orgolab_lab07 - o 1997 Cengage Learning'I'ECI'I Identifying...

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Unformatted text preview: o 1997 Cengage Learning 'I'ECI'I Identifying an Unknown Compound by Infrared Spectroscopy Prepared by Moses Lee, Furman University PURPOSE OF THE EXPERIMENT Prepare IR samples. Obtain and interpret IR spectra of organic compounds. BABKGROUHD REQUIRED You should have experience in IR spectroscopy, including theory and interpretation. BIOKGR'OUHD INFARMQT‘I'ON Infrared (1R) radiation is radiation in the energy range between the visible and.microwave regions 'of the electromagnetic spectrum. The portion of the IR spectral region between 4000 and 400 fin—‘1 is of greatest practical use to the organic. chemist. An IR spectrum,.as shown in Figure 1, is a plot of the percentage of [R radiation that passes through a sample versus the frequency of the radiation: The radiation that passes through the sample is measured as percent transmission. The frequency of the radiation is measured in wave— lengths per centimeter, which is known as a wavenumber. In IR spectroscopy, a simple moleculé‘ can produce a very complex spectrum. An organic chemist takes advantage of this complexity by com- paring the sped-ruin of an unknown compound with that of a reference compound. It' 15 unlikely that any 'two compounds would produce exactly the same IR spectrum. Although an IR spectrum is characteristic of the entire molecule, certain groups of atoms, called functional groups, give rise to particular absorption bands, or peaks. These absorption bands occur at or near the same frequency, regardless of the structure of the rest of the molecule. The persistence of these characteristic bands permits chemists to obtain useful structural information by comparing the absorption bands for a sample to tables of functional group absorption frequencies. ’CENGAGE oinrmgemmgmnmsmsoHumanswatcmmmmmnmmammm on Learning“ unedited.shrewusadhanytnrmorhyanymnrsgraptiulenmmornectamatrchndmbmmtnliedtopmmmm remmmmmimmmdisrmmmmmemmmmwmwmmpm permittedmierSectluflDl'ur1mmm1mwwsmcmmmmmwmwmflmm 102 percent transmittance 3 Figure 1 An infrared radiation spectrum of polystyrene, showing percent transmission versus radiation fiequency, cm Studying Vibration: and Energy TECH0710: Identifying an Unknown Compound by Infrared Spectroscopy wavelength (um) wavenumber (cm-1) i —1 Understanding IR Spectroscopy theory requires the study of vibration mechanics. In a normal mode of vibration, each atom in a molecule executes a simple harmonic oscillatipn (vibration) about its equilibrium position. A ball-and-spring molecular model can be used to demonstrate the effects of Vibrations in stretching» and bending springs, together with the motions of the balls. According to classical mechanics, the frequency of vibration v ofrt‘viro balls of total massarn connected by a spring with a force constant kpis shown in Equation 1. v = £04m?” (Eq- 1) The methods of classical mechanics can be used to study the vibrational motions and frequencies of a model containing several balls (or atoms) of various masses connected by springs (or bonds) with different force constants. The results of these studies form the basis for the interpretation of vibrational spectra. Classical mechanics would indicate that there is a continuum of vibration levels, and that a molecule may undergo any of numerous vibrations. Quantum mechanics, however, places restrictions on microphysical systems. These restrictions limit a molecule to having discrete energy levels The difference in energy (AB) between the vibrational levels 15 given by Equation 2, in which I: 15 Planck’s constant and k' IS the force constant. h k as = hv = hkfi = E (E) ”2 (Eq. 2) The reduced mass a, defined by Equation 3, is given for a molecule AB, where MA and MB are the atomic masses of atoms A and B. MAME = —— . 