Spectroscopy Experiment (NMR)
Determination of the Identity of an unknown
organic compound using selected physical
properties, Infrared Spectroscopy, and NMR
Spectroscopy
Unknown List: pp. 126 – 127 in Slayden Lab Manual
References
 Slayden,
 Pavia,
et al. – pp. 59 – 60
et al.
 Schornick
3/11/2016
– pp. 909
– 964
– http://classweb.gmu.edu/jschorni/chem318
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Spectroscopy Experiment (NMR)

Overview

Physical Properties

Boiling Point & Purification (Simple Distillation)

Physical Characteristics of Unknown

Refractive Index with Temperature Correction

Solubility (Relative to Water)

Density (Relative to Water)
 Infrared
 NMR
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Spectra Analysis
Spectra Analysis
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Spectroscopy Experiment (NMR)

The Laboratory Report:

Determination of an Unknown

Procedures

Title – Use Short, Concise terms
Simple Distillation, Refractive Index, IR
Spectrum, etc.

Materials & Equipment (2 Columns in list (bullet)
form)
Note: include all reagents & principal equipment
used
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Spectroscopy Experiment (NMR)

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The lab Report (Cont’d)
 Procedure Description:
 Descriptions must be detailed, but concise
 Use list (bullet) form
 Use your own words (don’t copy book)
 Results – Neat, logically designed template to
present results
 Summary of Results
 Paragraph summarizing experimental results,
computed results, and principal absorptions from
the IR & NMR spectra.
 Analysis & Conclusions
 A logical step by step set of arguments, utilizing
selected results, to support the proposed identity
of the Unknown compound
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Spectroscopy Experiment (NMR)

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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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Spectroscopy Experiment (NMR)

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Vapor Pressure / Boiling Point
 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),
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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Spectroscopy Experiment (NMR)

Distillation / Boiling Point Measurement
Note: The temperature range you obtain for your boiling
point may be inaccurate for three (3) reasons
1. The atmospheric pressure in the lab may not be:
1 bar (0.98692 atm)
2. The thermometers used in the lab may not
reflect the actual temperature
3. 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
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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Spectroscopy Experiment (NMR)

Simple Distillation Setup
Note: Equipment used in distillation is
expensive. Use care to avoid breakage.
ASK BEFORE YOU ACT!!
Note: The distillation procedure description includes
the equipment setup process and the
steps taken to collect the data
 Equipment
 Heating Plate (from cabinet to right of entrance
door)
 Sand Bath to hold distillation flask
 25 mL Distillation Flask
 Distillation Head
 Thermometer & Thermometer Adapter
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Spectroscopy Experiment (NMR)

Simple Distillation Setup (Con’t)


Condenser (with rubber tubing for cooling water)

Receiving containers

50 mL beaker for low boiling point and high
boiling point impurities.

50 mL Erlenmeyer flask for purified sample
Procedure

Use Ring Stands to support apparatus
Note: Adjust height of apparatus so that sample
receiving containers (small beaker & small
Erlenmeyer flask) can sit on bench about an
inch below where the condensate emerges
from the condenser
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Spectroscopy Experiment (NMR)

Simple Distillation Setup (Con’t)

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Procedure (Con’t)

Use tablets, books, etc., to adjust height of Heating
Plate

Use blue clamp to attach distillation flask to
distillation head

Use blue plastic clamp to secure condenser tube to
distillation head

Tighten tongs of clamp supporting the condenser
tube just enough to ensure condenser tube waterinput/output ports point outward or slightly up

Insert thermometer through adapter so that bulb is
positioned just below elbow opening to condenser
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Spectroscopy Experiment (NMR)

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Typical Distillation Setup (Simple Distillation)
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Spectroscopy Experiment (NMR)

Simple Distillation - Data Collection

Set Hot Plate Heat control unit to 4-5

Use 50 mL beaker to collect low boiling point
impurities

Collect drops of condensate in waste beaker until
temperature rise slows down and becomes constant
Note: It may be necessary to increase the
temperature setting on the controller

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Collect purified sample condensate in 50 mL
Erlenmeyer flask throughout the period when the
temperature is relatively constant
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Spectroscopy Experiment (NMR)

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Simple Distillation - Data Collection

If the temperature begins to increase again, turn the
heat off, remove the Erlenmeyer flask, and collect any
remaining condensate in the waste beaker

