Lab 1 - Melting Point & IR

Lab 1 - Melting Point & IR - CER Modular 1 Laboratory...

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Unformatted text preview: CER Modular 1 Laboratory TECH Program - in Chemistry PURPOSE OF THE EXPERIMENT BACKGROUND REQUIRED BACKGROUND INFORMATION 15° liquid X+ liqutd Y 145 liquid + solid Y mass‘laxwo BO 60 4U 2O 0 mass%Y 0 20 40 60 30 10 temperature. WC 130 701 editor: Joe Jellerr. Measuring the Melting Points of Compounds and Mixtures prepared by Joseph W. LeFevre, SUNY, Oswego Measure the melting points of pure benzoic acid and pure mandelic acid. Determine the eutectic composition and the eutectic temperature of benzoic acicl—mandelic acid mixtures. Identify an unknown compound using mixture melting points. N one The melting point of a compound is the temperature at which the solid is in equilibrium with its liquid. A solid compound changes to a liquid when the molecules acquire enough energy to overcome the forces hold- ing them together in an orderly crystalline lattice. For most organic com. pounds, these intermolecular forces are relatively weak. The. melting point range is defined as the span of temperature from the point at which the crystals first begin to liquefy to the point at which the entire sample is liquid. Most pure organic compounds melt over a narrow temperature range of 1—2 °C. The presence of a soluble impurity almost always ca uses a decrease in the melting point expected for the pure compound and a broadening of the melting point range. in order to understand the effects of impuri- ties on melting point behavior, consider the melting point—mass percent composition diagram for two different fictitious organic compounds, X and Y, shown in Figure l. The vertical axis represents temperature and the horizontal axis represents varying mass percent Compositions of X and Y. Both compounds have sharp melting points. Compound X melts at 150 “C, as shown on the left vertical axis, and Y melts at 148 "C, as shown on the right vertical axis. As compound X is added to pure Y, the melt- Figuret Melting point—mass percent . . . . . . .. . t 0 tot txtu’ ‘J' i r- - composmon diagram tor a MO‘ tng p In t he in“ ‘ rt. decreascs alt ng cum I; CB until a nuntmum tent » .erature of 130 L is reached. t-mnt B corres ends to 40 mass ercent X component mixture P _ and 60 mass percent Y and is called the eutectic composition tor com- pounds X and ‘1’. Item. both solid X and solid Y are in equilibrium with the liquid. The eutectic temperature of [30 “C is the lowest possible melting point for a mixture of X and Y. At temperatures below 130 "'C. mixtures of X and Y exist together only in solid form. than; i rii.:.p.:r|n,..mn“ m1 UtfiHt'. Ul-HIR‘Jl-I'l Nuflflfll'lllhlhwlilL reveredhvtilersmrrlght lieu-In maybete-prndurrtd.lummrtleil_\tmnl.or new! litany '. C E N G E farm at In JP11- mnm 5::Iphu..a-tmtmiiii_, nr mm harm at. in. toiling tmt not IIIiIitI-Ii l-i pliutm limit“; I'H'l'llljll'l“. u .iriiiing.t1igitiimg, taping, ernlntllliiilinti. In ll]|'|'1'|,|l|||llltflflulh\,1]r||lftlfllh|l|ntl“Ilia”! .fllil It'ltll'val \ysll'nimmil Ffll “plum!!le until-r ScCIIuII In) or IuB ul the IwalluteLl EIJIEsCupyrith flILL willinlil ‘" Learning‘ the III'IlIl w-Ium. pflln'u\|||llll at itm potent...- Pa Ti“ ill Till; Measuring lllc Melting I‘nints Di“ Conuxiunils and Mixtures Consider a loo-microgram (pg) mixture composed of 20 and 80 ug of Y. In this mixture, X acts as an impurity in Y. As the - ,, ture is heated, the temperature rises to the eutectic temperature of 13ft “C. At this temperature, X and Y begin to melt together at pail“ 3' the eutectic composition of 40 mass percent X and 60 mass percent y_ The temperature remains constant at 130 “C until all 20 pg at x mafia At the eutectic temperature, X and Y will melt in the ratio of 40 parts X to (all parts Y. If 2t} pg of X melts, then 30 ug of Y {20 ug X x 60/40 ratio = 30 up, Y) also melts. At this point, the remaining 50 pg of solid Y (80 pg — 30 pg : 50 pg) is in equitibrium with a molten mixture of the enter:- tic composition. As more heat is applied to the mixture, the temperature begins to rise, and the remaining Y begins to melt. Y