INTRODUCTION TO
`
`Organic
`Laboratory
`Techniques
`
`A CONTEMPORARY APPROACH
`
`Third edition
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`Argentum EX1032
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`Page 1
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`INTRODUCTION TO
`
`Organic
`Laboratory
`Techniques
`
`A CONTEMPORARY APPROACH
`
`Third edition
`
`DONALD L. PAVIA
`GARY M. LAMPMAN
`GEORGES. KRIZ
`l-V'estern Washington University, Bellingham, Washington
`
`SAUNDERS GOLDEN SUNBURST SERIES
`SAUNDERS COLLEGE PUBLISHING
`Philadelphia New York Chicago
`San Francisco Montreal Toronto
`London Sydney Tokyo
`
`Page 2
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`

`

`Copyright © 1988, 1982. 1976 by W. B. Saunders Company
`
`All rights re erved. No part of this publication may be reproduced or transmitted in any form or by any means ,
`electronic or mechanical, including photocopy, recording, or any information storage and retrieval system , without
`permission in writing from the publisher.
`
`Requests for permission to make copies of any part of the work should be mailed to: Permissions , Holt, Rinehart
`ew York, ew York 10003
`and Winston, 111 Fifth Avenue,
`
`Text Typeface: Times Roman
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`Production: York Production Services
`
`Cover Credit: © COMSTOCK , Inc.
`
`Printed in the United States of America
`
`I TRODUCTION TO ORGANIC LABORAJORY TECHNIQUES , Third Edition
`
`ISB 0-03-014813-8
`
`Library of Congre s Catalog Card Number: 87-28688
`
`789 039 987654321
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`Page 3
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`To All of Our Students in Chemistry 354 and 355
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`Page 4
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`508
`
`The Techniques
`
`An oil bath with ordinary mineral oil cannot be used above 200 to 220 °C.
`Above this temperature the oil bath may ' 'flash ,' ' or suddenly burst into flame. A hot
`oil fire is not extinguished easily. If the oil starts smoking, it may be near its flash
`temperature; discontinue heating. Old oil, which is dark, is more likely to flash than
`new oil is. Also, hot oil causes bad burns. Water should be kept away from a hot oil
`bath, since water in the oil will cause it to splatter. Never use an oil bath when it is
`obvious that there is water in the oil. If there should be water present, replace the oil
`before using the heating bath. An oil bath has only a finite lifetime. New oil is clear and
`colorless but, after extended use, becomes dark brown and gummy from oxidation.
`Besides ordinary mineral oil, a variety of other types of oils can be used in an
`oil bath. Silicone oil does not begin to decompose at as low a temperature as mineral oil
`does . When silicone oil is heated high enough to decompose, however, its vapors are
`far more hazardous than mineral oil vapors. The polyethylene glycols may be used in
`oil baths. They are water-soluble, which makes cleaning up after using an oil bath
`much easier than with mineral oil. One may select any one of a variety of polymer sizes
`of polyethylene glycol, depending on the temperature range required. The polymers of
`large molecular weight are often solid at room temperature. Wax may also be used for
`higher temperatures, but this material also becomes solid at room temperature. Some
`workers prefer to use a material that solidifies when not in use since it minimizes both
`storage and spillage problems. Vegetable shortening is occasionally used in heating
`baths.
`
`1.5 HEATING MANTLES
`A useful source of heat for situations that require temperatures above 100 °C is the
`heating mantle, illustrated in Figure 1-3. The heating mantle consists of a blanket of
`spun fiberglass with electric heating coils embedded within the blanket. This blanket
`fits snugly around the flask, providing an even source of heat. The temperature of a
`heating mantle is controlled by a Variac. Some heating mantles are designed to fit only
`
`J
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`,f /
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`'.t'.'· ·~iii/
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`FIGURE 1-3. Heating mantle
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`Page 5
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`Technique 1: Solvents and Methods of Heating Reaction Mixtures
`
`509
`
`specific ~izes of flasks ; other types fit a range of flask sizes. The heating mantle is safe ,
`1
`because 1t does not produce flames. It is rapid, since it does not have the h' h th
`erma
`1g
`.
`.
