2/26/2015 1 Infrared Spectroscopy (IR) Lab Infrared Spectroscopy – Identification of Unknown The use of selected physical properties and Infrared Spectroscopy to determine the identity of an unknown compound Text Materials Slayden – pp. 33 - 44 Pavia – pp. 851 – 88 (Infrared Spectroscopy) – pp. 941 - 959 (Mass Spectrometry) – pp. 719 - 729 (Simple Distillation) Solomons – Sec. 2.16; 9.12 - 9.14 Lecture Slides (Dr. Schornick Web Site) URL: http://mason.gmu.edu/~jschorni/irlecture.ppt 1
Infrared Spectroscopy (IR) Lab. Infrared Spectroscopy – Identification of Unknown The use of selected physical properties and Infrared Spectroscopy to determine the identity of an unknown compound. Text Materials Slayden – pp. 33 - 44 Pavia– pp. 851 – 885` (Infrared Spectroscopy) - PowerPoint PPT Presentation
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2/26/2015 1
Infrared Spectroscopy (IR) Lab Infrared Spectroscopy – Identification of Unknown
The use of selected physical properties and Infrared Spectroscopy to determine the identity of an unknown compound
Several experiments in Chem 315/318 (Org Lab I & II) involve the identification of an unknown compound
Liquid samples that students receive in Lab may contain some impurities in addition to the unknown compound that could produce ambiguous results when determining the chemical or physical properties of the compound
Simple Distillation is used to purify the sample by separating the pure compound that comes over in a narrow temperature range – corresponding to its boiling point – from impurities that have boiling points either lower than or higher than the compound
Distillation, like Spectroscopy, Melting Point, Refractive Index, etc. is one of the tools we introduce you to in the Organic I & II courses
Simple Distillation will be introduced here as a means to purify the unknown sample and to obtain an approximate boiling point of the compound
Simple Distillation and Fractional Distillation will be used in a future experiment as a means of separating compounds in a mixture
Boiling point is one of several physical properties – solubility & density relative to water, refractive index, melting point – that you will determine routinely on sample unknowns
Single Vaporization / Condensation cycle of a liquid sample
The Distillate for a mixture is always impure at any temperature range between the range of boiling points of the components
Therefore, it is impossible to completely separate the components in a mixture with simple distillation.
Relatively pure substances can be obtained from a mixture with Simple Distillation, if the boiling points of the components differ by a large amount (>100oC)
Redistilling the distillate from multiple sequential vaporization-condensation cycles would produce increasingly pure substances, but this is a very tedious process
Boiling PointThe normal boiling point (also called the atmospheric boiling point or the atmospheric pressure boiling point) of a liquid is the temperature at which the vapor pressure of the liquid is equal to 1 atmosphere (atm), i.e., the atmospheric pressure at sea levelAt that temperature, the vapor pressure of the liquid becomes sufficient to overcome atmospheric pressure and allow bubbles of vapor to form inside the bulk of the liquid.The standard boiling point is now (as of 1982) defined by IUPAC as the temperature at which boiling occurs under a pressure of 1 bar1 bar = 105 Pascals = 0.98692 atmospheres = 14.5038 psi (pounds per square inch) = 29.53 in Hg (inches of mercury) = 750.06 mm
Note: The temperature range you obtain for your boiling point may be inaccurate for three (3) reasons
1. The thermal inefficiency of the glassware used for the boiling point determination may result in a lower than expected measured value by as much as2 – 5oC
2. The thermometers used in the lab may not reflect the actual temperature
3. The atmospheric pressure in the lab may not be:
1 bar (0.98692 atm)
You should take this potential temperature differential into account when you compare your measured results with the list of possible unknowns in lab manual tables
Set up Simple Distillation apparatus (previous slide) Use 25 mL or 50 mL Distillation flask Place a Corundum or Teflon boiling chip in the flask Start gentle water flow through condenser Put a waste receiving container (small beaker) into
an ice water bath – especially for low boiling liquids. Begin heating sample
Note: The sample may appear to be boiling, but the actual boiling point is not reached until the temperature of the boiling liquid and the vapor surrounding the thermometer bulb reach equilibrium. At this point the vapor will start to condense in the condenser
Confirm with instructor that “Background” has been scanned in.
Select Memory location (X, Y, or Z) Press “SCAN” button Verify No. of Scans is “4”; if not, notify instructor Press “Execute” (last (far right) “soft” button If spectrum absorptions bottom out, remove Salt
Plate holder and reload Salt Plate with less sample. Rerun Scan / Execute again Push “Plot” to produce chart Remove Cell Holder and disassemble Clean Salt Plate with acetone and dry with ChemWipe Place Salt Plate in desiccator
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Infrared Spectroscopy (IR) Lab The Laboratory Report
Analyze and label the IR Spectrum to determine the type of compound
Select compounds of similar type from the Unknown List in the Slayden Manual, Appendix 3, p. 125-127
Analyze the physical property results to narrow down the possible candidates for the compound’s identity
Verify measured physical properties with appropriate literature or online resources - Merck, CRC, etc.
Cite the compound resource in the literature citation section of the report using the format described on slide 27 of the Organic Lab Handout.
Be sure to include the item number, if applicable, and the page number