continues to melt as the tem- perate re increases, shown by curve BC. Finally, at 1-12 “C, point C, where the liquid composition is 20 mass percent X and 80 mass percent Y, all of Y is melted. At temperatures higher than 142 “C, liquid X and liquid Y exist together with a compo- sition of 20 mass percent X and 80 mass percent Y. Thus, the melting point at which the entire mixture liquefies is 1-12 nC, six degrees lower than the melting point of pure Y. Also, the melting point range 130—142 “C is quite broad. In the previous example, X acts as an impurity in Y. Compound Y can also act as an impurity in X, as indicated in Figure 1 earlier in this experiment. For example, in a mixture composed of 80 pg of X and 20 tig of Y, the mixture begins to melt at the eutectic temperature of 130 °C. As before, at this temperature, the eutectic composition is 40 mass per- cent X and (in mass percent Y. The temperature remains at 't 30 “C until all 20 pg of Y melts. At the eutectic temperature, X and Y will melt in the ratio of 40 parts X to on parts Y. Thus, if 20 pg of Y melts, 13 pg of X (20 pg Y x 40/(10 ratio: 13 pg X) also melts. The remaining:I 67 ug of X (80 ug — 13 up, = 67 pg) melts over the range of 130—146 "'C, shown by curve BA. At 146 "C, the last traces of X melt. This melting range is larger than the range over which 20 mass percent X and 80 mass percent Y melts. If a mixture has exactly the eutectic composition of 40 mass percent X and on mass percent Y, the mixture shows a sharp melting point at l3tt “C. Observing this melting point could lead to the false conclusion that the mixture is a pure compound. Addition of either pure X or pure Y to the mixture causes an increase in the melting point, as indicated by curve BA or BC, respectively. Observing this melting point increase indicates that the original sample is not pure. The initial melting that occurs at the eutectic temperature is some- times very difficult to observe. This difficulty is especially true if only a small amount of an impurity is present, because the quantity of liquid produced at the eutectic temperature is very small. However, the tem- perature at which the last trace of solid melts can be accurately meas- ured. Hence, a sample with a small amount of impurity will have an observed melting point much higher than the eutectic temperature, but lower than that of the pure compound. Because the melting point of a compound is a physical constant, the melting point can be helpful in determining the identity of an unknown compound. A good correlation between the experimentally measured melting point of an unknown compound and the accepted melting point ‘-'——1—-""‘-'-"'-'-—--' TliCll 70 |: Measuring the Melting Poian ofCompounds and Mixtures 3 Figure 2 Flowchart for mixture melt- Ing point determination 01 an unknown mp << 133 “C. unknown is not trans-cinnamic acid Measuring Melting Points of a known compound suggests that the compounds may be the same. However, many different compounds ha ve the same melting point. A mixture melting point is useful in confirming the identity of an unknown compound. A small portion of a known compound, whosc melting point is known from the chemical literature, is mixed with the unknown compound. If the melting point of the mixture is the same as that of the known compound, then the known and the unknown com- pounds are most likely identical. A decrease in the melting point of the mixture and a broadening of the melting point range indicates that the compounds are different. A flowchart for using a mixture melting point to identify an unknown compound is shown in Figure 2. unknown mp = 133-135 “0 possible compounds benzoin: mp = 135—137 "C trans-cinnamic acid: mp = 133—134 r’C mix unknown and benzoin .