`-1 b
`f h
`·
`·
`1~ertia o. t e 01 . ath. It 1s convement because it is not subject to contamination as the
`011 bath 1s, and it does not produce the problem of oil clinging to the outside of the
`flask. One m_ust be careful, however, in using heating mantles , since there is the danger
`that_ the ~e~tmg ma~tle ~ay bum out if it is used to heat an empty flask. One should
`avoid spilh_ng chemicals mto the heating mantle. Finally , one additional disadvantage
`to the heating mantle is its great initial cost.
`
`1.6 HOT PLATES
`
`Occasionally , a hot plate is useful for heating small quantities of solvents when temper(cid:173)
`atures around 100 °C are needed. Care must be taken with flammable solvents to
`ensure against fires caused by " flashing, " when solvent vapors come into contact with
`the hot plate surface. One should never evaporate large quantities of a solvent by this
`method; the fire hazard is too large.
`Some hot plates also have built-in magnetic stirring motors . Their use is de(cid:173)
`scribed in Section 1. 8.
`
`1.7 HEATING UNDER REFLUX
`Often it is desired to heat a mixture for a long time and to be able to leave it untended.
`The reflux apparatus (Figure 1-4) allows such heating. It also keeps solvent from
`evaporating. A condenser is attached to the boiling flask, and cooling water is circu(cid:173)
`lated to condense escaping vapors. One should always use a boiling stone or a magnetic
`stirrer (see Section 1. 8) to keep the boiling solution from ''bumping.' ' The direction of
`the water flow should be such that the condenser will fill with cooling water; the water
`should enter the bottom of the condenser and leave from the top. The water should flow
`fast enough to withstand any changes in pressure in the water lines but should not flow
`any faster than absolutely necessary. An excessive flow rate greatly increases the
`chance of a flood, and high water pressure may force the hose from the condenser.
`When a flame is the source of heat, it is convenient to use a wire gauze beneath the
`flask to provide an even distribution of heat from the flame. In most cases , a heating
`mantle or a steam cone is preferable. It is essential that the cooling water be flowing
`before the heating has begun! If the water is to remain flowing overnight, it is advisable
`to fasten the rubber tubing securely with wire to the condenser.
`If the heating rate has been correctly adjusted, the liquid being heated under
`reflux will travel only partly up the condenser tube before condensing. Below the
`condensation point, solvent will be seen running back into the flask; above it, the
`condenser will appear dry. The boundary between the two zones will be clearly demar(cid:173)
`cated, and a reflux ring or a ring of liquid will appear there. In heating under reflux ,
`the rate of heating should be adjusted so that the reflux ring is no higher than a third to
`a half the distance to the top of the condenser.
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`Page 6
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`510
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`The Techniques
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`,,
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`7 ~
`~ I~ flame, heating mantle, or steam cone
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`FIGURE 1-4. Heating under reflux
`
`It is possible to heat small amounts of a solvent under reflux in an Erlenmeyer
`flask. With gentle heating, the evaporated solvent will condense in the relatively cold
`neck of the flask and return to the solution. This technique, illustrated in Figure 1-5,
`requires constant attention. The flask must be swirled frequently and removed from the
`heating stage for a short period if the boiling becomes too vigorous. When heating is
`in progress, the reflux ring should not rise above the base of the flask's neck.
`In Figure 1-5, still another technique for heating small amounts of solvent
`under reflux is illustrated. A cold-finger condenser is inserted into a test tube or a
`small flask. As the vapors rise, they touch the cold surface of the condenser. The
`vapors are condensed, and the resulting liquid drips back to the bottom of the container.
`Some commercial cold-finger condensers are designed to rest on top of the test tube or
`flask. If pressure builds up in the container, it is released when the condenser is lifted
`slightly. With cold-finger condensers that fit through cork stoppers, such as the one in
`the illustration, it is necessary to provide a slot in the stopper to prevent pressure from
`building within the container. Without the slotted stopper, one would be heating a
`closed system, creating a potential "bomb"!
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`Page 7
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`Technique 1: Solvents and Methods of Heating Reaction Mixtures
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`511
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`cold finger
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`FIGURE 1-5. Tended reflux of small quantities
`
`1.8 BOILING STONES
`A boiling stone, also known as a boiling chip or Boileezer, is a small lump of porous
`material that produces a steady stream of fine air bubbles when it is heated in a solvent.
`This stream of bubbles, and the turbulence that accompanies it, breaks up large bubbles
`of gases in the liquid. It also reduces the tendency of the liquid to become superheated,
`and it promotes the smooth boiling of the liquid. If the liquid becomes superheated,
`very large bubbles may erupt violently from the solution; this is called bumping. The
`boiling stone, by its action, decreases the chances for bumping.
