Swim found the following information -- inscribed in invisible ink -- on a transparency concealed within an opaque fish-tank.
After rinsing with toluene and brushing off all the deceased toucans, swim was able to read: =============================================
"You can reduce 2,5-dimethoxyacetophenone to 2,5-dimethoxyethylbenzene with Zn/Hg and HCl in 50% yield too, see JOC 25, 1245 (1960)"
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Surprisingly, the text of that very reference constituted the next paragraph Swim read:=============================================
"Mossy zinc (140g) was amalgamated by a standard procedure* and the amalgam refluxed with 263 mL of concd. hydrochloric acid and 175 mL of water while 35 g. of 2,5-dimethoxyacetophenone was added over a 1-hr perod. Reflux was continued for 22h during which time 97 mL of additional concd. hydrocloric acid was added in 10-mL increments. The oily upper layer was then separated, dried with calcium chloride and fractionated under reduced pressure. The main fraction, b.p. 65-67.5°C/ 5mm, consisted of 15.7 g (49%) of a pale yellow liquid. Refractionation gave a colorless analytical sample, b.p. 68-70°C/ 7mm, f.p. -4 to -6.5°C."
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Swim was tickled to find that the *'ed reference to standard zinc amalgamation was the next item quoted on the transparency:=============================================
"The procedure originally used by Clemmensen is satisfactory for the reduction of many carbonyl compounds which are appreciably soluble in the acid mixture, or which melt below the boiling point of the reaction. The exact proportions of zinc and hydrochloric acid employed are not of great importance provided that both are present in large excess...
The use of mechanical stirring has been reported, but in most cases sufficient agitation is provided by the ebullition of the hot acid.
The physical form of the zinc appears not to be critical, since zinc turnings, zinc wool, granulated zinc, zinc powder, and mossy zinc have given good results. Mossy zinc has been most commonly used...
The zinc is ordinarily amalgamated by treatement with 5% - 10% weight of mercuric chloride in the form of a 5 - 10% aqueous solution. The time required for amalgamation can be diminished by employing a solution of mercuric chloride in very dilute hydrochloric acid. In order to obtain a homogenous amalgam, it is advisible to shake or stir the mixture during tha amalgamation.
Preparation of Zinc Amalgam: A mixture of 100g mossy zinc, 5 to 10 g. of mercuric chlorie, 5 cc. concentrated hydrochloric acid and 100 to 150 cc. of water is stirred for five minutes. The aqueous solution is decanted, and the amalgamated zinc is covered with 75cc of water and 100cc of concentrated hydrocloric acid. The material to be reduced, [usually about 40g], is then added immediately and the reaction is started."
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a brief additional reference to 2,5-dimethoxy-1-ethylbenzene was included:=============================================
"A pale straw colored liquid was distilled at 120°-140°C at the water pump to give 2,5-dimethoxy-1-ethylbenzene as a white fluid product."
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This confirmed the description of a white fluid, but did not give much insight into the pressure at which the boiling point was measured.
It was here that the transparent text launched into a puzzling continuation, which piqued my interest. It did not appear to be a reference to published material:=============================================
"140 g mossy zinc -- [aside: mossy zinc == jagged, solid, bitesized nuggets of zinc, as though molten metal drops had been gleefully splattered onto a cold irregular surface] -- was weighed out and placed into a large three necked roundbottomed flask equipped with overhead stirrer, addition funnel & reflux condensor. 175 mL of water was measured and poured in with the zinc.
7 g of mercuric chloride was cautiously weighed out and tapped into the roundbottomed flask. 7 mL of concentrated hydrochloric acid was measured and poured into the roundbottommed flask.
The various apparatii (addition funnel, reflux condenser, stirrer shaft / adapter) were placed securely in the necks of the roundbottomed flask and stirring was initiated immediately, and continued for five minutes. The zinc turned dull grey, followed by an identical color change in the water (with slight fizzing observed), and subsequently bright patches of exposed elemental zinc could be seen.
After five minutes, the aqueous solution was decanted into an appropriate waste receptacle. The amalgamated zinc was retained in the roundbottom, which had become warm to the touch.