__ _.L ' __. :_ _; ;L mp = 135-137 "C. mp << 133 “C. unknown is benzoin unknown is not benzoin mix unknown and trans-cinnamic acid _ _ i;_“_ _f—’— 1 mp : 133—134 “C. unknown is trans-cinnamio acid Melting points can also be used to assess compotmd purity. A melt- ing point range of 5 0C or more indicates that a compound is impure. Purification of the compound causes the melting point range to narrow and the melting point to increase. Repeated purification ma)r be neces— sary before the melting point range narrows to 1—2 “G and reaches its maximum value, indicating that the compound is pure. in practice, measuring the melting point of a crystalline compound involves several steps. First, a finely powdered compound is packed into a melting point capillary tube to a depth of 1-2 mm. Then the Cap- illary tube containing the sample compound is inserted into one of sev- eral devices used to measure melting points. Figure 3 Difierent types of melting point apparatus; (a) Thiele tube; to} Thomas—Hoover; {c} Met-Temp I't-Ztiil Till: Measuring the Melting I'oints ot'( Inmimunds and Mixtures =I-1-‘EJ ll 5 ll ll ll it I Figure 3(a) shows the Thiele tube apparatus, filled to the base of the neck with silicone oil or mineral oil. The capillary tube is attached to a thermometer so that the sample is located next to the middle of the thermometer bulb, The thermometer is inserted into the oil and then the side arm of the Thiele tube is heated with a Bunsen burner flame. The Thomas—Hoover Uni-Melt device, shown in Figure 30:), con— tains silicone oil that is stirred and heated electrically. Silicone oil can be heated to temperatures up to 250 0C. With this device, up to seven sam- ples can be analyzed at one time. The Mel-Temp apparatus. shown in Figu re Bic). consists of an alu- minum block that is heated electrically. The aluminum block can be heated easily to temperatures up to 400 “C, and can tolerate tempera— tu res up to 500 "C for brief time periods. A thermometer and up to three samples can be inserted into the block at one time. A light and magni- fier permit easy viewing of the sample(s). If the melting point of the compound is unknown, it is convenient to first measure the approximate melting point of the compound, called the orientation melting point. The sample is heated at a rate of 1045 “C per minute until it melts. Then the melting point apparatus is cooled to approximately 15 “C below the orientation melting point. A new sample is healed, increasing the temperature at a much slower rate of 1—2 “C per minute, to accurately measure the melting point. A slow heating rate is necessary because heating a sample too rapidly may cause the thermometer reading to differ from the actual temperature of the heat source. The result Would be an observed temperature reading that differs from the actual melting point temperature. If the melting point of the sample is known, the sample can be quickly heated to within Ill—15 “C of its melting point. Then the heating rate can be slowed to increase 1—2 °C per minute until the sample melts. .--““AA‘A“‘A‘-‘-‘-.—---“--‘---- p P P p I I I I r I I I I I I I I I I I I I I I l I I I l TH ill Till: Measuring the Melting. Point» of { fumpounds and Mixtures ’3 lirrors in observed melting points often occur due to a poor heat transfer rate from the heat source to the compound. One ca use of a poor heat transfer rate is the placement of too much sample into the capillary tube. Finely ground particles of the compound are also necessary for good heat transfer. 1f the particles are too coarse, they do not pack well, causing air pockets that slow heat transfer. Sometimes slight changes, such as shrinking and sagging, occur in the crystalline structure of the sample before meltng occurs. Also, traces of solvent may be present due to insufficient drying and may appear as droplets on the outside surface of the sample. This phenom- enon is called sweating and should not be mistaken for melting. The initial melting point temperature always corresponds to the first appearance of liquid within the bulk of the sample itself. Some compounds decompose at or near their meltng points. This decomposition is usually characterized by a darkening in the color of the compound as it melts. If the decomposition and melting occur over a narrow temperature range of 1—2 “C, the melting point is used for identification and as an indication of sample purity. The melting point of such a compound is listed in the literature accom- panied by d or l'ft'i'flni'p. If the sample melts over a large temperature range with decomposition, the