3 fl MA + Ms (Eq ) When the frequency of infrared light applied to a compound is exactly the same as the natural vibrational frequency of an interatomic bond, the molecule absorbs the light and the amplitude of the bond vibration increases. As 0 1997 Cengage Learning TECH0710: Identifying an Unknown Compound by Infrared Spectroscopy 103 Figure 2 Normal modes of CH2 vibrations Interpreting Spectra symmetrlc asymmetric HCH bend rocklng stretching stretching motion (—2850 cm") (—2925 cm“) (-4450 orn'l) (—750 cm") indicated in Equation 2, the force constant for the deformation determines the vibrational frequency. Therefore, frequency, and thus the frequency of the radiation absorbed, is related to the rigidity or strength of the bond and the masses of the bonding atoms. Specifically, the vibrational frequency is higher for stronger bonds and for fighter atoms. Two types of vibrations, stretching and bending, are responsible for most of the important peaks used to identify organic compounds. For example, Figure 2 shows a few types of vibrations for the CH2 group. Bending motions require less energy than stretching motions, so the bending motions absorb at lower frequencies, and, therefore, have smaller wavenumbers. The magnitude of infrared absorption bands is proportional to the change in dipole moment, or separation of charges,‘that a bond undergoes when it stretches. Thus, the more intense bands in an infrared spectriim are often produced by $320 and C-0 stretching vibrations. In contrast, the CEC stretching band for a symmetrical alkyne is almost nonexistent because the molecule undergoes no net change in dipole moment when it stretches. It is usually not possible to assign Specific molecular vibrations to the majority of bands in an infrared spectrum. However, it is helpful to divide an IR spectrum into two parts. The 4000 to 1500 cm'1 portion is useful for identifying various functional groups. Bands in the 1500 to 600 crn'1 portion, called the‘fingerprint region, are the result of many typesiot vibrations that are characteristic of the molecule as a whole. This complex fingerprint region represents a unique pattern for each organic compound. The region is useful for comparing the spectrum of an unknown compound with the quctra of known compounds for identification purposes Two examples, 2-methy1—2—propanol and 2-butanone, illustrate the application of IR spectroscopy to identify organic functional groups. In the IR spectrum of 2—methyl-2-propanol, shown in Figure 3(a) on the next page, the intense band from 3500 to 3100 cm‘1 can be attributed to the OH group. Specifically, this band illustrates the stretching frequency of the 0—H bond. The stretching frequency! of a 9-H bond is near 2900 cm”, and the stretching frequency of a C— 0 bond is near 1150 cm”. The stretching frequency of ’the 9:0 group in 2—butanone results in an intense band near 1700 cm“, as shown in Figure 3(b) on the next page. Clearly, the fingerprint regions in Figures 3(a) and 3(b) are different, reflecting the differentstructures of the two compounds. Table ’1 gives a correlation of thermore common structural units and their characteristic vibrational frequencies. I I =1 I} 104 l l I l percent transmittance p‘ercent transmittance Figure 3 Measuring IR Spectra TECHOTIO: identifying an Unknown Compound by Infrared Spectroscopy wavelength (um) i warvenumber (orn'l) wavelength (11m) 1‘: ii Eé 1!? ..."... .'..-._.:.v..... .._.:._...E ............. "..‘. ..._;... ....\_ 3000; 3500-" ....2036.m._...1..é36 . 1600 {£56 wavenumber (om*‘) The IR spectra of (a) 2—methyl—2—propanol and (b) 2-butanone Currently, there are two types of instruments used to record IR spectra. dispersive double-beam and Fourier transformed (FT) spectrophotometers. In a double-beam spectrophotometei, the IR radiation is emitted into a monochromator, which separates the‘wavelengths of light. The monochro- matic radiation goes into a beam splitter composed of a mirror and a prism. The beam splitter allows equivalent beams ‘of relatively narrow wavelength range to pass simultaneously through a sample and a reference cell. Another pnsm and mirro'r system focuses the emergent light into a rotating-sector mirror. An electronic bridge system detects the radiant energy transmitted by the sample and the reference as a voltage difference. A recording of the percent transmission of the IR radiation through the sample with varying wavelengths produces an IR ‘spectrum. The FT-IR method splits the electromagnetic radiation into two beams One beafn travels over a longer path inside the spectrophotometer than the other beam. A recombination of the two beams creates an inter- ference pattern or interfemgram. Fourier transformation, a computerized gm.