Lab 34 - IR Analysis (1)

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Dec 6, 2023

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Experiment 3: IR Analysis of Organic Liquids Purpose: In this experiment, you will:  Demonstrate the procedure for collecting an infrared spectrum  Obtain spectra of various liquid samples  Compare peak values to the table and identify the unknown compounds Reference: Modified by Dr. Gautreaux from – Juniata College Chemistry Department. Background Theory: Infrared light has a wavelength of 770nm to 1000nm, which corresponds to a wavenumber range of 12,900 to 10 cm -1 . Infrared light with wavenumbers from 400 to 4000 cm -1 will be used to identify various identify various organic compounds. When a beam of infrared radiation is passed through a substance, the radiation can be either absorbed or transmitted, depending on its frequency and the structure of the molecules it encounters. Energy is absorbed when its frequency matches the natural frequency of the bond motion. The energy absorbed by the molecules may bring about increased vibration, stretching, bending or rotating. The IR spectrophotometer measures the frequency of the energy causing the bond motion. Each type of bond absorbs specific wavenumbers of infrared light depending on its structure, nature of atoms bonded and effects of surrounding atoms. In this experiment, various samples will be placed into a beam of infrared light. Some of the light will be absorbed. The absorption of infrared light will be indicated by a valley (called a peak) in a graph called a spectrum . You will compare the wavenumbers of the peaks to the table of known values to determine the identity of the unknown samples. Liquid samples can be prepared for IR analysis by simply placing an undiluted sample on the IR Spectrophotometer. Before running the sample, a background should be collected. This will make the computer subtract any peaks from the surrounding atmosphere when you collect the data for your sample. Chemical Hazards: Name Formula/Structure Hazards PPE CHEM 3320 – IR ORGANIC LIQUIDS 1

Procedure: Part I: Collecting the Background Spectrum 1. Make sure there is nothing on the surface of the IR. Clean with a few drops of acetone and a KimWipe. 2. Click on “Collect  Background”. (Instructor will assist) 3. Wait for the scanning to complete. A background spectrum should appear. It will be automatically saved and used to set a ratio for your sample(s). Part II: Collecting a Liquid Sample Spectrum 4. Use a disposable pipette to place 2 or 3 drops of the your assigned unknown onto the crystal of the IR spectrometer. 5. Click on “Collect  Sample”. a. Enter the name of your sample (include group name or initials) for example: Mark, Jane, Chris : L1 or MJC: L1 b. Click enter. 6. When the scanning is done, the spectrum should appear on the screen. Move on to Part III: Processing and Printing an IR Spectrum for the next part of the process. Part III: Processing and Printing an IR Spectrum 1. When the data collection is complete, the spectrum should appear on the screen. 2. If the peaks appear too small, add more sample. Collect data again. 3. If the peaks appear too large, wipe off some with a Kimwipe. Collect again. 4. Print the Spectrum. 5. Move on to Part IV: Cleaning Up . Part IV: Cleaning Up USE ACETONE ONLY 1. Wipe the crystal with a Kimwipe. 2. Wash both the surface of the instrument and the top with acetone – dry with Kimwipe. 3. If you are going to do another sample, repeat the steps starting with Part II. If you’re done, follow the directions below. Analyzing the IR Spectra 1. List the wavenumbers of the major peaks from the spectrum in your data table. 2. Identify the functional groups (as many as possible) represented by the peaks using the Peak Table below. 3. Identify each unknown by comparing the functional groups found with the possible samples shown below. 4. Give a brief explanation of why you picked that molecule as the identity of the sample. 5. Answer the questions. CHEM 3320 – IR ORGANIC LIQUIDS 2