data cannot be used for identification PUF‘PUSES. Some compounds pass directly trom solid to vapor without going through the liquid phase, a behavior called sublimation. When subli- mation occurs, the sample at the bottom of the capillary tube vaporizer; and recrystallizes higher up in the capillary tube. A sealed capillary tube is used to take the melting point of a compound that sublimes at or below its melting point. The literature reports the melting point for these compounds accompanied by .4, salt, or suiwl. In this experiment you will measure the melting points of benzoic acid, mandelic acid, and mixtures of these two compounds. Both com- pounds melt near 122 "C. You will use these data to construct a melting point—mass percent composition diagram. From this diagram, you will estimate the eutectic temperature and eutectic composition for benzoic acid and mandolic acid. Finally, using, the mixture melting point method, you will identify an unknown compound. Measuring the Melting Points of Compounds and Mixtures Equipment graph paper metric ruler {mm} marking pen microspatnla melting point capillary tubes 2 watch glasses Reagents amt Properties titular nmss substance rjtittt'tttly {Inf/moi) mp {"C) lip (DC) benzoic acid It) mg 122.12 122—123 249 mandeIic acid ll) mg 152.15 120—122 {I PROCEDURE 1. Measuring Melting Points of Benzoic Acid and Mandelic Acid NOTE 1: Make certain that no more than 1—2 mm of compound is placed in the capillary tube. A larger amount will give a meltng point range that is too large. NOTE 2: HeatingI the capillary tube too quickly near the melting point will result in an inaccurate melting point measurement. 'l‘liCl'l 7'01: Measuring the Melting Points ol Compounds and Mixtuer Preview - Measure the melting point of benzoic acid O Measure the melting point of mandeiic acid 9 Measure the melting point range of four mixtures containing various amounts of benzoic acid and mandelic acid - Obtain a sample ot an unknown compound 0 Measure an orientation meltinpI point and an accurate inciting point of your unknown compound - Obtain a sample of each of two substances appearing in Table 1 that have melting points similar to your unknown I Prepare a mixture of your unknown compound and each of your selected compounds ' Measure the melting point of each mixture '- Identify your unknown compound Caution: Wear departmentally approved safety goggles at all times while in the chemistry laboratory. Always use caution in the laboratory. Many chemicals are potet'tv tially harmful. Prevent contact with your eyes, skin, and clothing. Avoid ingesting any of the reagents. Caution: Benzoic acid is an irritant. l’lace 2—3 mg of benzoic acid on a clean, dry watch glass. it the com- pound is not a fine powder, pulverize it using a microspatula. Caution: Capillary tubes are fragile and easily broken. Load a melting point capillary tube by pressing the open end of the tube into the powder. I’ack the. powder into the closed end of the tube by tapping the closed end against the bench top. Repeat the cycle of loading and packing until you can see 1-2 mm of benzoic acid through the tube. more ll To ensure good packing, drop the capillary tube with the open end up through a l-m-long piece of glass tubing, onto the bench top. Repeat several times. Place the capillary tube in the melting point apparatus provided by your laboratory instructor. Because pure benzoic acid melts at 122—123 “C, heat the capillary tube rapidly to ill] “C. ‘l'hen slow the heating rate to 1—2 "C per min. more :1 Record the temperature at which liquid first appears in the bulk of the sample and the temperature at which the entire sample becomes liquid. Caution: The capillary tubes are hot. Allow them to cool enough to avoid burning your fingers. When finished, remove the capillary tube. Place all used capillary tubes in the container labeled “Discarded Capillary Tubes", provided by your laboratory instructor. 