`Boiling stones are generally made from pieces of pumice, carborundum, or
`marble. Wooden applicator sticks are also used.
`Because boiling stones act to promote the smooth boiling of liquids, one should
`always make certain that the boiling stone has been placed in the liquid before the
`heating is begun. If one waits until the liquid is hot, it may have become superheated,
`at which time a boiling stone would cause all the liquid to try to boil at once. The
`liquid, as a result, would erupt entirely out of the flask, or at least froth violently. As
`soon as boiling ceases, the liquid is drawn into the pores of the boiling stone. When this
`happens, the boiling stone no longer can produce a fine stream of bubbles; it is spent. A
`new boiling stone must be added each time the boiling stops.
`Magnetic stirrers provide much the same action as boiling stones, since they
`produce turbulence in the solution. The turbulence breaks up the large bubbles that
`form in hot solutions. A magnetic stirring system consists of a magnet that is rotated by
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`512
`
`The Techniques
`
`an electric motor. The rate at which this magnet rotates can be adjusted by a potentiom(cid:173)
`eter. One places a small bar magnet, which is coated with some nonreactive material
`such as Teflon or glass, into the flask. The magnet within the flask rotates in response
`to the rotating magnetic field caused by the motor-driven magnet. The result is that the
`inner bar magnet stirs the solution as it rotates. As previously mentioned, some avail(cid:173)
`able hot plates incorporate a magnetic stirring motor, so that heating and stirring can be
`done simultaneously.
`
`1. 9 EVAPORATION TO DRYNESS
`Often one wants to evaporate a solution to dryness or to concentrate a solution by
`removing the solvent either completely or by a particular amount. This can be done by
`evaporating the solvent from an open Erlenmeyer flask. Such an evaporation must be
`conducted in a hood, since many solvent vapors are toxic or flammable. A boiling
`stone must be used. A gentle stream of air directed toward the surface of the liquid will
`remove vapors that are in equilibrium with the solution and accelerate the evaporation.
`An eyedropper tube or capillary pipet, connected by a short piece of rubber tubing to
`the compressed air line, will act as a convenient air nozzle. A tube or an inverted funnel
`connected to an aspirator may also be used. In this case, vapors are removed by
`suction. Both methods are illustrated in Figure 1-6. It is better to use an Erlenmeyer
`flask than a beaker for this procedure, since deposits of solid will usually build up on
`the sides of the beaker where the solvent evaporates. The refluxing action in an Erlen(cid:173)
`meyer flask does not allow this build-up.
`It is also possible to remove low-boiling solvents under reduced pressure. In
`this method, the solution is placed in a filter flask, along with a wooden applicator
`
`I
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`FIGURE 1- 6. Rapid evaporation of a solvent
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`Technique 1: Solvents and Methods of Heating Reaction Mixtures
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`513
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`FIGURE 1-7. Reduced-pressure solvent evaporation
`
`stick. The flask is stoppered, and the sidearm is connected to an aspirator (by a trap), as
`described in Technique 2, Section 2.3, p. 518. Under reduced pressure, the solvent
`begins to boil. The wooden stick serves the same function as a boiling stone. By this
`method, solvent can be evaporated from a solution without heat. This technique, illus(cid:173)
`trated in Figure 1-7, is often used when heating the solution might decompose ther(cid:173)
`mally sensitive substances. The method has the disadvantage that when low-boiling
`solvents are used, solvent evaporation cools the flask below the freezing point of water.
`When this happens, a layer of frost forms on the outside of the flask. Since frost is
`insulating, it must be removed to keep evaporation proceeding at a reasonable rate . The
`solution must take heat from the surrounding air to evaporate. The frost prevents this
`necessary heat transfer. Frost is best removed by one of two methods: either the flask is
`placed in a bath of warm water (with constant swirling) or it is heated on the steam bath
`(again with swirling). Either method promotes efficient heat transfer.
`Large amounts of a solvent should be removed by distillation (see Technique 6).
`ONE SHOULD NEVER EVAPORATE ETHER SOLUTIONS TO DRYNESS, except
`on a steam bath or by the reduced-pressure method. The tendency of ether to form
`explosive peroxides is a serious potential hazard . If peroxides should be present, the
`large and rapid temperature increase in the flask once the ether evaporates could bring
`about the detonation of any residual peroxides. The temperature of a steam bath is not
`high enough to cause such a detonation.
