Yesterday late afternoon-evening, I had what I thought was a major crisis on my hands. I was right in the middle of a vacuum distillation, and at that of a high-boiling compound that is BARELY suitable for distillation at atmospheric pressure, needing over 250 'C to distill at 1atm, it could be done, but is known to result in a far less pure result for the compound in question with significant degradation taking place.
I'd tried distillation using a water aspirator venturi-type vacuum (basically consists of a short length of glass tube, or in some cases metal, which attaches to the water input, and is surrounded by a bulb of glass or metal having a T-connection hose-barb sticking out of the side, to which the system to be placed under vacuum is connected by means of suitable tubing, preferably with a flask and distillation head situated in between to serve as a water trap in case of suckback due to vacuum loss which would otherwise, if the venturi isn't isolated from the water flow first before opening the system back up to atmospheric pressure again the vacuum in the system will pull water in from the flow into the flask. Having a second flask placed between the receiving flask used to catch the distillate and the venturi, connected by a distillation head and claisen adapter, etc. serves as a reservoir to contain any suckback and give the user of the aspirator time to disconnect the water flow before turning it off. The top glass tube connects to this bulb, and tapers into a cone with a thin narrow opening forcing the water into a narrower jet at higher pressure, which then is forced through a narrowing of the long glass stem, the bottom end of which being open is connected to the exit hose through which the water is discharged, and the pressure differential created when the water is fired through the section of the lower stem (which comes out of the bottom of the bulb and begins just below the conical water outlet situated inside it, with a small air gap through which the hose barb for the vacuum connection to system to be evacuated, end of the conical taper from the water inlet, and the narrowing of the bore of the exit tube (which widens afterwards, with a bit more than an inch or so of narrow region, kind of like a wasp-waist in profile) so they are able to communicate in terms of pressure change, I.e an open system (with the exception that its closed at the far end, of whatever glassware arrangement is being used, so as to form a vacuum), which takes place by the venturi effect as the water rushes through the narrowed bore at high pressure and expands afterwards when it reaches the end of it at the exit portion of the tube.
Its intended for applications involving stripping low-boiling solvents, or when higher vacuum would cause a compound to boil off TOO low (in temperature), where a mechanical, more powerful vac pump is overkill.
But, after stripping the DCM off what I was distilling at aspirator vacuum level negative pressure (DCM-dichloromethane, methylene chloride, CH2Cl2, a volatile chlorinated hydrocarbon solvent similar in nature to chloroform although it boils at a much lower temperature, around 36 'C IIRC.) heating using a steam bath, after the dichlor was stripped, I switched over to using my rotary vane pump, a mains-powered mechanical pump capable of creating much lower pressure than is the venturi because the substance to be distilled, once freed of solvent, bar a ml or two of lower-boiling fore-run which was stripped and discarded, the distillation flask was heated by a bath of ethylene glycol which boils at just under degrees 'C and the rotary vane pump vacuum applied.
Unfortunately, the weather was searing, baking, roasting hot. A real scorcher of a bastardly day, to the extent it was making me feel a bit dizzy and shaky. I'd just given my vac pump (the rotary vane one, the venturi doesn't use any) a fresh oil change, completely drained it and refilled the oil reservoir with nice clean fresh stuff, turned it on, and left my product to distill. Unfortunately somewhere along the vac line there seems to have been a leak and I couldn't for the life of me resolve it. So will have to redo it using alternative pieces of glassware in case its the grind of the joints themselves, since I'd clamped everything up, applied some strong, stainless steel keck clips to hold everything in place, after applying some quality Dow-Corning high vacuum grease to all joint interfaces between glass pieces (I.e 1 liter distillation flask, an air bleed (basically fine capillary tubing, which can be pulled from a glass pasteur pipette held in a blowtorch flame, which connects to the outside and lets a tiny stream of bubbles in, the end being dipped into the distillate, this stops the substance being distilled from undergoing severe bumping under vacuum, since boiling stones don't work. For air sensitive substances, a nitrogen feed can be connected, running at low positive pressure, just a slight flow, or an argon tank if reactive with nitrogen, like lithium metal is for example although reactivity with N2 is very uncommon. Glass stoppers, since the distillation flask was of the kind having more than one neck. One neck connected to the capillary bleed tube, the other to a distillation adapter and thermometer inlet, with thermometer in place. Temperature set, condenser rigged up to one of my water pumps and the water pump turned on, and vacuum applied.
The problem, was, IMO, the heat. Because I'd left it for a while to distill over, and came back to check on the progress of the distillation, but instead of coming over as desired it was failing to distill, so I disconnected the vac pump, turned the pump itself off, and made a few adjustments. Then reattached the vac line to the receiver flask and turned on the vacuum. Or rather, I tried to turn it on. it wouldn't start. Could tell that the motor itself, or at least the circuitry hadn't burnt out, since it was making a humming noise plus slight vibration, the mechanism trying to work. But it wouldn't start up at all, even with a bit more oil being poured into the reservoir. The pump itself, on further inspection was so furiously, searing hot that it was barely touchable with bare fingers on any of the metal parts and the exhaust port felt like the muzzle of a flamethower after use by a small army of compulsive pyromaniacs.
So, I had no choice. I had to leave the compound in the distillation flask with only a few drops of distillate collected.
