. I
observed firsthand the trials and tribulations of Mr. A as he struggled to
match up the correct combination of ratios and conditions that would allow
a smooth, consistent reaction and predictable results every time. Finally,
after lots of frustration, confusion, losses, and---in the end---a
revelation, the perfect set of elements was hit upon and recorded.
The scale Mr. A chooses to perform this reaction on is half-scale to
the scale in the Ritter writeup, which was 55g aluminum and 50g MDP-2-P.
Therefore this writeup will illustrate the reaction on a scale of 27.5g
aluminum and 25g MDP-2-P. The subject found for his own personal reasons
that this smaller scale was much easier to manage (not the least of which
is that even with a huge 4-liter separatory funnel, at this smaller scale
it gets pretty filled up!). There's no doubt that the original larger
scale can be successfully applied, although it would require adjustments
in the glassware capacity, stirring method, and probably other elements.
MATERIALS and APPARATUS:
* 27.5g Reynolds Wrap Heavy Duty aluminum
foil
* 25g MDP-2-P
* 20 mL MeNO2 of 99+% purity
* 750 mL MeOH +
50 mL more for addition funnel + additional small amounts that will be
needed later to thin the mixture
* 400mg HgCl2
* 2-liter 2-neck flat
bottom flask
* reflux condenser (400mm preferable)
* 250mL or 500mL
addition or separatory funnel
* cooling setup (bucket, water pump,
tubing, 1 bag ice)
METHOD:
1. Weigh 27.5g of Reynolds Wrap Heavy Duty aluminum foil
(NOTE: it HAS to be Reynolds and it MUST be the heavy duty stuff) and then
tear it by hand or cut it with scissors into small rectangles
approximately 1" by .75". Settle down with this task with a good CD or TV
show because it is tedious and will take about 30-40 minutes.
2. With a
coffee grinder, "grind" these pieces of foil for durations of about 10
seconds. Fill the coffee grinder only loosely (about two thirds
full---don't stuff it! That will adversely change the consistency of the
ground foil). It will probably take about 4 to 5 "loads" in your grinder
to do the whole 25g of foil, depending on the size of your grinder. (In
actuality, the foil does not get "ground," but rather, each individual
piece just gets compacted and compressed. If it is compressed too heavily,
the inner surfaces of the foil nuggets may be rendered inaccessible to the
Hg/MeOH solution, changing the timing of the amalgamation and maybe even
causing an incomplete or failed reaction.) When properly done, the foil
should be in gnarled little nuggets about the size of sunflower seeds
(shell included) and should NOT look super-tight and small.
3. Place a
3" stirbar in your 2L flat bottom flask and onto your stirplate. Add the
foil nuggets to the flask and then proceed to set up your glass, support
and clamps so that the reflux condenser and addition/sep funnel are
securely affixed and your flask is well-centered on the stirplate (this
will be critical when you begin to attempt stirring!). Also, prepare your
cooling, i.e. attach the inflow and outflow tubes to the reflux
condenser.
4. Carefully add the 400mg HgCl2 to 750mL MeOH to a tightly
sealable bottle and shake to dissolve all HgCl2. Set this solution
aside.
5. Combine the 25g MDP-2-P, 20mL MeNO2, and 50mL MeOH and pour
them into the addition/sep funnel. Rinse your beaker (or whatever you
used) with a tiny bit of additional MeOH to get the residual ketone and
add it to this MDP-2-P/MeNO2/MeOH solution.
6. Very slowly and
carefully (w/gloves, glasses, long sleeves and a Hail Mary if you're
Catholic), using a large funnel, pour the HgCl2/MeOH solution from step 4
down the condenser.
7. Turn the stirring on full blast for a 5-second
burst to intimately mix the solution and the foil. If you have prepared
the foil as described above, it will easily stir. Give it a few more
5-second stirs over the next few minutes. I believe that doing this really
helps facilitate the amalgamation process that is about to occur.
8.
After about 5 minutes or so, you will begin to see bubbles popping up on
the surface of the MeOH solution. At first they will be tiny, like
champagne bubbles. Then after a few minutes you will see them joined by
larger bubbles closer to the size of those seen in boiling water. It is
around this same time that the appearance of the aluminum will change from
its normal shiny silver color and start to take on a dull gray look,
accompanied by a gray cloudy look that begins forming in the MeOH. This is
the magic moment when you want to begin dripping in your
MDP-2-P/MeNO2/MeOH mixture. Set a drip rate of approximately 1 drop per
second at this point and no faster. You can speed it up a bit later to
accelerate the reaction if desired.
9. Place about 3 lbs ice into your
bucket. When you can feel exothermic warmth begin by feeling the outside
of the flask, quickly add about 2.5 liters water to the bucket (or an
appropriate amount to make very ice-heavy ice water) and plug in the
pump.