`
`1.10 COLD BATHS
`At times, one may need a medium in which a flask or some other piece of a~paratus is
`to be cooled rapidly to a temperature below room temperature. A cold bath 1s used for
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`514
`
`The Techniques
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`this purpose. The most common cold bath is an ice bath. An ice bath is a highly
`convenient source of 0 °C temperatures. The ice bath should actually be called an
`ice-water bath, since it requires water to work well . An ice bath made up of nothing but
`ice is not very effective, because the large pieces of ice do not make good contact with
`the outside walls of the vessel immersed in the bath. Some liquid water must be added
`to the ice to create an efficient cooling medium. There must be enough water to ensure
`that the flask being cooled is totally surrounded by water but not so much that the
`amount of ice present is no longer enough to keep a temperature of 0 °C . If too much
`water is added to the ice bath, the buoyancy of a flask resting in the ice bath may cause
`it to tip over. There should be enough ice in the bath to permit the flask to rest firmly .
`When temperatures lower than 0 °C are desired, one may add some solid so(cid:173)
`dium chloride to the ice-water mixture. The ionic salt lowers the freezing point of the
`ice, so that temperatures in the range Oto -10 °C can be reached. The lowest tempera(cid:173)
`tures are reached with ice-water mixtures that contain relatively little water.
`Very low temperatures can be secured with solid carbon dioxide, or Dry Ice,
`whose temperature is -78 .5 °C. The large chunks of Dry Ice do not provide uniform
`contact with a flask being cooled. Liquid is required , along with the Dry Ice, to provide
`uniform contact with the flask and efficient cooling. The liquid used most often with
`Dry Ice is isopropyl alcohol, although acetone or ethanol can also be used. One should
`be cautious when handling Dry Ice or cooling baths made from it, since Dry Ice can
`inflict very severe frostbite.
`Extremely low temperatures can be obtained with liquid nitrogen (-195.8 °C).
`It is not used in organic chemistry as often as Dry Ice.
`
`Technique 2
`Filtration
`
`Filtration is a technique used for two main purposes . The first is to remove solid
`impurities from a liquid or a solution . The second is to collect a solid product from the
`solution from which it was precipitated or crystallized. Two different kinds of filtration
`are in general use: gravity filtration and vacuum (or suction) filtration.
`
`2.1 GRAVITY FILTRATION
`
`I
`
`The most familiar filtration technique is probably filtration of a solution through a
`paper filt_er_ held in a funnel , allowing gravity to draw the liquid through the paper. In
`general, It IS best to use a short-stemmed or a wide-stemmed funnel. In these types of
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`Technique 2: Filtration
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`515
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`funnels there is less likelihood that the stem of the funnel will become clogged by solid
`material accumulated in the stem. This clogging is a particular problem if a hot solution
`saturated with a dissolved solid is being filtered. If the hot saturated solution comes in
`contact with a relatively cold funnel (or a cold flask, for that matter), the solution is
`cooled. The rapidly cooled solution will be supersaturated, and crystallization will
`begin. Crystals will form in the filter, and either they will fail to pass through the filter
`paper or they will clog the stem of the funnel.
`Four other measures are feasible for preventing clogging of the filter. The first
`is to keep the solution to be filtered at or near its boiling point at all times. The second
`measure is to preheat the funnel by pouring hot solvent through it before the actual
`filtration. This keeps the cold glass from causing instantaneous crystallization. The
`third way is to keep the filtrate (filtered solution) in the receiver hot enough to continue
`boiling slightly (by setting it on a steam bath, for example) . The refluxing solvent heats
`the receiving flask and the funnel stem and washes them clean of solids. This boiling of
`the filtrate also keeps the liquid in the funnel warm. And fourth, it is useful to acceler(cid:173)
`ate filtration by using fluted filter paper, as described below. A gravity filtration is
`shown in Figure 2-1.
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`jT\
`'\
`.
`fl)
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`FIGURE 2- 1. Gravity filtration
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`516
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`The Techniques
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`FIGURE 2-2. Folding a filter cone
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`A. Filter Cones
`
`The simplest way to prepare filter paper for gravity filtration is to prepare a filter cone,
`as indicated in Figure 2-2. The filter cone is particularly useful when the solid material
`being filtered from a mixture is to be collected and used later. The filter cone, because
`of its smooth sides , can easily be scraped free of collected solids. Because of the many
`folds, fluted filter paper, described in the next section, cannot be scraped easily .