Put the purified sample back into the original vial for
further processing

Dispose of any waste organic liquid in the jar in the
hood
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Spectroscopy Experiment (NMR)
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Solubility/Density (Relative to Water)
 Add 2-3 mL Distilled water to a small to medium
test tube
 Add 4-5 drops of the purified distillate or 3 mg of
solid sample to the test tube
 Shake the test tube vigorously
 Note the following:
 Did the sample dissolve, i.e., single clear
solution?
 Determine relative density
 Does the sample float on top of the liquid?
 Does the sample float in the middle of the
liquid or disperse throughout the liquid?
 Does the solid settle to the bottom?
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Spectroscopy Experiment (NMR)
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Refractive Index
Abbe Refractometer (Bausch & Lomb)
 Clean prisms with tissues & methyl alcohol –
BE GENTLE!!
 Do not touch prism with fingers or other hard
objects, use tissues
 Use sufficient drops of sample to cover pridm
surface
 Close hinged prisms together - Gently
 Use switch on left to turn on the light
 Move hinged lamp up into position
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Spectroscopy Experiment (NMR)

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Refractive Index (Con’t)

Rotate coarse and fine adjustment knob on the right
side of instrument until horizontal dividing line (may
not be sharp at first) between the light (Top) and dark
(Bottom) halves of the visual field coincide with the
center of the cross-hairs

Use eyepiece to focus cross-hairs

If horizontal line dividing light & dark areas appears as
a colored band (chromatic aberration), adjust with the
knurled drum knob on the front of the instrument

Press switch on left side of instrument down to make
the scale visible

Read Refractive Index value to 4 decimal places
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Spectroscopy Experiment (NMR)
Refractive Index - Reading the Instrument
 Index of Refraction (ND) is the ratio of the speed of
light in a vacuum (air) vs. the speed of light in a
medium
 The speed of light in a medium increases with
decreasing density and decreases with increasing
density
 Thus, Nd increases with increasing temperature (lower
density) and decreases with decreasing temperature
(higher density)
 Measured values of “nD” are adjusted to a standard
20oC.
 Read the room temperature using the thermometer
attached to the instrument or use the one in your set.
Correction Factor = t * 0.00045 = (Rm Temp – 20) * 0.00045
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(Con’t on next slide)
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Spectroscopy Experiment (NMR)
If temp > 20oC then t is positive - correction factor is added
If temp < 20oC then t is negative - correction factor subtracted
Ex: For an observed value of 1.5523 at 25oC, the correction is:
1.5500
1.5523
1.5550
1.5580
1.5600
Instrument is read to “4” decimal places
The value corrected for temperature is:
nD20 = nDrm temp + t (0.00045)
t = Room Temp - 20.0oC
nD20 = 1.5523 + (25 – 20)*(0.00045) = 1.5523 + 0.00225 = 1.5546
Note: nD at 25oC, as expected, is lower than nD at 20oC
Typical Range of Values for Organic Liquids: 1.3400 - 1.5600
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Spectroscopy Experiment (NMR)

IR Spectrum

Salt Plate Preparation

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 – do not use
water or methanol to clean plates


Solid Samples soluble in Acetone

Dissolve sample in acetone

Evaporate on Salt Plate
Solid Samples not soluble in acetone

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Make Potassium Bromide (KBR) pellet
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Spectroscopy Experiment (NMR)

IR Spectrum

Salt Plate Preparation (Con’t)


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Plate Holder

Place Salt Plate sandwich into base of Plate
Holder

Place Plate Holder top over base and press down
Obtain Spectrum

Instructor - At the start of session enter new
“background”

Student - Insert Plate Holder into beam slot

Select Memory location (X, Y, or Z)
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Spectroscopy Experiment (NMR)

IR Spectrum

Obtain Spectrum (Con’t)

Push “SCAN” button

Verify “No. of Scans” box shows “4”

Press “Execute”
Note: If spectrum bottoms out - remove Cell
Holder and reload Salt Plate with less sample;
rerun Scan again
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Spectroscopy Experiment (NMR)

When the “Scan” box in lower right corner
indicates “Ready”, press “Plot” to produce chart

Remove Cell Holder

Clean & dry Salt Plates with Acetone

Place in desiccator, replace desiccator cap

An NMR Spectrum of your unknown will be handed out
during the laboratory session.

Summarize the principal NMR peaks in the :

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
Results section of the report opposite the procedure
description

The Results Summary Section
Analyze the NMR spectrum as part of the “Analysis &
Conclusion” section
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