__m.m_ menu—“.1 o 1997 Cengage Learning TECH0710: Identifying an Unknown Compound by Infrared Spectroscopy 105 Table 1 Absorption frequencies of some common bonds (shown in bold type) Bond Type of compound Frequency {is-H (stretch) alkanes 2800—3000 =(II—H (stretch) alkenes, aromatics 3000—3100 -:7C—H (stretch) alkynes 3300 —O—I-I (stretch) alcohols, phenols 3600—3650 (free) 3200-3500 (H-bonded) (broad) —O—H (stretch) carbbxylic acids 2500-3300 —1{l—H (stretch) amines 3300—3500 (doublet for NH2) 0 _&_H (stretch) aldehydes 2720 and 2820 —C!= .. (stretch) alkenes 1600—1680 — =(IZ— (stretch) aromatics 1500—1600 —CEC—H (stretch) alkynes 2100—2270 0 (stretch) aldehyde, ketones, 1680-1740 _&_ carboxylic acids —CE (stretch) nitriles 2220—2260 C—N (stretch) ‘ amines 1180—1360 -C—H (bending) alkanes 1375 (methyl) —C—H (bending) alkanes 1460 (methyl and methylene) ~C—H (bending) alkanes 1370 and 1385 (isopropyl split) —C—H (bending) R—CH=CH2 1000-960 and 940—900 —C—H (bending) R2C=CHg 915—870 ..—C—H (bending) cis RCHr-CHR 290—650 —C—H (bending) trans RCH=CHR 990—940 —C—H (out of plane bending) mono subst. benzenes 770-730 and 710-690 -C—H (out of plane bending) 0- subst. benzenes 770-735 -C—H (out of plane bending) m- subst. benzenes 810-750 and 710—690 —C—H (out of plane bending) p- subst. benzenes 860—800 —C—O (stretch) primary alcohols 1050 —C—O (stretch) secondary alcohols 1100 —C—O (stretch) tertiary alcohols 1150 —C-O (stretch) phenols 1200 mathematical manipulation of data from the inter-ferogram, converts the interferogram into the usual Hi spectrum. The instrument does not use a monochromator. The radiation from the entire IR spectrum passes through the sample |simultaneously, savjpg much time. FF-[R instruments can have very high resolution (<0.001 cm"). Moreover, because the data undergo analog-to—digital'conversion, FT-IR data can be easily manipulated. Combina- tions of results of several scans average out random artifacts, allowing excellent spectra from very small samples. 106 Preparing IR Samples TECH07IO: Identifying an Unknown Compound by Infrared Spectroscopy Infrared spectra of liquid samples are prepared by placing neat, or pure, undiluted samples between two salt plates. Glass is opaque to IR radiation and cannot be' used. Instead, the sample is prepared using potassium bromide (KBr), sodium chloride (NaCl), or silver chloride (AgCl) plates, {which are transparent to IR radiation. A solid sample can be mixed with solid KBr. The mixture is then pressed into a very thin pellet. Alternatively, a solid sample can be dissolved in a solvent‘ such as dichloromethane, tn'chloromethane, tetrachloromethane, or carbon disulfide. In a third method, solid samples may be ground into a very fine powder and mixed with Nujol mineral oil to form a mixture called a Nujol® mull. Some solvents do absorb IR at certain frequencies, so those parts of a spectrum are not useable. Typically, neat samples for liquids and KBr‘pellets for solids are used because neither method produces extraneous absorptions to obscure the spectra. Consequently,these methods have been chosen for this experiment. Equipment 35-min 0- d- agate mortar and 2 KBr, NaCl, or AgCl plates, with pestle a sample holder gloves lint-free tissues IR spectrophotometer, either microspatula double-beam or FT ‘ 6 Pasteur pipets, with latex bulb KBr hand press, with a die set and an appropriate KBr pellet holder Reagents and Properties