Teachers guide in spectra of liquids

INFRARED SPECTROSCOPY TEACHERS' GUIDE IR SPECTRA OF LIQUIDS LEVEL This lab is aimed toward second-year chemistry or the more advanced first-year students. In order to understand the principle objectives, students should be familiar with functional groups and structural formulas of molecules and the basics of organic nomenclature. Unless students are given a thorough introduction to IR spectroscopy, they will not appreciate or even understand the concept of how molecules absorb radiation. The lab should be preceded by an introduction to the basics of spectroscopy. The principles of spectroscopy can be introduced using the Spec-20, or by doing IR spectroscopy of films. The lab is designed to have students performing Part 1 while others are using the instrument. Part 1 is nicely done in groups of two to four. Part 2 may be done singly or in pairs. Throughout the procedure students should be free to collaborate with others. NOTE: You may want to give your students a list of the Part 2 substances and ask them to identify their unknown from this list. The follow-up questions at the end of the lab are arranged in order of increasing difficulty. Select and assign only the ones which are appropriate to the level and prior experience of your students. This lab will fit into the curriculum with the study of physical properties, organic compounds and functional groups, or electromagnetic radiation. IR spectroscopy may also be introduced as a separate unit in and of itself once the prerequisites have been covered. PRE-LAB DISCUSSION A number of points will be confusing to students, so take some time beforehand to help them understand some of the basic terminology. Specifically address the following: Absorbance peaks: The spectra the students will be examining will be transmittance spectra. When radiation is absorbed, there is a decrease in the radiation transmitted. This is exhibited by a downward deflection of the pen in that region of the spectrum. This downward INFRARED SPECTROSCOPY deflection is called an absorbance peak. Some may want to argue that it is a trough or valley. They have historically been referred to as peaks. They could also be called absorption bands. (In the context of course of a stripe, not a musical ensemble.) It may be simpler to explain and stick to one term, peak. Wavenumbers versus wavelengths: The wavenumber is a new term to students. It is used interchangeably with frequency, but wavenumbers are really not frequencies. They are directly proportional to frequency and therefore to energy of absorption. Wavenumbers are used in lieu of frequency largely as a matter of convenience. Calculate, for example, the frequency corresponding to an IR wavelength of 6200 nm: This is a large and inconvenient exponential number. IR spectroscopists, recognizing that wavelength and frequency are inversely related have elected to use wavenumber, the reciprocal of the wavelength in cm. Reciprocal cm are proportional to frequency and are units of a more convenient size. If you think it is important that students understand the term beyond the verbal definition, work through some examples using equations they should be familiar with. Select a wavelength in nm, convert it to cm and take the reciprocal. Demonstrate that as wavelength increases wavenumber decreases, proportional to the frequency and energy. Percent transmittance: An initial intensity of incident radiation enters the sample. Some of it is absorbed, and the remainder is transmitted. The percent transmittance is the ratio of the two. Intensity of absorbance: When interpreting the spectra, students will be asked to qualitatively determine the intensity of the peak - how big it is! They should recognize that the greater the magnitude of the absorption peak, the greater the absorbance (and the less the transmittance). The terms used to designate intensity in this lab are strong (s), medium (m) and weak (w). INFRARED SPECTROSCOPY Show your students some IR spectra and how absorbances vary with functional groups, bond strengths and atomic masses. Show them spectra which demonstrate the difference between, for example, the C-H stretch in an alkane (sp3 carbon) and alkene (sp2 carbon) or the differences and similarities among the various carbonyl compounds. After all of the prerequisites have been covered, spend a day prior to the delivery of the instrument devoted to interpretation of sample spectra. Use overheads of spectra and have students participate in completing a data table like the ones they will be using. Demonstrate the logic involved. They will have a much greater degree of success going into the lab if they are comfortable with the procedure and the unfamiliar terms have been demystified. PREPARATION Twenty-four spectra (six sets of four) are provided with this lab for use in Part 1. These are spectra of compounds each of which have one fairly easily identifiable functional group. Some of the spectra in each group may have common elements among them. The standard spectra for Part 2 will also be provided as well as the unknown liquids. The liquids will be supplied in glass bottles with a dropper which can be stored and reused. TIME ALLOTMENT A class should get through both parts in about three to four 50 minute lab periods. SAFETY / DISPOSAL Students should wear gloves and goggles during this lab. With a wide variety of organic compounds, some may be dermally absorbed, lacrimators or potentially allergens to some students. Depending on the number of students involved and the cleaning solvent used, handling of the liquid samples and cleaning the salt plates could be done in a hood. Trichlorotrifluoroethane works well and is less hazardous than dichloromethane. 