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
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Infrared Spectroscopy (IR) Lab

Elements of the Experiment

1st Week



Lecture on Theory of:

Infrared Spectroscopy

Mass Spectroscopy

Ultraviolet / Visible Spectroscopy
2nd Week

Determination of Unknown Physical Properties

IR Spectrum of Unknown
Spectroscopy Problem Set:

Will be E-Mailed by Instructor

Due Date to be Determined
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Infrared Spectroscopy (IR) Lab

IR Spectroscopy


The following spectrographic techniques are introduced
to the Chem 315 and Chem 318 organic lab courses:

Mass Spectrometry

Ultraviolet Spectroscopy

Ultraviolate (IR) Spectroscopy

Nuclear Magnetic Resonance (NMR)
Other Instrumental Techniques

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Gas Chromatography
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Infrared Spectroscopy (IR) Lab

Physical Properties of Spectroscopy Unknown

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The following physical properties of organic compounds
are routinely determined in various combinations in
most experiments

Melting Point

Refractive Index (Corrected for Temperature)

Boiling Point & Purification via Simple Distillation

Physical Description of Purified Unknown

Solubility Relative to Water

Density Relative to Water
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Infrared Spectroscopy (IR) Lab

Organic Lab – Unknowns, Purification, Boiling Point

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
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Infrared Spectroscopy (IR) Lab

Simple Distillation - Background

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
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Infrared Spectroscopy (IR) Lab

Simple Distillation – Background (Con’t)

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
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Infrared Spectroscopy (IR) Lab

Simple Distillation – Background
 Boiling Point
The 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 level
At 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 bar
1 bar = 105 Pascals = 0.98692 atmospheres
= 14.5038 psi (pounds per square inch)
= 29.53 in Hg (inches of mercury) = 750.06 mm
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Infrared Spectroscopy (IR) Lab

Simple Distillation – Procedure
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 as
2 – 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
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Infrared Spectroscopy (IR) Lab

Typical Distillation Setup
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Infrared Spectroscopy (IR) Lab

Simple Distillation – Procedure

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
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Infrared Spectroscopy (IR) Lab

Simple Distillation - Procedure
 Note the temperature when the distillate begins to drip into
the waste receiving container
 Continue to collect distillate in the waste container until the
temperature begins to level off
 Remove the waster container and begin collecting the
distillate in a small clean Erlenmeyer flask
 Note the temperature when you start to collect the purified
sample
 Continue to collect the sample until the temperature begins
to rise again (it may not change before the all of the
sample has come over)
 Note the temperature just before the temperature begins
to change
 The first and last temperatures recorded in the narrow
boiling range represent the boiling point range of your
sample
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Infrared Spectroscopy (IR) Lab

Solubility & Density Relative to Water

Place about 2 mL Distilled Water in a medium test tube

Add 4-5 drops of the solid or purified liquid unknown

Shake test tube firmly

Observe solubility of Sample in Distilled Water

If sample is insoluble, observe whether sample:
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
Floats on top of water

Floats in the middle of the water

Sinks to the bottom of the test tube
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Infrared Spectroscopy (IR) Lab

Refractive Index

Clean prisms of ABBE’ Refractometer with tissues &
Methyl Alcohol – BE GENTLE!!

Do not touch prism with fingers or other hard objects,
use tissues

Place 3-4 drops of sample on Prism.

Close Prism and raise lamp in front of Prism Portal.

Flip switch on left side to turn on light.

Use large dial on right to bring light/dark image into
view.

If image cannot be found, flip switch on left down and
use large dial on right to bring the Scale into view
around 1.4000
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Infrared Spectroscopy (IR) Lab

Refractive Index (Con’t)

itch on left down and read value to 4 decimal
placesRelease switch on left and use large dial on right
to bring light/dark image into view

Sharpen line of demarcation using Drum dial on front of
instrument.

Use Eyepiece to sharpen Cross-Hairs

Align the line of demarcation with the Cross-Hairs

Flip sw

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Ex. 1.5523
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Infrared Spectroscopy (IR) Lab
 Refractive Index (Con’t)
Light Half
1.5500
1.5523
1.5550
1.5580
1.5600
Dark Half

The following equation adjusts the raw Refractive
Index value to 20oC:
ND20 = NDRm Temp + (Rm Temp – 20) * 0.00045
Ex: For an observed value of 1.5523 at 16oC, the
correction is:
ND20 = 1.5523 + (16 – 20) * 0.00045 = 1.5523 + (-4)
* 0.00045 = 1.5505
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Infrared Spectroscopy (IR) Lab

Infrared Spectroscopy

Liquid Samples

1 to 2 drops of liquid sample are placed between two
single crystals of sodium chloride (Salt Plates)
Note: NaCl plates are water soluble – keep dry


Solid Samples Soluble in Acetone

Dissolve sample in acetone

Evaporate on Salt Plate
Solid Samples Not Soluble in Acetone

Make Potassium Bromide (KBR) pellet

Put Salt Plate sandwich in Plate Holder

Place Plate Holder into Beam Slot of IR Spectrometer
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Infrared Spectroscopy (IR) Lab

Infrared Spectroscopy – Con’t

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
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