I thought to myself, 'oh shit, I've really had it here, my vacuum pump just kicked the bucket and I'm going to have to buy a new one. But in the morning, after going to bed, albeit a fairly hot and sweaty overly stuffy night I did manage to sleep shallowly with the aid of a few strong sleeping pills (15mg nitrazepam in total, 3x5mg tabs plus some adrenal blocking agents of the autoreceptor agonist type, clonidine and one of its relatives, tizanidine, a muscle relaxer and a bloody strong one at that)
Woke up early in the morning, about 7-7:30am and more or less bolted out from under the covers to see whether it would be called upon for me to administer the last rites to my vac pump...pressed the 'on' switch briefly.....and couldn't help but crack a broad grin from ear to ear, practically beaming with delight when I heard the familiar low, loud buzzing hum of the motor spinning up to full speed. The vacuum pump works!!!!
I won't be forced to buy a new one (although I'm going to be getting a new one anyway, going to buy a few heating mantles of different sizes and build my own magnetic stirrer around them, so I've got them of sizes suitable for heating all my different flask sizes, all the way upwards from tiny 5ml microscale kit to full size synthesis-scale glass.) first, and now I can afford to save up for a spare, and more powerful vacuum pump, and not have to blow away a sizeable portion of my money on it. I could afford to buy one right now, I've a few hundred pounds to spend on glassware and will be getting another £80-something to £90-something tomorrow too, for my disability spazz pay/mobility assistance allowance due to my fucked up joints, so I'll get myself the heating mantle size most useful for me first, and gradually move up and down in sizes until every one is covered. And of course build the magnetic stirrers to go beneath them. Plus now I can buy myself some more glassware. I want some new distillation adapters, some size reduction adapters to enable me to connect 24/40 or other 24/XYZ taper size glassware joints to smaller bored glassware pieces. Another Vigreaux column would be nice. Couple of spare thermometers (already have one on order), and I'd quite like some calcium metal, more magnetic stirbars, rubber septa for transferring highly air-sensitive and/or pyrophoric reagents via nitrogen-purged syringe and cannula through rubber seals fitted to glass joints into systems kept under vacuum or under argon/nitrogen/helium atmospheres that would otherwise degrade, be destroyed or burst into flames on contact with oxygen or water vapor traces in the air. A new vacuum pressure gauge and manifold allowing me to run multiple vac lines independently of each other would be lovely, and I need a new 1l, 3-neck flask in 24/40 taper plus a couple of small separatory funnels . Would like some pyridine, a few liters of diethyl ether, some tetrahydrofuran (an etherial solvent, a cyclic ether) and both some lithium aluminium hydride (aka 'LAH' in chemist shorthand, LiAlH4, a fairly powerful hydride type reducing agent and although expensive, some Red-Al (Sodium bis(2-methoxyethoxy)aluminumhydride ). Another hydride reducing agent, pretty pricey and unfortunately, in atom-economy terms also expensive due to having to use some 7 molar equivalents of Red-Al (another name for it, being Vitride, both commercial brand type names) per functional group on a molecule to be reduced, but with the advantage of being quite a gentle, yet powerful reducing agent and very clean in doing its work. Takes a lot, expensive, but a very good reagent in practical terms for the uses at which it is suitable for, it does its job quickly, cleanly, effectively and gently, whilst unlike the cheaper (although still not exactly growing on trees, and for most hobbyists very difficult to purchase, although thankfully I am not amongst those who cannot do so, if I've the money to spend, I can get it if I want any) LAH, Sodium bis(2-methoxyethoxy)aluminumhydride/Red-Al/Vitride is not pyrophoric, whilst lithium aluminium hydride is violently reactive with water, reacts nastily with air too, being very pyrophoric and moisture sensitive, needing handling under inert gas atmospheres, in solvents freshly dried, preferably in THF or other ethers after they have just been freshly distilled off of sodium metal under a current of scrupulously dried inert gas, nitrogen, helium or argon and used in glassware that has been baked out in the oven or flame-dried immediately before use. Otherwise it has a nasty tendency to spontaneously ignite on contact with atmospheric water vapor/O2 or traces of water in solvents used or inert gas streams And that is the last thing any chemist wants to have happen, a product torched, equipment potentially damaged, a visit to the burns ward after a few liters of ether or THF etc. just burst into flames with something similar to a fire in metal dusts in the middle of said highly volatile, flammable solvents. Red-Al ain't cheap, but it is much safer to handle and gentle on substrates to be reduced. LAH always strikes me as the chemical equivalent of a rottweiler straining at its leash, growling evilly just waiting for the opportunity to bite a chemist on the nackers. (I.e set something or someone on fire) and vitride more of a show dog, some fancy bloodline pedigree fucking show dog that does tricks and wins expensive prize ribbons but demands its dinner served to it on a gold-rimmed plate with its name inscribed on whilst sat at the family dining table and a napkin around its neck, tied just-so:P (whereas the tamer, but rougher sodium borohydride is a useful, benign working dog that is well behaved, but certainly no pedigree. A mutt, but a good natured, hard working mutt, with STAB (sodium triacetoxyborohydride) being somewhere in the middle of prize winning and working mutt, whilst cyanoborohydride is more of a pedigree with a deep desire to rip your face off nonetheless (releases cyanide gas in acidic conditions and is, in terms of reduction ability, quite selective, although at the same time, pretty sensitive to PH. So a pedigree face-eating rottweiler, although at least that I'm aware of, not one that spontaneously bursts into flames.
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Yes, I admit, I get some weird mental imagery when it comes to my hobby sometimes, I really do. And the synaesthesia is even weirder. Ever smelled green, rounded and wide for example? or greenish white, thin and mobile with corners? thats what chloroform and dichloromethane respectively smell like to me.They smell of colored shapes with specific ways of moving, as do other elements and chemical compounds. Its strange as fuck a sensory modality and way tu feel the world but I'd not trade it, without, it would be just the same as losing the sense of touch, or other main sense mode and I'd definitely miss it.