3 INFRARED SPECTROSCOPY VARIATIONS It may be of interest to obtain the spectrum of a clean salt plate to show students that plates are relatively transparent to IR radiation in this region of the spectrum. Plates that have been used may show absorbances due to contaminations from handling, absorption of chemicals or improper cleaning. SAMPLE RESULTS You may want to be fairly liberal in grading. This will likely be students' first exposure to using a correlation chart and trying to differentiate one peak from another. They should come away with a feeling for the usefulness of the instrument, and a recognition for some practical applications. Below are correct responses to the questions. ANSWERS TO PRELAB QUESTIONS 1. Typical definitions of neat: unmixed with anything, undiluted, straight. Advantages of neat samples are: easier preparation because no mixing is required; no extraneous peaks from added solvents; no solvent effects. 2. Equations are: E = hc / ë and õ=c/ ë A comparison between infrared and visible radiations can be tabulated: visible infrared frequency greater greater less less energy shorter wavelength longer 3. imparts organic organic A functional group is a particular arrangement of atoms in a molecule which a characteristic set of properties to the molecule. The functional groups an molecule contains enable it to be classified into general types or families of compounds. 4. salt Wearing gloves will help prevent: liquids from being absorbed through the skin; plates from absorbing oil and moisture from fingertips. 4 INFRARED SPECTROSCOPY 5. The aqueous solution will dissolve the salt plates! ANSWERS TO FOLLOW -UP QUESTIONS 1. Refer to the key for spectra in Part 1. 2. Refer to the key for spectra in Part 1. 3. Refer to the key for spectra in Part 2. 4. Answers will vary. 5. (something A desiccator is an air-tight device, containing a water absorbing material hygroscopic) used to dry or store substances. 6. The two possibilities for this formula are dimethyl ether, CH3-O-CH3 and ethyl alcohol, CH3CH2OH. The peak at 1200 cm-1 and the absence of an O-H stretch 3200 cm-1 indicates that the first compound is ether. The second compound, exhibiting the alcohol peak is ethyl alcohol. above 7. radiation. Percent transmittance is the ratio of the transmitted radiation to the incident Mathematically, %T = I / Io x 100, where I is the transmitted intensity and Io is the initial intensity. 8. The background can be defined as the natural conditions in which something is experienced - in this case, the environment between the source of radiation and detector. It is what the instrument "sees" when no sample is present. Absorbance from the background in IR spectra is due primarily to water vapor and carbon dioxide. Since absorbance in this region of the spectrum can only be measured resulting changes in dipole moment, diatomic molecules such as N2 and O2 do contribute to the background. If absorbance due to the background were not subtracted, interpretation of the spectrum would be complicated by the additional peaks. the from not 9. Let students be creative. The best response is that glass is not transparent to IR radiation. This could be demonstrated with two thermometers, one of which is 5 INFRARED SPECTROSCOPY inside test a test tube, set an equal distance from an incandescent bulb. The one inside the tube will not rise to the same temperature. This could also be demonstrated by recording the spectrum of a glass slide. 10. Free response. 11. NaCl begins to absorb strongly below 500 cm-1. 12. The values given are in wavenumbers or the reciprocal of the wavelength in centimeters (reciprocal centimeters). Given 4000 cm-1 (4000/cm) , take the and convert to nanometers. reciprocal 13. The triple bond is stronger than the double bond. A stronger bond energy corresponds to a greater vibrational energy. 6 INFRARED SPECTROSCOPY KEY - PART 1 SAMPLE SETS OF SPECTRA PROVIDED BY THE CHEMOBILE These are the correct names corresponding to the numbers for each of the spectra. Group A (green) Group B (purple) 1 2 3 4 1 cyclohexanol isopentyl acetate 3 2-heptanone 4 3-aminopentane n-butanol bromobenzene butyl phenyl ether benzaldehyde 2 Group C (red) 1 2 3 4 Group D (blue) isopropyl alcohol 2-heptanone 1-heptyne 2-buten-1-ol 1 2 3 4 cyclohexene isopentyl alcohol hexane benzaldehyde Group E (orange) Group F (violet) 1 2 3 4 1 2 3 4 oleic acid cyclohexene heptane 1-heptyne 7 ethyl acetate bromobenzene methylbenzene 1-propanol INFRARED SPECTROSCOPY KEY - PART 2 FUNCTIONAL GROUP IDENTIFICATION CHART Unknown Compounds a l k a n e a l k e n e heptane CH3(CH2 )5CH3 1-heptyne CH3(CH2 )4C≡CH 2-heptanone CH3(CH2 )4COCH3 benzaldehyde C6H5CHO cyclohexanol C6H11OH ethyl acetate CH3CO2C2H5 oleic acid CH3(CH2 )7CH=CH(CH2 )7CO2H butyl phenyl ether C6H5O(CH2 )3CH3 2-buten-1-ol CH3CH=CHCH2O H X cyclohexene C6H10 X a l k y n e h a li d e a l c o h o l a ro m a ti c a l d e h y d e k e t o n e e t h e r a m i n e es t e r c a r b a c i d X X X X X X X X X X X X 8 INFRARED SPECTROSCOPY Unknown Compounds a l k a n e bromobenzene 3aminopentane isopentyl alcohol n-butanol methyl benzene acetone 10-undecenoic acid isopropyl alcohol 1-propanol a l k e n e C6H5Br (C2H5 )2CHNH2 (CH3 )2CHCH2CH2OH CH3(CH2 )3OH C6H5CH3 CH3COCH3 CH2=CH(CH2 )8COOH (CH3 )2CHOH CH3CH2CH2OH a l k y n e h a li d e a l c o h o l X a r o m a ti c a l d e h y d e k e t o n e e t h e r a m i n e es t e r c a r b a c i d X X X X X X X X X X NOTE: This table is just to help to clarify the functional groups for you. You need to carefully consider how you are going to use it. Consider that absorptions by other parts of the molecule (excluding the primary functional group) may also be present. However, for example, an organic chemist would say that isopropyl alcohol is an alcohol, NOT an alkane and an alcohol. KEY - PART 2 UNKNOWN LIQUID NUMBERS These are the correct numbers corresponding to the liquid unknowns. 9 INFRARED SPECTROSCOPY Unknown Number 1 Identity heptane Unknown Number 13 Identity 3-aminopentane 2 1-heptyne 14 isopentyl alcohol 3 2-heptanone 15 n-butanol 4 benzaldehyde 16 2-heptanone 5 cyclohexanol 17 methylbenzene 6 ethyl acetate 18 acetone 7 oleic acid 19 cyclohexene 8 butyl phenyl ether 20 10-undecenoic acid 9 2-buten-1-ol 21 isopentyl alcohol 10 1-heptyne 22 isopropyl alcohol 11 cyclohexene 23 bromobenzene 12 bromobenzene 24 1-propanol 10