`With filtrations using a simple filter cone, solvent may form seals between the
`filter and the funnel and between the funnel and the lip of the flask. When a seal forms,
`the filtration stops because the displaced air has no possibility of escaping. To avoid the
`solvent seal, you can insert a small piece of paper or a paper clip or other bent wire
`between the funnel and the lip of the flask to let the displaced air escape. Alternatively,
`you can support the funnel by a ring clamp fixed above the flask, rather than by placing
`it in the neck of the flask.
`
`B. Fluted Filters
`
`The technique for folding a fluted filter paper is shown in Figure 2-3. The fluted filter
`increases the speed of filtration in two ways. First, it increases the surface area of the
`filter paper through which the solvent seeps; second, it allows air to enter the flask
`along its sides to permit rapid pressure equalization. If pressure builds up in the flask
`from hot vapors, filtering slows down. This problem is especially pronounced with
`filter cones. The fluted filter tends to reduce this problem considerably, but it may be a
`good idea to use a piece of paper, paper clip, or wire between the funnel and the lip of
`the flask as an added precaution against solvent seals . Fluted filters are used when the
`desired material is expected to remain in solution. These filters are used to remove
`undesired solid materials, such as dirt particles, activated charcoal , and undissolved
`impure crystals.
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`Technique 2: Filtration
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`517
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`2
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`3
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`4
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`5
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`6
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`7
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`9
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`FIGURE 2-3. Folding a fluted filter paper, or origami at work in the organic lab
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`518
`
`The Techniques
`
`TABLE 2-1. Some Common Qualitative Filter Paper Types
`and Approximate Relative Speeds and Retentivities
`
`Fine
`
`High
`
`Slow
`
`0
`
`·;;; e 0
`
`0..
`
`TYPE (by number)
`
`SPEED
`
`E&D
`
`S&S Whatman
`
`Very slow
`Slow
`Medium
`Fast
`Very fast
`
`610
`613
`615
`617
`. . .
`
`576
`602
`597
`595
`604
`
`5
`3
`2
`1
`4
`
`Coarse
`
`Low
`
`Fast
`
`2.2 FILTER PAPER
`
`Many kinds and grades of filter paper are available. Paper is generally available in fine,
`medium, and coarse porosities. Fine-porosity paper will catch very fine solid particles
`but generally gives very slow filtration. Coarse paper increases the rate of filtration but
`may not catch all the particles. The paper must be correct for a given application. In
`choosing, one should be aware of the various properties of filter paper. Porosity is a
`measure of the size of the particles the paper allows through. Highly porous paper does
`not remove small particles from solution; paper with low porosity removes very small
`particles. Retentivity is a property that is the opposite of porosity. Paper with low
`retentivity does not remove small particles from the filtrate. The speed of filter paper is
`a measure of the time it takes a liquid to drain through the filter. Fast paper allows the
`liquid to drain quickly; with slow paper, it takes much longer to complete the filtration.
`Since all these properties are related, fast filter paper usually has a low retentivity and
`high porosity, and slow filter paper usually has high retentivity and low porosity.
`Table 2-1 compares some commonly available qualitative filter paper types and
`ranks them according to porosity, retentivity, and speed. Eaton-Dikeman (E&D),
`Schleicher and Schuell (S&S), and Whatman are the most common brands of filter
`paper. The numbers in the table refer to the grades of paper used by each company.
`
`2.3 VACUUM FILTRATION
`
`Vacuum, or suction, filtration is more rapid than gravity filtration, but without spe(cid:173)
`cially prepared filter media it does not catch fine particles without clogging the paper
`pores. In this technique, a receiver flask with a sidearm, a filter flask, is used. The
`sidearm is connected by heavy-walled rubber tubing to a source of vacuum. Thin(cid:173)
`walled tubing will collapse under vacuum, due to atmospheric pressure on its outside
`walls, and will seal the vacuum source from the flask. A Buchner funnel (see Figure
`2-4) is sealed to the filter flask by a rubber stopper or a rubber gasket (Neoprene
`adapter) cone. The flat bottom of the Buchner funnel is covered with an unfolded piece
`of circular filter paper, which is held in place by suction. To keep the unfiltered mixture
`from passing around the edges of the filter paper and contaminating the filtrate in the
`
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`Technique 2: Filtration
`
`519
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`pressure tubing
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`Hirsch funnel
`
`FIGURE 2-4. Vacuum filtration
`
`flask below, it is advisable to moisten the paper with a small amount of solvent before
`beginning the filtration. The moistened filter paper adheres more strongly to the bottom
`of the Buchner funnel. Since the filter flask is attached to a source of vacuum, a
`solution poured into the Buchner funnel is literally "sucked" rapidly through the filter
`paper. To prevent the escape of solid materials from the Buchner funnel, one must be
`certain that the filter paper fits the Buchner funnel exactly. The paper must be neither
`too big nor too small. It must cover all the holes in the bottom of the funnel but not
`extend up the sides of the funnel.
