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Write contributing (important) resonance structures for each of the following compounds
and predict their relative C=O vibrational frequencies based on the importance of the
contributing resonance structures. List in them order from highest to lowest frequency.

cyclohexanone, 2-cyclohexenone, cyclohexanecarbaldehyde
CHEM 51LB Page 1 Rev 4/21/10 EXPERIMENT 5 SPECTROSCOPIC METHODS: INFRARED SPECTROSCOPY (IDENTIFICATION OF FUNCTIONAL GROUPS) Reaction: None Technique: Infrared Spectroscopy Category : Identification This experiment is designed to provide you with more experience using infrared spectroscopy. In this experiment we will analyze the infrared spectra of several unknown compounds. You will use your knowledge of infrared spectroscopy to identify these unknowns. Reading Assignment for Experiment 5: Techniques 18, pp. 228-262 Additional background on IR spectroscopy at the end of this handout Supplementary information in Janice Gorzynski Smith (2 nd ed), Chapter 13, Pre-lab Questions: 1. Based on the discussion of Resonance Contributors and the Relative Vibrational Frequencies for C=O Bonds outlined below, explain why a thioester absorbs at a lower frequency than an ester in IR spectroscopy. Use inductive and resonance arguments in your explanation. R C OR' O R C SR' O ester 1735 cm -1 thioester 1690 cm -1 2. Why should aqueous solution not be used in cleaning NaCl plates? CAUTION The compounds you are assigned may be hazardous if inhaled or absorbed through the skin. Avoid contact and inhalation. Methylene chloride is toxic. Avoid contact and inhalation.
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CHEM 51LB Page 2 Rev 4/21/10 Experimental Procedure: You will be assigned to a team of three to four students and be given four or more organic compounds with various functional groups such as alcohol, aldehyde, ketone, carboxylic acid, amine, and alkene. Each student in the team should take at least one IR. 1. Operation of the IR Spectrophotometer A small group of students will be brought into the instrument room and be given instruction on how to take an infrared spectrum of a liquid or solid sample. Liquid samples will be run neat between two salt plates, while solid samples will be run as a dichloromethane thin film, a Nujol mull or a KBr pellet. Read the FT-IR instruction section carefully. 2. Instrumentation: The Perkin Elmer 1600 FT-IR Operating Summary 1) Check that the instrument is “ON”. 2) Mount the salt plates containing sample on the sample holder (Handle the salt plates only by the edges.) 3) Press the “execute” button or “scan” followed by 4. The instrument will automatically obtain the spectrum over the full wavelength range of the instrument within a short period of time. 4) Adjust the transmittance by pressing button with arrow signs. 5) Mark peaks using “peakcur” and “mark” functions. 6) Press “ Plot ” to print a copy of the spectrum. You may plot extras for your lab partners. DO NOT PRESS PRINT! 7) Remove the salt plates and clean them. 3. Obtain IR Spectra for your Unknown Compounds Your team will receive a set of the unknown compounds shown below in a labeled vial with the Unknown Number written on the lid. Obtain an infrared spectrum for each sample, then make copies for your teammates. OH O OH O CH 3 O N CH 3 H H O N O H
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