10. While monitoring the growing intensity of the bubbling
amalgamation, turn on/off the stirring intermittently as you did earlier.
This time it is to assure distribution of the added ketone/nitromethane in
the reaction flask but also because the amalgamation seems to gain its
vital momentum more effectively if given some significant blocks of time
(meaning about 30 seconds at a time) in between "stirring bursts." When
the reaction is clearly starting to get vigorous and hot, crank the
stirring to 10 and leave it on.
NOTE: This is where you can take advantage of Mr. A's trial and error
regarding this reaction's parameters. If you used the kind of foil
specified, prepared it as specified, used no more than the specified 400mg
HgCl2, and used a 2-liter and NOT a smaller flask, you can breathe easy
knowing that the reaction is going to hum along nicely but will not get
out of control, and will result in perfectly processed aluminum amalgam
sludge. You may think that a 2-liter flask is oversized for this reaction,
but that is precisely the point. The extra headroom in the glass provides
a nice zone of "breathing room" for the reaction and facilitates good
refluxing. I've seen this reaction get out of hand in a 1000mL flask, and
it isn't pretty, believe me. Use the 2-liter.
11. As the reaction progresses only a few minutes after the addition
was started, you will observe that the aluminum is breaking up fairly
rapidly. This is good, as long as you have the ketone/nitro mixture
dripping in at a good rate of about 1drop per second. But be careful with
the addition rate at this point, as a rate that is much faster than this
could easily send the reaction into overdrive (not good). Your reflux
should be unnervingly vigorous as the amalgamation really starts to pick
up speed, with the MeOH literally pouring down out of the condenser. I
know it's hard to believe, but this is what you want, this is good. I'm
telling you, LOTS of trial and error came before this writeup. Trust me.
You will also see sludge already starting to settle at the bottom and
forming a ring on the glass around the top surface of the spinning mess.
The consistency will get thicker by the minute. Add more ice to your
bucket as needed.
12. At this point you can sort of control the
reaction rate by slowing down or speeding up the addition rate a bit. Of
course the reaction is already barreling along, so you won't want to speed
it up much. The concept here is that you want the addition of the
ketone/nitromethane to be paced neck-and-neck, as it were, with the
breakdown of the foil as it amalgamates and gets turned into sludge. In
other words, you have to watch those two things and sort of adjust the
addition so that they proceed at approximately the same rate. It's tricky,
and imprecise, but with a little experience and intuition you'll get the
hang of it. Sure, you could be lazy and just leave the addition at a
steady 1 drop/second the whole time, but if the amalgamation peters out
way before your addition is finished, and you find yourself adding your
beautiful ketone to impotent sludge, don't cry to me. The addition should
take about 40-45 minutes in total, and as it's finishing, the state of the
aluminum should be about 95% broken down. In fact the reaction should by
now (~45 minutes after addition was started) look like a really thick,
steely-gray chowder with only minor small slivers of undissolved aluminum
visible if any at all. You will probably even need to add an extra 20-30mL
of MeOH down the condenser at this point (or before) to help it keep
stirring effectively. This is no problem.
A note about color at this point is helpful too. Comparing successful
reactions to failed ones, I have observed that there is a distinctive
color to the mixture early on that indicates healthy amalgamation and
foretells a successful run. At a point maybe 30 minutes or so
post-addition, the reaction takes on a color that I would describe as
being "light steely gray with blue overtones." It is a hard thing to
describe shades of gray, but I will try. It is a light shade, akin to the
color of common gray sweat pants, but like I say with a very slight
suggestion of a blue hue in there as well. This is in contrast to what I
saw in failures resulting from using too thick of aluminum and not enough
HgCl2, where a dark metallic gray with definite green overtones (from
unreacted ketone) was noted.
NOTE: Another point I would like to make about the timing of the
addition against the breakdown of the aluminum is that Mr. A found that
there was a definite "spike" curve to the amalgamation reaction which was
easily observed by watching the reflux rate. That is to say, there is a
peak that it builds up to and then comes down from. At this scale, and
using the exact materials described herein, that buildup to peak and
subsequent slowdown occurs over approximately 25 minutes or so---very
fast. So at only about 20-25 minutes after you first started feeling the
amalgamation heating up, it will have slowed to a reflux of about 2 drops
per second, after having been at a peak with a reflux rate so furious it
is a stream, not drops. At one hour and 15 minutes after you first started
the addition, the reflux will have slowed to a very calm 1 drop per 2
seconds or so. Finally, when...
A) the reflux has slowed to almost no reflux at all
B) if you stop
the stirring you do not see any small bubbles anymore
C) no "uneaten"
aluminum is visible and the solution is a thick, uniform gray soup,
...the reaction has essentially finished. It will reach this state at
about one hour 45 minutes to two hours after addition was started.