`Two types of funnel are useful for vacuum filtration. The Buchner funnel,
`which has already been considered, is used for filtering a large amount of crystals from
`solution. The Hirsch funnel, which is also shown in Figure 2-4, operates on the same
`principle as the Buchner funnel , except that it is smaller and its sides are sloped rather
`than vertical. The Hirsch funnel is used for isolating smaller quantities of solid materi(cid:173)
`als from a solution (smaller sizes of the Buchner funnel are also available for this
`purpose). In the Hirsch funnel also, the filter paper must cover all the holes in the
`bottom but must not extend up the sides.
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`520
`
`The Techniques
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`2.4 FILTER AID
`It was mentioned that specially prepared filter beds are needed to separate fine particles
`when using vacuum filtration. Often, fine particles either pass right through a paper
`filter or they clog it so completely that the filtering stops. This is avoided by using a
`substance called Filter Aid, or Celite. This material is also called diatomaceous earth,
`because of its source. It is a finely divided inert material derived from the microscopic
`shells of dead diatoms (a type of phytoplankton that grows in the sea).
`
`WARNING: LUNG IRRITANT
`When using Filter Aid, take care not to
`breathe the dust.
`
`Filter Aid will not clog the fiber pores of filter paper. It is slurried, or mixed with a
`solvent to form a rather thin paste, and filtered through a Buchner funnel (with filter
`paper in place) until a layer of diatoms about 3 mm thick is formed on top of the filter
`paper. The solvent in which the diatoms were slurried is poured from the filter flask,
`and if necessary, the filter flask is cleaned before the filtration is begun. Finely divided
`particles can now be suction-filtered through this layer and will be caught in the Filter
`Aid. This technique is used for removing impurities and not for collecting a product.
`The filtrate (filtered solution) is the desired material in this procedure. If the material
`caught in the filter was the desired material, one would have to try to separate the
`product from all those diatoms! Filtration with Filter Aid is not appropriate when the
`desired substance is likely to precipitate or crystallize from solution.
`
`2.5 THE ASPIRATOR
`The most common source of vacuum (approximately 10-20 mrnHg) in the laboratory
`is the water aspirator, or "water pump," illustrated in Figure 2-5. This device passes
`water rapidly past a small hole to which a sidearm is attached. The Bernoulli effect
`causes a reduced pressure along the side of the rapidly moving water stream and creates
`a partial vacuum in the sidearm.
`A water aspirator can never lower the pressure beyond the vapor pressure of the
`water used to create the vacuum. Hence, there is a lower limit to the pressure (on cold
`days) of 9 to 10 mrnHg. A water aspirator does not provide as high a vacuum in the
`summer as in the winter, due to this water-temperature effect.
`A trap must be used with an aspirator. A trap is illustrated in Figure 2-4. If the
`water pressure in the laboratory line drops suddenly, the pressure in the filter flask may
`suddenly become lower than the pressure in the water aspirator. This would cause
`water to be drawn from the aspirator stream into the filter flask and would contaminate
`the filtrate. The trap stops this reverse flow. A similar flow will occur if the water flow
`at the aspirator is stopped before the tubing connected to the aspirator sidearm is
`disconnected. ALWAYS DISCONNECT THE TUBING BEFORE STOPPI~G THE
`ASPIRATOR. If a "back-up" begins, disconnect the tubing as rapidly as possible
`
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`

`Technique 2: Filtration
`
`521
`
`FIGURE 2-5. Aspirator
`
`before the trap fills with water. Some workers like to fit a stopcock into the stopper on
`top of the trap. A three-hole stopper is required for this purpose. With a stopcock in the
`trap, the system can be vented before the aspirator is shut off. If the system is vented
`before the water is shut off, water cannot back up into the trap.
