Microscale Organic Laboratory II: The Benefits Derived from Conversion to the Program and Representative Experiments Dana W. Mayo, Samuel S. Butcher, Ronald M. Pike1, Caroline M. Foote, Janet R. Hotham, and David S. Page Bowdoin College, Brunswick, ME 04011
Part I nresented a nreliminarv studv of the emissions from selected processes and the mixing of these emissions within the lahoratory atmosphere ( I ) . This paper describes the reduction to practice of microscale lahoratory procedures and outlines benefits derived from this anoroach to im~rovine lahoratory air quality. In order to illus&te the style a i d scaG of the laboratow.ex~eriments which mav he incoroorated into . such a program two representative examples are given in detail in the discussion. We are currently in the process of developing and testing a wide variety of experiments a t the microscale level. These exercises will eventually provide the core of a lahoratory text. We are also in the process of evaluating the reduction of harmful emissions from these microexperiments (I). The results of this latter work and an illustrated discussion of the microtechniques involved will be incorporated in an instructor's manual to accompany the new student text. Mlcroscale Organic Laboratory Experiments Two reaction sequences which have been reduced to practice in our lahoratory program are described. These experiments represent a variety of introductory transformations ranging from the classical to the rarely performed. In all cases the quantity of starting material does not exceed 150 mg. The compelling arguments for undertaking the microapproach are as follows: (1) reduction of the scale of starting material by 100-fold (nearly 1MX)-foldin certain cases) leads to a parallel reduction in organic soluents required for these experiments. This contraction from the usual level (50-500 g) of solvent/student/experiment reduces the demands on the
Presented at the 187th National ACS Meeting. St. Louis, MO, April 1984. Permanent address: Merrimack College. North Andover, MA 01845.
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lahoratory ventilation system (1); (2) regardless of the quality of a lahoratory atmosphere, microscale reactions simply reduce student contact with toxic materials of all types; (3) the threat to lahoratory safety from explosion or fire is largely eliminated; (4) the cost of chemicals parallels the 100- to 1000-fold reduction in use; (5) conversely, the very large reduction in chemical costs Dresents an onnortunitv to exoand the available variety of experiments. u'dtil the introduction of the microscale reactions. elimination of ex~erimentsbased on cost has been a recurring theme over the past few years in this lahoratory, and it appears that the complete elimination of the undergraduate introductory organic laboratory at many institutions is alikely response to advancing budgets; (6) the option of being able locally to synthesize adequate supplies of reagents and starting materials, not commercially available, for use in a particu~arkxperiment,is an additional bonus of the microapproach. If a relatively simple synthesis of the materials is available in the literature, a multiple yearsupply can usually he obtained from a single synthetic preparation (see experiment B: Preparation of an Aromatic Nitrile); (7) in our experience microglassware (Ace Glass, Inc.) (2) is considerably more durable than macro- or semimicro-ground glass equipment. This stability translates into lower lahoratory breakage costs and. thus. is welcome news to students. whose costs f& organic chemistry are already very high. It shbuld he em~hasizedthat. while commerciallv available microelassware kits (2) are attradive for programs undertaking full conversion to this scale of experimentation, manv of the microscale experiments can becarried out utili~ingbrdinar~ organic laboratory equipment. For exam~le,the Cannizzaro reaction descrihid l&w can t,c run q ~ ~ jsatisfactorily te employing two 10 X 75-mm test tuhes and one 12-mL crntrifugr tul)e plus a Pastrur piprt (whirh ran function hoth as a separator). funnel and as R means ro transfer solutions):and finally, (8)we perceive a signifirant pedagogic advantage in this latr~rarory concept at the sophumore level. For example, ihe close relationship of the techniques required to those of analytical chemistry reinforces parallel laboratory programs at this stage
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in the student's development. Related to this point is our observation that students quickly recognize that small accidental losses of inaterial cannot b e tol&ated when working with millieram quantities. The fol~owing~are two experiments currently being carried out hv a \dunteer section in our sophomore laboratory program: Cannizzaro Reaction The Cannizzaro reaction of p-chlorohenzaldehyde, (I), represents an interesting example of a classical oxidationreduction reaction not often carried out in the introductory organic laboratory program. The procedure represents a particularly difficult experimental challenge as the quantity of startine aldehvde has been reduced 100-fold from the conventional ( 3 )quantity and ultimawlg becomes divided into two products which are hoth isolated and characterized.