To this approximately 140g of amalgamated zinc, was added 175 mL of water and then, gradually, 263 mL of conc. hydrochloric acid. Moderate amounts of heat and bubbling hydrogen gas were generated during the addition of the hydrochloric acid.
Once all the hydrochloric acid had been added through the addition funnel, stirring and heating were started. After about 20 minutes, reflux had been established
At this point, 35 g. of pale yellow, pungent 2,5-dimethoxyacetophenone was measured out and added to the addition funnel. While placing the 2,5-dimethoxyacetophenone in the addition funnel, it was observed that Residual traces of concentrated hydrochloric acid on the addition funnel caused the initial few droplets of the acetophenone to rapidly darken to an orange-brown color. Most of the acetophenone was unaffected.
The addition funnel stopcock was adjusted so that the acetophenone dripped out at the rate of about a drop every two seconds into the refluxing reaction mixture...(introduced to the reaction mixture dropwise over the course of 1 hour.)
Once the addition of acetophenone was complete, reflux was continued for an additional 22 hours. At two hour intervals, 10mL of concentrated HCl was added.
During the course of the reaction, the aqueous layer progressed from pale gray to dark brown and finally to a clear, dim bluish color [?]. It is surmised that the unfortunate use of a stainless steel stirring rod (which was corroded by the HCl liquid & fumes) was responsible for the brown / blue discolorations of the aqueous phase.
Throughout the course of the reaction, the added acetophenone was an oily upper layer... initially pale yellow, gradually becoming a water-white, oily fluid, and then in the last several hours, increasingly brown and viscous oil that began to coat the sides of the roundbottom. It was hypothesized that the clear white oil observed in the middle stretch of the reaction was mostly desired product, and the viscous brown material that developed towards the end consisted of unwanted products of polymerizations and side reactions from prolonged reflux.
After 22 hours, the reflux was stopped and the aqueous layer was decanted into a separatory funnel.
There was some residual zinc amalgam remaining in the flask's round bottom. It was cautiously rinsed out and disposed of in the same waste container as previous mercury-containing waste.
The separatory funnel now contained: lower dim blue aqueous layer, and and upper, viscous brown oily layer of about 35mL volume, which clung to the glass sides of containers it was in, and broke into rubbery droplets when agitated in water.
The viscous brown oil was fully separated but not washed or dried. It was left to sit in the dark at room temperature in a sealed container for about a day.
The brown oil was then placed in a small round-bottomed flask. A vacuum distillation setup was assembled with an oil-bath as the heat source & a jacketed condensor leading to another small round-bottomed recieving flask.
A vacuum of 1.8 - 2.0 mm Hg
(measured in the Vacuum Line, NOT inside the distillation apparatus) was established. (i.e., a vacuum of 1800 to 2000 microns, which was the
highest pressure that could be measured with available equipment) and immediately there was bumping of the crude product, probably due to the absense of boiling chips & presence of residual water (since product had not been dried).
The bumping was not severe enough to be problematic, and after several minutes of vacuum, subsided. There was no appreciable decrease in the volume of the crude product.
After consultation of a nomograph plus calculation of theoretical b.p. using Clausius / Cla-pey-ron equation, it was expected that the b.p. of desired product would be about 45°C at 2mm Hg.
With this in mind, the temperature of the oilbath was raised gradually. As minutes passed, the oilbath temperature rose through 30°C, then 40°C and 50°C without any discernable activity in the distillation flask.
Finally, with the oilbath temperature above 60°C, sporadic bumping / bubbling was once again observed in the brown oily crude product.
It became apparent that the size of the distillation setup was too large for the modest amount of liquid being distilled, since despite the occasional bump, no liquid was seen condensing in the condenser, (and very little even on the sides of the roundbottom distilling flask.)
Also of concern was the oilbath's high temperature relative to the calculated theoretical boiling point of the desired product at the measured pressure (65 to 70°C oil vs. about 45° expected distillation temperature). Due to such concern, the oil temperature was maintained at no more than 70°C, to avoid potential severe bumping.
However, with only sporadic bloops in the distillation flask, no significant vapor was being produced and no progress was made in vacuum distillation. Eventually distillation was terminated and plans were made to alter the setup to better conform to the small scale of distillation being performed.