`Aspirators do not work well if too many people use the water line at the same
`time, since the water pressure is lowered. Also, the sinks at the ends of the lab benches
`or the lines that carry away the water flow may have a limited capacity for draining the
`resultant water flow from many aspirators. Care must be taken to avoid floods.
`
`2.6 CRUDE FILTRATIONS
`Often one wants to make a very rapid filtration to remove dirt or impurities of large
`particle size from a solution. This is accomplished most easily by laying a loose mat of
`glass wool in the bottom of an ordinary funnel and pouring the solution through the
`mat. It may be helpful to decant, or pour off, the clear liquid gently before performing
`this crude filtration on the solid residue at the bottom of the flask.
`Small amounts of solution can be filtered in a similar manner. A plug of glass
`wool is packed loosely into an eyedropper pipet, and the solution is dropped into the
`packed pipet from a second pipet.
`
`Page 18
`
`

`

`Technique 3
`Crystallization: Purification of
`Solids
`
`f'
`
`Organic compounds that are solid at room temperature are usually purified by crystalli(cid:173)
`zation. The general technique involves dissolving the material to be crystallized in a
`hot solvent (or solvent mixture) and cooling the solution slowly. The dissolved mate(cid:173)
`rial has a decreased solubility at lower temperatures and will precipitate from the
`solution as it is cooled. This phenomenon is called crystallization if the crystal growth
`is relatively slow and selective and precipitation if the process is rapid and nonselec(cid:173)
`tive. Crystallization is an equilibrium process and produces very pure material. A small
`seed crystal is formed initially, and it then grows layer by layer in a reversible manner.
`In a sense, the crystal "selects" the correct molecules from the solution. In precipita(cid:173)
`tion, the crystal lattice is formed so rapidly that impurities are trapped within the
`lattice. Therefore, in any attempt at purification, too rapid a process should be avoided.
`Too slow a process should also be avoided. The time scale for crystal formation should
`cover tens of minutes or hours, rather than seconds or days. The two principal mistakes
`that can be made are (1) cooling the solution too rapidly and (2) suddenly adding an
`"incompatible" solvent to the solution. Both of these mistakes will be considered in
`this technique.
`
`3.1 SOLUBILITY
`
`The first problem in performing a crystallization is selecting a solvent in which the
`material to be crystallized shows the desired solubility behavior. Ideally, the material
`should be sparingly soluble at room temperature and yet quite soluble at the boiling
`point of the solvent selected. The solubility curve should be steep, as can be seen in line
`A of Figure 3-1. A curve with a low slope (line B, Figure 3-1) would not cause
`
`C. (poor solvent)
`-very soluble at
`all temperatures
`
`v-A. (good solvent)
`-very soluble at elevated tem peratures
`-sparing ly so luble at room temperature
`
`_ j~ - - - - - - - - - -8 . (poor solvent)
`
`Q)
`..0
`::,
`0
`<fl
`<fl
`E
`....
`(I)
`
`CJ)
`
`I
`
`temperature ----___,
`FIGURE 3- 1. Graph of solubility versus temperature
`522
`
`Page 19
`
`

`

`Technique 3: Crystallization: Purification of Solids
`
`523
`
`TABLE 3-1. Solvents, in Decreasing Order of
`Polarity
`
`H 20
`RCOOH
`RCONH2
`ROH
`RNH 2
`RCOR
`RCOOR
`RX
`ROR
`ArH
`RH
`
`Water
`Organic acids (acetic acid)
`Amides (N, N-dimethylformamide)
`Alcohols (methanol , ethanol)
`Amines (triethylamine, pyridine)
`Aldehydes, ketones (acetone)
`Esters (ethyl acetate)
`Halides (CH2Cl 2 > CHC1 3 > CC14)
`Ethers (diethyl ether)
`Aromatics (benzene, toluene)
`Alkanes (hexane , petroleum ether)
`
`significant crystallization when the temperature of the solution was lowered . A solvent
`in which the material was very soluble at all temperatures (line C, Figure 3-1) would
`not be a suitable crystallization solvent. The basic problem in performing a crystalliza(cid:173)
`tion is to select a solvent (or mixed solvent) that will provide a steep solubility-versus(cid:173)
`temperature curve for the material to be crystallized; that is , a solvent that allows the
`behavior shown in line A is an ideal crystallization solvent.
`The solubility o