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Experiment 1: The Schlenk Line 1.1 Overview

Many reagents used for organic syntheses are sensitive to oxygen, moisture, or both. These compounds to be protected from the ambient atmosphere. You will use several of these reagents during this course and you will become acquainted with a number of techniques to protect materials from the atmosphere. You will likely use these techniques in the inorganic laboratory as well. One of the best references for the handling of air-sensitive compounds is the book The Manipulation of Air-Sensitive Compounds by D. F. Shriver and M. A. Drezdzon. I have a copy of this book that you’re welcome to look through any time. The main technique that we will use for excluding air and water from our reactions in this class is a Schlenk line.

1.2 Schlenk Techniques

The centerpiece of the defense against atmospheric intrusion is the double manifold, or Schlenk line. In the illustration below, the double manifold (or dual manifold) is in the middle, connected to various pieces of equipment on each side. The two long, horizontal tubes of the Schlenk line are called “manifolds”. Each manifold can be filled with a gas, or “un-filled” with vacuum. Reactions and manipulations are typically done under an atmosphere of an inert gas, usually nitrogen, admitted into one of the manifolds (the

Diagram of a Schlenk Line http://www.chemistryviews.org/details/education/3728881/Tips_and_Tricks_for_the_Lab_Air-Sensitive_Techniques_1.html

Chem 339, Spring 2016, Hoover Adapted from J. M. Mayer, Chem 317

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“nitrogen side”). The other manifold (the “vacuum side”) is connected to a vacuum pump through a liquid-nitrogen cooled trap. [Our vacuum pump is housed in the equipment room, so you won’t usually see it.] Any solvents that make it into the vacuum line condense in the trap before they get into the pump. The trap protects the pump and the pump oil from these solvents. Glassware (see below) is connected to the Schlenk line via rubber hoses. The glassware is exposed to the vacuum or nitrogen manifold using the two-way stopcocks. For safety reasons, our Schlenk lines will be set up in the hoods. Be sure to study this diagram before coming to class. Your first ‘experiment’ will be to setup and test either a dual manifold (like the one pictured) or a single nitrogen manifold. Be sure all stopcocks are lightly greased. If you forget this, the glass pieces may stick together and it can be a real pain getting them apart. The function of the thin film of grease is to allow the glass pieces to slip over each other. If you use too little grease, the glass pieces will bind. If you use too much grease, it will ooze into the holes and into your line and will degrade more rapidly. When you’re done, the ground-glass part of the stopcock should look clear, with no streaks in the grease. Your TA or Instructor will help you get the hang of this. The vacuum gauges we are using in this class are analog Pirani-type gauges. The gauges are attached to the vacuum manifold and are your way of seeing what the pressure is in the manifold. The pressure is measured in units of torr. Recall that one atmosphere is 760 torr. The gauge has a range of 1-760 torr. In this class, these gauges will be used mostly to tell you qualitatively what is going on inside your Schlenk line. Whenever you do anything on the Schlenk line, the pressure inside will change. Be sure you know what you expect to happen whenever you open or close a valve on your line – and watch the gauge to be sure that this is what happens. If anything surprising occurs, STOP, close the valve, and think again. NOTE: go slowly and wait for the gauge to respond. The nitrogen inlet that we will be using is the nitrogen inlet in your hood. This line is plumbed to two nitrogen tanks in the equipment room. The Nujol bubbler at the end of the nitrogen line is there to prevent air from getting in. Be careful, if you pull vacuum on the nujol bubbler, you will suck Nujol back into your line and make a big mess. Many students do this at least once and then have to clean their line (not fun!). If you need to pull vacuum on the nitrogen line, use a pinch clamp on the hose or squeeze it with your fingers to prevent the backflow of Nujol into the Schlenk line in case you make a mistake. When you’re using the line, there should be a slow and steady flow of nitrogen through the nitrogen manifold and out the bubbler. The stream of nitrogen bubbles tells you what’s going on in this manifold.

Chem 339, Spring 2016, Hoover Adapted from J. M. Mayer, Chem 317

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You have an array of equipment to help you manipulate solutions without exposing them to air. The Schlenk flask is an ordinary round-bottom flask with a sidearm with a stopcock (if it is a glass stopcock, be sure it’s greased! Teflon stopcocks shouldn’t be

greased.) You can connect this sidearm to the Schlenk line with thick rubber tubing and use it to admit nitrogen to the flask or to evacuate it. The tubing needs to be thick so that it won’t collapse under vacuum. You will put something in the neck of the flask, such as a glass stopper (greased), a septum, or another piece of apparatus such as a Schlenk addition funnel. The Schlenk addition funnel is a convenient means for adding solutions to a reaction vessel. With the lower stopcock closed, it is completely sealed off from the flask below it, so you can put solutions in the funnel without contaminating the atmosphere in the bottom flask. To move a solution from the funnel to the flask, first connect both to the nitrogen manifold via the sidearm stopcocks. Then just open the valve at the bottom of the funnel. Connecting everything to nitrogen is critical because this equalized the pressure between the top and bottom; without it, pressure would build up in the lower flask as the liquid flows down, and the flow would stop. In all air sensitive work, you will always have to worry about pressures – what the pressure is in each piece of apparatus and how this will affect what you’re trying to do. A more common addition funnel is the pressure-equalizing addition funnel. This type of addition funnel has a side-arm that connects the reaction vessel to the funnel, to avoid the build-up of pressure. The drawback of this type of funnel is that the reaction vessel cannot be closed off from the funnel. Another common way to cap a Schlenk flask is with a septum, an air-tight rubber membrane. By piercing the septum with a variety of sharp instruments, you can add or remove liquid from the flasks without exposing them to the air. And septa (the plural of septum) seal back up (pretty well) when you remove a needle. Since septa will not