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Cannizzaro Reaction: Student Preparation of p-Chlorobenzyl Alcohol (M . . and 0-Chlorobenzoic Acid (Ill) h e e d w e : In a 1-mL vial is placed p-chlorohenzaldehyde (I) (150 mg, 1.07 mmol) followed by methanol (0.4 mL). In a 5-mL microreaction cone vial is dissolved potassium hydroxide (0.24 g, 4.3 mmol) in distilled water (0.4 mL). This alkaline solution is then cooled to room temperature. The aldehyde solution is transferred by Pasteur pipet to the gently swirled reaction vial containing the base. A micro-fefluxcondenser is attaehed to the flask and the reactants are maintained at 55-65' for 1h by use of a hot water bath. After cwling, 2 mL of distilled water are added and the aqueous methanolic solution extracted with methylene chloride (three 0.5-mL portions). [Note: The alkaline phase is saved for further workup.] The methylene chloride layers are transferred by Pasteur filter pipet (PFP12to a 3-mL vial. These combined extracts are wsshed with saturated sodium bicarbonate solution (two 0.25-mL portions). The aqueous upper phase is removed and discarded (a PFP is used in this operation). The methylene chloride solution is dried over granular anhydrous sodium sulfate (150 mg). The dried solution is then transferred by a PFP to a tared 5-mL micro-reaction cone vial. The sodium sulfate drying agent is rinsed with fresh methylene chloride (0.3 mL) and the wash combined with the dried organic phase. The solvent is then evaporated using a stream of dry nitrogen gas under the hwd to yield the crude alcohol product, 11. After weighing, this crude product is recrystallized from a solution of 4% acetone and hexane (-0.25 mL). Collection of recrystallized I1 by use of a Hirsch filter (11.5-mm plate diameter) followed by washing with heaane (0.2 mL) yields p-chlorobenzyl alcohol, (II),m.p. 68-69O (lit. value 75') (4). The alkaline aqueous phase (2.8mL) remaining from the original extraction procedure is acidified with concentrated hydrochloric acid (0.4 mL). The voluminous white precipitate is collected under reduced pressure on the Hirsch filter and rinsed with distilled water (2 mL). Air drying yields para-chlorohenzoie acid, (Ill), m.p. 237-239' (lit. value 243") (5).The product may he recrystallized from methanol. Characterization of the reaction oroducts. 11and 111,is carried out I,y rumparison of rheir IR spectra wirh n reference stunddrd. Typical prduct yreld.i for the alcohol, 11, are in the rangc of 4R mg (63T). For the acid, Ill, typical yicldr oround 62 rng t;S%) nrr uhtained
micro-scale experiments. First. the Pasteur fdter ninet is used extensively fo; the transfer of small quantities ofii6uids. The PFP is an essential means of successfully carrying out manipulations of small quantities of liquids required in the reactions undertaken a t this scale and its use should be introduced early in the program. Second, the sepnrntion of acidic from neutral products is effectively demonstrnted. Third, the technique of recrystallization is demonstrated by purification of the alcohol.
Preparation of an Aromatic Nltrile This is a conversion usually requiring the use of an aromatic amine (0-tohidine. 11 e ) . dinzotizarion ICUSOI~HIO. e: . - . 31 -. N~NO;, 7.3 g) and cyan;de (NaCN, 16 g)in a Sandmeyer reaction (6). We start with an aldehyde, (for example, piperonal, (IV), a known flavoring material, 30 mg) which is converted to an 0-phenylated oxime, (V). This crystalline intermediate is isolated, characterized, and then treated with alcoholic base to yield the nitrile, (VI) (10 mg) by an elimination reaction. This approach is based on the work of Miller and Loudon (7).
v
V Z ( X vi The reagent 0-(2,4-dinitrophenyl)hydroxylamine,(X), is not commercially availahle, but the synthesis from ethyl Nhydroxyacetimidate (VIII) is s t r a i g h t f o m d (8,9). A slightly modified and reliable route is included below: NO.
NO.
IX
ix
NO,
-+
o a N d N H s
X
The above experiment involves the utilization of a number of techniques which will be useful to the student carrying out
Preparation of Nitrile Reagent: Instructor Preparation of O(2.4dinitmphenyl)-hydroxylamine, (8 This preparation illustrates the synthesis of a reagent used in the reaction sequence that is not commercially available. On the scale described below, the amount of product I X ohtained would be sufficient for a ~ ~ r o x i m a t e150 l v students. Since the intermediate M stores &ily for long periods of time sufficient material for several vears can be readilv produced in one preparation.
The Pasteur filterpipet is avery useful means of efficiently transferring small quantities of solution. These pipets are easily assembled from standard Pasteur glass pipets (available from Fisher ScientificCo.) by shortening the capillary tip (from 125 to 50 mm) and inserting a prewashed cotton plug (3-5 mm) into the tip end.
Procedure: Ethyl 0-(2,4-dinitrophenyl)acetohydroxamate, (IX). In a 250-mL three-necked round-bottomed flask fitted with a condenser protected by a calcium chloride drying tube, magnetic stirrer and addition funnel is placed ethyl N-hydroxyacetimidate, (VIII), (3.1g, 0.03 mol) dissolved in absolute ethanol (40 mL). Potassium hydroxide (pellets,1.7 g, 0.03 moll is suspended in this solution. A solution of 2,4-dinitrochlorohenzene,(VII),(6.1g, 0.03 mol)
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in warm absolute ethanol (15 mL) is added dropwise (20 min) with stirring while maintaining the reaction mixture at %lo0 by aid of an ice bath. The solution rapidly becomes homogeneous followed by precipitation of the light yellow hydroxamate. After stirring for an additional 45 at the mixture is filtered under reduced pres. sure to yield the product, IX. ~ ~ ~ ~ ofthis ~ ~material t ~froml l absolute ethanol gives ethyl O-(2,4-dinitrophenyl)acetohydro~~mate, (1x1, as light yellow, flat plates, m.p. 111-112° (lit. value 111-112°) 191 ,"I.
stored in the freezer under nitrogen this material is stable for a long period of time. Typical crude yields average 6.1 g (83%)and recrystallized yields are in the range 6.0 g (75%).