A mini-scale vacuum distillation setup was assembled, which used a two-necked, standard joint "test tube" as the distillation flask and a small roundbottom as the recieving flask. The crude product was placed in the test tube, attached to which was a thermometer/inlet adapter, and a three-way distillation adapter. The distillation adapter was connected directly to the receiving flask, without a jacketed condenser intervening. Although no boiling stones were available, a small stirbar was included in the distillation vessel and the oilbath was also stirred magnetically using a stirrer/hotplate.
This time, the oilbath was heated more agressively and the vacuum was strengthened to 0.4 mm Hg (400 microns) - again, measured in the vacuum line, not within the distillation apparatus. The oilbath again reached a temperature of about 60°C before any activity was observed in the distillation "flask". Again, the crude product began bumping despite the stirbar. Nevertheless, heating of the oilbath continued and the temperature of the distilling vapor was monitored. Once the oilbath had reached a temperature of about 80°C, the rate of bumping increased and a pale/clear, low-viscosity oily fluid began condensing on the side of the distillation "flask" (test-tube) and also on the thermometer bulb. ...the thermometer inside the distillation container shot up to 65°C.
As the oilbath temperature was increased to 100°C, enough vapor was generated that pale/clear oil began condensing in the distillation adaptor and dripping into the recieving flask. The temperature of the distilling vapor remained steady at 65°C but the rate of distillation increased as the oilbath temperature was raised.
The attainment of successful vaccuum distillation was heartening, but the unexpectedly high temperature of the distilling product was disheartening. Although the references suggested a boiling point of about 65°C, it was at a pressure of 5mm, not at 0.5mm. Equations were double-checked, but consistently it was calculated that the boiling point of the expected product, 2,5-dimethoxy-1-ethylbenzene, should have been much lower at the pressure of 0.4 mm Hg.
Nevertheless, all other aspects of the distilling substance matched description, and distillation was continued over the course of about an hour, yeilding about 10mL of clear/pale oil in the recieving flask and about 25mL of extremely viscous, tacky black muck in the distillation vessel (similar to taffy / cool tar in consistency).
It is of concern that the 10mL of pale oil obtained is not the desired 2,5-dimethoxy-1-ethylbenzene, although in most respects it matches descriptions of the substance... the boiling point of the liquid at low pressure does not correspond to the calculated boiling point."
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Here, Swim abruptly encountered the lower edge of the transparency.
After reading through all that verbigarbage, I felt a sense of cameraderie and empathy for the unknown author, and took a few minutes to scribble out my own calculations of the boiling point of 2,5-dimethoxy-1-ethylbenzene
The most accurate data point provided in the references was "68°C to 70°C at 7mm Hg."
The Clausius Cla-pey-ron equation for calculating phase transition to gas is given as p = p° exp ( (-
deltaH / R) * (1/T - 1/T°) )
substituting knowns and assuming the heat of vaporisation for 2,5-dimethoxy-ethylbenzene is ~50 [J/mol-°K], this yields0.4 torr = 7 torr * e ^ { (-50/8.314)[ J/mol-°K ] * (1/ (273+69)°K - (1 /
)°K) }
From this calculation, Swim gets an expected boiling point of around 33°C at 0.4 mm Hg. This does not match very well with the author's reported distillation temperature of 65°C to 70°C at 0.4 mm Hg.
Thus, Swim also wonders if the mysterious author of the transparent saga is justified in his concern about the identity of the product.
Swim notices that the author is measuring his vacuum outside the vacuum apparatus, AFTER the distilling vapor has been condensed. Swim wonders if the pressure conditions INSIDE the distillation flask may have been different -- the pressure higher due to vapor of the distilling crude product.
It makes sense that the pressure should drop after the vapor is condensed at low temperature in the recieving flask. Thus, the measured pressure in the vacuum line could be lower than the actual pressure experienced by the distilling substances, perhaps explaining the higher than expected boiling point?
As a last note: it occurs to Swim that the f.p. of 2,5-dimethoxy-1-ethylbenzene was provided in one of the references (-4 to -7 °C), but that the author of the bizzarre, concealed transparency made no mention of checking the f.p. of his unknown product.
Swim thinks that perhaps this is the next thing the author should try, if he hasn't already done so.
What do y'all think?