2. Determining the Eutectic Temperature and Composition of a Benzoic Acid— Mandelic Acid Mixture NOTE 3: It you are using a Thiele tube. place the samples to the lett and right of the thermometer bulb. Secure them in place with .1 small ring of rub- ber tubing. .is shown in l’lgure 4. Make certain thiI bottom of the capillary tube is positioned vertically near the mid- point of the thermometer bulb. Also, be certain lite rubber tubing and pen marks are 2—3 cm above the oil surface because ll‘lt' oil expands when heated. NOTE -t: It you are using a Mel-Temp apparatus. you will need to tilt the samples a few millimeters above the base and slowly rotate the samples to see the last trace of crystals melt. Be caretut not to break the capillary tubes. 3. Identifying an Unknown Compound by Mixture Melting Point capillary tubes with samples Figure 4 Attachment of two capillary tubes to a thermometer TECH Fill: Measuring the Melting l’nlnts I'll-Lil'll'l'llll'l-lnd.‘ and Mixtures i Obtain 2—3 mg of mandolic acid and measure the melting point fol- lowing the procedure described for benzoic acid. Pure mandelic acid melts at 120—122 “C. From your laboratory instructor, obtain four benaoic acid—mandelic acid mixtures of the following compositions: pt'nn'ltl percent _l:r'u:oic acid itinndclic acid mixture 1 80 EU mixture 2 60 4t) mixture 3 4t} 6t} mixture 4 2t] 80 Using a marking pen, carefully label a capillary tube for each mix- ture. For example, near the top of the tube, mark the tube that will con- tain mixture 1 with one horizontal line. Similarly, mark the tubes for mixtures 2-4 with two, three, and four lines, respectively. Load each mixture into its capillary tube as previously described. Place the capillaries containing mixtures l and 2 into the melting point apparatus. fNo'I't-r .1] Heat the samples rapidly to 80 “C. Then slow the rate of increase to l~2 uC per min. Carefully observe and record the temperature at which the crystals first begin to melt and the tempera- ture at which the last trace of crystals melts. [NUTF 4} Allow the apparatus to cool to 80 "'C and repeat the melting point measurements, using the capillaries containing mixtures 3 and 4. Caution: Unknowns may be flammable, toxic, and irritating. Obtain It} mg of an unknown compound trout your laboratory instruc- tor and record its identification code. Pulverize the sample, label and load a capillary tube, and take an orientation melting point. Cool the apparatus to 15 “C below its orientation melting point. Prepare a new sample, and accurately measure the melting point. From Table l (on the next page), identify the two compounds that have melting points closest to the melting point of your unknown com- pound. Obtain a few milligram of each of these compounds. Place one knowu compound on a clean, dry, labeled watch glass. Add an approx- imately equal amount of your unknown compound. Similarly, place the other known compound on a second watch glass and add an approximately equal amount of your unknown. l’ulverize and mix each sample thoroughly, using a clean microspatula each time. Load the samples into separate, labeled capillary tubes. Also, load two capillary tubes with pure unknown. Take the melting point of one of the mixtures and the pure unknown simultaneously. Quickly heat the samples to within 30 "C of the pure compound’s melting, point. Then slow the heating rate increase to 1-2 "C per min. Repeat the procedure using the other mixture. Compare your data and identify your unknoum. 8 TECH 7lll: Measuring tlllz‘ Multing Points of Compounds and Mixturcs Table 1 Melting points of com- t‘Or::po:_uid_ mg { c:C)_ compom-ul __mp ( T) pounds “sad as unknowns bcnzhydrol 65—67 trans-cinnamic acid 133—134 biphenyl 69—72 benzoin 135-137 phcnanthrcnc 99—101 benzilic acid 150—153 o-toluic acicl 103—105 adipic acid 152—154 acetanilide 113—115 benzanilide 164—166 fluorunc 114—116 4-bromoacetanilido 167469 (R,S)-mandolic acid 120—122 4-23;$oxybcnzoic 215—217 bunzoic acid 122—123 anthracene 216—2] 8 4. Cleaning Up Usu- lho labeled collection containers provided by your laboratory instructor. Wash your glassware with soap or detergent. Caution: Wash your hands thoroughly with soap or detergent before leaving the laboratory. ...
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Lab 1 - Melting Point &amp; IR - CER Modular 1 Laboratory...

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