Procedure: O-(2,4-dinitrophenyl)hydroxylamine, (X). In a 50-mL three-necked round-bottomed flask fitted with a magnetic stirrer, addition funnel and condenser is placed IX (2.9 g, 0.01 mol). Dissolution of IX is then accomplished by dropwise addition of lO%perehloricacid (10 mL) at 0' (icebath) with stirring. After the addition of the acid the reaction mixture is held at 0' for an additional 60 min. The temperature of the solution is then allowed to rise to ambient values and stirring continued for an additional 24 h. Filtration of the precipitate under reduced pressure yields crude X. Recrystallization from hot ethanol gives O-(2,4-dinitropheny1)hydroxylamine, (X), m.p. 111-112' (lit. value 111-112°) (7,9). Typical crude yields average 1.8 g (89%).Recrystallized yields range in the vicinity of 1.6 g (80%). Aromatic Nitrile: Student Preparation of Piperonylonitrile (VI). In a 5-mL micro-reaction flask fitted with a reflux condenser and drying tube, is placed O-(2,4-dinitrophenyl)hydroxylamine,(X), (40 mg, 0.20 mmol). Dissolution of X occurs on the addition of absolute ethanol (3 mL) with gentle warming. The condenser is removed and piperonal, (IV), (30 mg, 0.02 mmol) is added to the reaction mixture. Following dissolution of the aldehyde, 12M HCI (twodrops) is added through the reflux condenser (Pasteur pipet). Complete precipitation of the oxime, (V),is accomplished by swirling the reaction flask in an icehath. Collection of the precipitate by use of a Hirsch filter (11.5-mm plate diameter), fallowed by washing with cold absolute ethanol (0.5 mL, saue filtrate) and air drying gives piperonal O-(2,4-dinitrophenyl)oaime, (V), (lit. value, m.p. 19P195O) (7). Refrigeration of the filtrate for at least 24 h produces a second crop of mime crystals. This second crop, collected by the same technique, may he combined with the initial product if its m.p. is above 180". Overall oaime yields in excess of 90%are generally obtained. Characterization of the oaime, (V), is carried out through comparison of its infrared spectrum to a reference standard. Provided there is a close spectral match and the m.p. is in the range (>181°), proceed to the second step in the reaction sequence (a minimum of 50 mg or a 75%yield of V is necessary to continue). In a 10-mL micro-reaction flask fitted with reflux condenser is placed the oxime, V, (50 mg, 0.15 mmol) suspended in 95% ethanol (5 mL). Ethanolic KOH (0.2 N, 2 mL) is then added. The reaction mixture is slowlv heated to reflux temoerature hv use of a hot water bath. During this warming period thesolution ripidly turns a deep yellow. The reaction mixture is maintained at mild reflux far a period of 1h. Fallowing the reflux period, the condenser is removed, and the reaction mixture concentrated to approximately a 0.5-mL volume employing a grnrlc stream 0 1 nitn,g,en Ens. An rxtraction sdution is prepared hy diluung 5T %OH 11.5mL, with distilled water 1:.5ml.). This bmic ,dutwn (three 3-rill. portions) i.4 used to lransfer theahwe reaction residue (0.5 mL) to a i 2 - m ~capped centrifuge tube. This suspension is extracted with methylene chloride (four2-mL portions). The organic fractions are comhined and dried over granular anhydrous sodium sulfate (-70 mg). By use of a PFP the dried solution is transferred into a 10-mL miera-reaction flask. The drying agent is rinsed with methylene chloride (two 1-mL portions) and these rinsings combined with those from the initial extraction. The solvent is eeaporated On the hnudl under a strewn of V 1 car to obtain the rrudr piptrunylonitrilr. ( V I I Thisreridue isdissolved in mrrhylenr chloridc/hrxanp 1:l 1-0.25 niL) Th~solutioni%.wdied .. tothr head af a micro-chromatographic column (neutral alumina, activity 1,0.3 g). The column is eluted with the CHzCIzhexane (Ll)solvent mixture (-1.0 mL). On evaporation under a stream of Nz gas the single call a d fraction yields the desired piperonylonitrile,(VI), as white, thin needles, m.p. 92-93O (lit. value 92-93°) (7). Characterization of the product is carried out by comparison of its infrared spectrum with a reference standard and a mixed m.0. determination with a refrrrnce ilanclard. Typical crude b irlds oiv1 are in the range 01 16 mr.