Substance Quantity Molar mass (g/mol) bp (°C) mp (°C) p-anisaldehyde" 0.3 mL 136.15 248 benzoic acid 2 mg 122.12 122—123 ethanol“ 20 mL potassium bromide 100 mg unknown‘ 0.3 mL ‘in a capped vial containing SA molecular sieves Preview 0 Prepare a KBr pellet of benzoic acid - Obtain an IR spectrum of benzoic acid 0 Prepare a sample of p-anisaldehyde using either KBr, NaCl, or AgCl plates 0 Obtain a spectrum of p-anisaldehyde 0 Prepare a neat IR sample of a liquid unknown 0 Obtain an IR spectrum of the liquid unknown 0 Deduce the structure of the unknown o 1997 Cengage Learning TECH0710: Identifying an Unknown Compound by Infrared Spectroseopy 107 Chemical Alert 1. Preparing a KBr Pellet Sample ol Benzoic Acid Wear departmentally approved safety goggles at all times while In the chemistry laboratory. I ,.. Benzolc acid and KBr are irritants. Prevent eye, skin, and clothing contact. Avoid inhaling dust and lngestlng these compounds. View the demonstration amounts of benzoic acid and KBr in the vials provided by your laboratory instructor. Place approximately 2 mg of benzoic acid and about 100 mg of dry KBr into a small agate mortar. Grihd the mixture into a fine powder with a pestle. n NOTE: When preparing lFi samples, keep everything dry. Do not expose samples to water. Always keep the bottle containing trip dry. spectroscopic grade KBr in an oven set at 100 r’C. When you take the bottle from the oven. cap the bottle and place it in a de'siccator to 060]. Place the lower anvil of the hand press with the shorter die pin on a clear area of the bench. Attach the collar to the anvil, oriented as shown in Figure 4. NOTE: A different type of pellet press may be employed in your laboratory. If so, your laboratory instructor will provide instructions for its use. Use a microspatula to quickly transfer about 75% of the finely grOund sample into the collar. Place the upper anvil with the longer die pin over the collar so that this die pin comes in contact with the sample, as showu in 'Figure 4. Place ‘the die“ set on the hand press plunger while holding the press in the vertical’ position. Slowly compress the2 sample by‘pulling down the lever and holding it for 20—30 s. Release the lever and remove the die set from the press. Carefullyseparate the upper and lower anvils. Leave the KBr pellet in the collar. Place the collar containing the KBr pellet onto a sample holder. Use the procedure described in either Part 3A or Part SB to obtain an IR spectrum. At the end of the experiment, use a microsPatula to remove the KBr pellet from, the collar. Place the pellet material into the container labeled “Recovered KBr Pellets”, provided by your laboratory instructor. Wash the metal apparatus carefully with water and dry the apparatus in the oven. 108 l l Figure 4 j .u A KBr hand press with a die set 2. Preparing a Sample of p-Anisaldehyde TECHOYIO: Identifying an Unknown Compound by Infrared Spectroscopy pressure die pins ] -A. Using KBr or NaCl Plates p-hnlsaldehyde Is an lrrltant. Prevent eye, skin. and clothing contact. Avoid lnhallng fumes and lngestlng’the compound. Use p-anlsaldehyde In a fume hood. . NOTE: Your laboratory instructor will tell_you which of the following procedures to use. Obtain a sample of p-anisaldehyde from your laboratory instructor. Keep the container capped except for the very brief time necessary to remove a sample. To prevent stirring up the molecular sieves, do not agitate the container prior to use. Remove a pair of KBr or NaCl plates from the desiccator. Use a Pasteur pipet to place half a drop of p—anisaldehyde' 1n the center of one of the plates. Gently press the plates together to remove any air bubbles. NOTE: KBr and NaCl plates are fragile and hygroscopic. Do not use water to Mp6 the plates Even moisture from your fingers will attack the plates. Use gloves and only handle the plates by the edges. , Place the plates on the sample holder, as shown in Figure 5. Place the metal cover over the top of the plates and replace all nuts. Gently tighten