Peak Table : Sample wavenumbers of infrared light absorbed by the bonds in organic compounds. Functional Group Structure Bond Wavenumber (cm -1 ) Alkanes CH 3 -CH 2(n) -CH 3 (single bonds) C-H stretch 2950 – 2850 (m or s) Alkenes CH 2 =CH 2 (double bonds) C-H stretch C=C stretch 3100 – 3010 (m) 1680 – 1620 (vs) Aromatic Rings C-H stretch C-H bending C=C bending ~ 3030 (vs) 860 – 680 (s) 1700 – 1500 (m,m) Alcohol/Phenol R-OH O-H stretch 3550 - 3200 (broad, s) Carboxylic Acid (organic acid) O OH R O-H stretch C=O stretch 3000 – 2500 (broad, vs) 1780 – 1710 (s) Aldehyde R H O C=O stretch 1740 – 1690 (s) Ketone R R O C=O stretch 1750 – 1680 (s) Ester R 1 O O R C=O stretch C-O stretch 1750 – 1735 (s) 1300 – 1000 (s) Possible structures for Liquid Unknowns C H 3 CH 3 hexane OH C H 3 CH 3 isopropyl alcohol O C H 3 CH 3 methyl ethyl ketone CH 3 toluene CH 3 O C H 3 O ethyl acetate CHEM 3320 – IR ORGANIC LIQUIDS 3

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Data and Conclusions Sample/Unknown #: ___1_____ Major Peaks (cm -1 ) – Peak Positions Functional Group 1750-1735 cm -1 Ester 1250 cm -1 1050 cm -1 Identity of Sample: Ethyl Acetate Why do you think this is the identity of the sample? Due to the three peaks in this graph, it correlates with the C=O and 2C-O group that fall under the functional Group. The ester infrared spectra are easily distinguished from isomeric carboxylic acids by the absence of the broad O-H Sample/Unknown #: __2 ________ Major Peaks (cm -1 ) - Peak Positions Functional Group 2950-2850 cm -1 Alkanes 1490-1350 cm -1 750 cm -1 Identity of Sample: Hexane Why do you think this is the identity of the sample? Since on the right side of the infrared spectrum of at 1500 cm -1 is which is the fingerprint area that is known for the identification of hexane. The bonds are C-H stretch single bonds. Sample/Unknown #: ____3 ________ Major Peaks (cm -1 ) – Peak Positions Functional Group 3200-3400 cm -1 Alcohols 2950-2850 cm -1 Carboxylic acids Identity of Sample: Isopropyl alcohol Why do you think this is the identity of the sample? alcohol carbon is attached to two other carbons sometimes shown as (CH3)2CHOH. It is a structural isomer of 1-propanol. The absorption of the broad O-H stretching caused by hydrogen bonding interactions, common to all hydrogen bonded molecules with a hydroxyl group. Sample/Unknown #: _____4 ______ CHEM 3320 – IR ORGANIC LIQUIDS 4

Major Peaks (cm -1 )- Peak Positions Functional Group 3000-2900 cm -1 Ketone 1710 cm -1 1390-1400 Identity of Sample: Methyl ethyl ketone Why do you think this is the identity of the sample? stretching vibrations of the carbonyl group bond C=O. There are multiple C-H absorptions but the non-particularly can be ketones. Sample/Unknown #: 5 _______ Major Peaks (cm -1 )- Peak Positions Functional Group 3050 cm -1 Alkenes 1500 cm -1 Aromatic Rings 1100 cm -1 730 cm -1 Identity of Sample: Toluene Why do you think this is the identity of the sample? Only alkenes and aromatics show a C–H stretch slightly higher than 3000 cm -1 . Compounds that do not have a C=C bond show C–H stretches only below 3000 cm -1 . the carbon-carbon stretches in the aromatic ring the in-plane C–H bending and the C–H oop. Post Lab Questions: 1. Why must water be kept away from the experiment? a. Solvents should be free of water to avoid damaging the sodium chloride cell surfaces. A reference spectrum of the blank solvent should be obtained and subtracted from the sample spectrum. 2. Why is it necessary to obtain a background spectrum? a. This is necessary because it provides a baseline that the instrument will subtract out of subsequent scans. It’s a way to compensate for the presence of water, CO 2 , impurities in salt plates, etc. CHEM 3320 – IR ORGANIC LIQUIDS 5