Nevertheless, you will leave it stirring happily for a total of three
hours after the addition was finished to assure that the reaction has run
its full course and the conversions that you desire have had ample time to
take place. You could add a bit of external heat at this point but it's
probably not even needed (Mr. A does it just to be sure). One reason I
bring this all up is that there has been lots of talk about how this
reaction needs 8 hours or 24 hours or even 36 hours to run! But those time
frames apply only in cases where much thicker aluminum is used, and/or in
variations using methylamine and not nitromethane. Mr. A was never
successful in using thicker aluminum, and doesn't want to be! Why would
someone want to make a reaction take any more time than it needs? Beats
me! I'm mystified! The approach illustrated in this writeup optimizes this
reaction to finish in 3 hours 45 minutes from beginning to end, and it
probably doesn't even need that much time.
13. If you chose to apply external heat, turn it off at 30 minutes
before the targeted finish time. Otherwise you will have to wait an extra
30 minutes for it to cool for the next steps.
14. When finish time has
arrived, dismantle your setup, set aside your reaction flask, and make 750
mL of a 35% NaOH solution (750 mL H2O + 262.5g NaOH) and let it cool to
room temp or below (safety glasses!).
15. Into a separatory funnel no
smaller than 2000mL capacity, pour your beautiful gray reaction mixture,
being very careful to KEEP THE STIRBAR FROM FALLING IN to the sep funnel
and breaking it (that would be ugly). If your mixture is really thick, you
may need to add small amounts of MeOH to thin it to a pourable
consistency. This is perfectly fine. Wash the final residue out of the
reaction flask with a few mLs of MeOH and add it to the funnel
also.
16. Slowly pour the NaOH solution into the sep funnel (gloves and
glasses! no excuses!). That's right, don't dump it in wholesale. Basifying
should be a gentle process. If you bully those molecules they may decide
they're being disrespected and choose not to cooperate. Adding the NaOH
will cause the mixture to warm up a bit as the very last bits of the
aluminum are dissolved, which is fine. Swirl it a couple times and give it
about 10 minutes to cool down to something closer to ambient temperature.
That yummy stinky methylamine smell tells you that the reaction was
successful.
17. When the mixture in the sep funnel has cooled down,
extract it once with 400mL toluene followed by once with 100mL toluene.
These are the critical moments for your yield now, so you be sure to shake
long and hard (at least 3 minutes) during these extractions (I don't have
to tell you to vent do I?!). The toluene/product layer will of course be
on top since toluene floats on water. Also, be sure to give the
separations ample time to happen (at least 15 minutes); it is easy to tell
when it's okay to separate because the interface of small toluene bubbles
finally resolves and you have a nice clean line between the layers. If you
like, do as Mr. A does and finish off with a final small extraction of
50-60 mL toluene just to get the last of the stuff.
NOTE: Your extractions will contain a tiny amount of the
base/metal/garbage from the bottom layer; this is inevitable but easily
worked around in this way: when you have collected your combined
toluene/product extractions in a bottle, chill that bottle in the freezer
for 30 minutes or so. When cold, the garbage gets a lot less mobile and it
is easy to decant the toluene away from it. Just be vigilant while pouring
the last 50 mL or so and avoid letting that glob of crap rejoin the
toluene. Yeah, you will lose the very last 2 or 3 mL, but that's life.
Alternatively, you could filter it through a paper towel, but you will
still lose the same amount when the towel absorbs it. Just get over it and
move on!
18. If you haven't already, drain the garbage layer out of your sep
funnel into a storage bottle or something, and wash the garbage residue
out your sep funnel with water.
19. Wash the toluene/product 4 times
(or more) in your sep funnel with 400 or 500mL H2O and a final time with
500mL of a saturated NaCl solution to remove any traces of solvated
HgCl2.
20. Dry your toluene/product solution with 30g of your favorite
drying agent (MgSO4 recommended) in an acetone-cleaned, heat-dried bottle
for no less than 30 minutes (Mr. A is superstitious so he lets it sit for
an hour). Shake it a few times during this period.
21. Filter the
solution and gas it with that good ol' HCl bubbler setup. Be smart and use
just enough muriatic (31% HCl) to wet the salt but not enough to make any
puddles, and put a wad of drying agent wrapped in tissue paper in line
somehow between the reaction flask and the tube leading to your pipette
end. Weep with joy as a bumper crop of white precipitate crashes out of
solution.
Expected yield: approximately 20-21.5g raw odoriferous product that
will purify via careful recrystallization to 17-18g of beautiful
snow-white MDMA! Ain't life grand?
------------------
O+O+O+O+O+O+O+O+O+O+O+O
"There's a methyl to
my madness"