the nuts to apply an even pressure to the top plate. o 1997 Cengage Learning TECHD710: Identifying an Unknown Compound by Infrared Spectroscopy 109 Figure 5 IR salt plates and holder ui hex huts salt plate ——— v one-half drop “u of sample salt plate -— placed here threaded pOSts fillets" 'fifi;\\\\l-' “1“ sample holder Use the procedure described in either Part 3A or Part SE to obtain an IR spectrum. At the end of the experiment, remove the sample holder from the sample compartment and unscrew the nuts. Remove the plates and separate them. Absolute ethanol ls flammable and toxic: Do"not use near flames or other heat sources: Avold Inhallng fumes and lngestlng the compound. Use a Pasteur pipet to rinse the plates with dry absolute ethanol. Then dry the plates with absorbent, lint-free tissues. Place the used tissues into a container labeled ”Used Tissues”, provided by your laboratory instructor. Return the plates _ to the desiccator, and return the remaining p—anisaldehyde to your laboratory instructor. 3. Using AgCl Plates p-Anlsaldehyde Is an lrrltent. Prevent eye, skin, and clothlng contact. Avold Inhallng fumes and lngestlng the compound. Use p-anlsaldehyde in a fume hood. ’ Obtain ,tyvo AgCl plates from the desiccator. Put half a drop of peanisaldehyde between the plates and gently press them together to remove any air bubbles. NOTE: AgCl plates are fragile. Use gloves when handling AgCI plates. Minimize exposure of the plates to light because light will darken them. 110 Figure 6 AgCI cell assembly 3. Obtaining a Spectrum TECHO710: Identifying an Unknown Compound by Infrared Spectroscopy Assemble the AgCl cell as shown in Figure 6. Place an O-rifig in the cell body and place the AgCl plates on top of the ring. Put the second O-ring on top of the AgCl plates and then put -on the nut. Gently tighten the nut by hand. Do not overtighten because the AgCl plates may distort or crack. Place the fully assembled cell on a sainple holder aligning the assembly so that light canapass through it. Use the procedure described in either Part 3A or Part SB to obtain an IR spectrum. At the end of the experiment, remove the sample holder from the sample compartment and unscrew the nut. ”Remove the AgCl plates and separate them. Absolute ethanol is flammable and‘ toxic. Do not use near flames or other heat sources. Avoid Inhallng fumes and lngestlng the compound. Use a Pasteur pipet to rinse the plates with dry absolute ethanol. Then dry the plates with absorbent, lint-free tissues. Place the used tissues into a container labeled "Used Tissues”, provided by your laboratory instructor. Return the KBr plates to the desiccator, and return the remaining p—anisaldehyde to your laboratory instructor. A. Using an F1" -IR Spectrophotometer Make certain that there is nothing in the sample compartment of the IR spectrophotometer. Use the instructions provided by your laboratory instructor to operate the spectrophotometer. Obtain a background spectrum. Place the‘sample holder in the sample compartment and start the data acquisition. Adjllst the output to maximize the display in both the x and y axes. Place a new sheet of chart paper on the plotter. Plot the spectrum. Check With your laboratory instructor to see if the spectrum that you have recorded is acceptable. If the spectrum is not acceptable, prepare a new sample and repeat the procedure until you obtain an acceptable spectrum. O 1997' Cengage Learning TECHU710: Identifying an Unknown Compound by infrared Spectroscopy 111 4. Preparing a Sample of an Unknown compound Using KBr, “act, or KBr Plates 5. Cleaning Up B. Using a Double-Beam IR Spectrophotometer Place the sample holder in the sample compartment. Use the instructions provided by your laboratory instructor to operate the spectrophotometer. Place a reference beam attenuator into the reference compartment to equalize the amount of energy in the two beams. Set the starting wavenumber to 4000 cm" and adjust the att...
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