Schlenk flask Schlenk tube Schlenk pressure-equalizing addition funnel addition funnel

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usually give you a truly air-tight seal, they should be swapped for a glass stopper if you need a flask to stay air-free for more than an hour or so. A syringe, which you’re probably familiar with, is one way of transferring liquids into or out of a septum-capped container. If the container is under a pressure of nitrogen, you can simply insert the needle into it, pull back the plunger to remove the desire amount of liquid, then withdraw the needle and inject the liquid into another flask. In the time it takes to move from one flask to another not much air can diffuse into the small bore of the needle. There are two important hints about using syringes. First, the “dead space” in the needle and syringe body will ordinarily have air in it. If this would be a problem, you should flush it out before using it. Suck nitrogen into the barrel from a nitrogen filled container (usually a flask connected to the nitrogen line), then expel the nitrogen out into the air. Repeat this two or three times to purge air from the syringe. Second, remember to always think about pressure when transferring via syringe. If you try to suck a liquid out of a sealed container, you wont get very far, since you’ll build up a partial vacuum in the container. You can avoid this by making sure the flask from which you are withdrawing is connected to a source of nitrogen, like your nitrogen manifold, or by injecting a volume of nitrogen gas to compensate for the volume of liquid that you are going to remove. Similar tricks can alleviate difficulties in injecting into closed flasks. A cannula is a hollow steel needle with two sharp ends. It can serve as a sort of express route for transferring liquids when setup as shown. If the pressure in the flask at right is greater than that in the other flask, the liquid will be pushed from the right flask to the left flask.

Cannula transfer http://www.chemistryviews.org/details/education/4360441/Tips_and_Tricks_for_the_Lab_Air-Sensitive_Techniques_3.html

Chem 339, Spring 2016, Hoover Adapted from J. M. Mayer, Chem 317

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1.3 Gas Handling: Tanks and Regulators

The handling of gases provides a number of challenges. First, all gases are “air sensitive” to the extent that if they get mixed with air they are no longer pure. (There are also gases that undergo chemical change because of reaction with oxygen or water.) Gases are purchased in thick (usually steel) tubes, either large cylinders or smaller “lecture bottles.” The gas inside is present under pressure, typically pressures much greater than one atmosphere. This represents the second challenge: a pressurized gas would love to get out of its container and will do so with some force if allowed to. This is a serious hazard and one that must always be kept in mind while working with compressed gases. In the mid-90s, at my alma mater, there was a serious incident when someone opened a lecture bottle to their Schenk line when the stopcock on the line was closed. The high pressure gas was released into the rubber hose, but had no place to go. So what did it go? The gas blew open the thick hose, with sufficient force to snap a piece of glass off the line. A fair amount of noxious gas was sprayed into the lab and everyone had to be evacuated. A similar problem could happen in our lab if the nitrogen gas line was opened without an appropriate outlet or bubbler in place. Let’s not have a problem like this in our class this semester! We use a series of valves and regulators to make sure that the flow of gases is controlled (by us). The cylinder or lecture bottle that comes from the vendor has a valve on top, essentially an on/off valve (there is a little bit of flow adjustment possible with this valve, but very little). The valve on the tank is your first line of defense. If this is closed, no gas can get out. But when this main valve is open, the gas will come out at a high rate (flow): our nitrogen tanks come pressurized to 2,000 psi (pounds per square inch) or ca. 150 atmospheres. Typically, if anything goes wrong while working with a gas, we would close off this valve. Our cylinders are stored in the equipment room and you won’t always have easy access to them. In this class, if anything goes wrong while working with your nitrogen line, close off the nitrogen at the hood valve. In this lab, you will use large cylinders of nitrogen as your source of inert gas. To control the flow of gas from a large tank, always use a regulator (such as the one pictured at right). One other precaution: these tanks should always be chained to a wall or a lab bench so that they cannot tip over. If one tipped over and fell on the regulator, it could snap off the valve on top of the tank. This would turn your gas cylinder into a steel torpedo, propelled by the high pressure gas inside.

Cylinder regulator http://www3.imperial.ac.uk/xraycrystallography/crystallography/helium

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The regulators we use for large cylinders typically have two valves and two gauges – and it is important that you know the function of all four things. The gauge closest to the cylinder tells you the pressure in the cylinder; it will be calibrated from 0-3,000 psi (it will also be calibrated in other units, just to be confusing). The valve that is in between the two gauges is a diaphragm valve, which regulates pressure (not flow). This amazing device enables you to set the pressure coming out of the regulator, which you can read on the second gauge, the one farther away from the tank. Typically we use low pressures, 0-5 psi (above atmospheric). Finally, the small valve leading out of the regulator is a needle valve that regulates flow, not pressure. In this lab, we also have an additional valve on the fume hood. This valve is your last line of defense – open this valve slowly to bleed gas into the line, then if you need more gas, open it wider. The vavlue is a needle valve and works as you would expect: screwing it down will close the valve. The diaphragm valve, however, is opposite: it is closed when screwed all the way out, and most open as you screw it in. Be sure that you understand this difference. You must be very careful not to let pressure build up in any piece of apparatus. Schlenk techniques can tolerate pressures only slightly greater than one atmosphere. If you have a pressure of two atmospheres in a flask (external pressure plus one atm more) that’s 14.7 pounds on every square inch of your apparatus. So a stopper with a one square inch opening will have 14.7 pounds pushing it open. This is equivalent to hanging a bowling ball off of it! Be sure – whenever you work with gases – that you know what will happen anytime you open a valve. Be sure you know where the gas is supposed to be going and where the gas will go if the pressure by accident gets too high. Let’s have a safe and fun lab!