CHEM 201
Sec C3A
Sp 2014
Name: Douglas Miller
Date: 09-23-14
Date of Experiment: 09-17-14
Date of Submission: 09-24-14
Experiment #3: Infrared Spectroscopy identification of Unknowns
Abstract: An unknown sample was identified via its boiling point, index of refraction and
infrared spectra. Boiling point was determined utilizing standard laboratory techniques. While
the index or refraction and IR spectra were determined via the use of an ABBE-3L
Refractometer and a Perkin Elmer 1420 respectively. The identity of the unknown sample was
determined to be cyclohexanone (C6H8O).
Introduction:
Infrared Spectroscopy makes use of the fact that the chemical bonds holding atoms
together are in constant motion. This motion is caused by both the bending and stretching of the
individual bonds within the molecule. By passing a beam of light, within the infrared (IR)
spectrum, through a sample some of the energy will be absorbed and the loss of energy along
with the corresponding frequency can be recorded. The frequency of the absorption corresponds
to particular bonding patterns within the molecule thus allowing us to determine which types of
bonds are present. This in conjunction with other information such as the molecular formula,
boiling point and index of refractions can often allow us to make determinations on the chemical
structure of an unknown samples. A major limitation of IR is its inability to measure bonds that
do not adsorb energy within the IR region, one such example is the C=C bond within ethylene.
In this experiment, an unknown sample, in this case #621, is to be identified via the use
of it boiling point, index of refraction and IR spectra. Boiling point was determined by following
the procedures as outlined in Ref (1) Experiment 1: Part B with a recorded result of 142-143OC.
The index of refraction was determined via the use of an ABBE-3L Refractometer at 20oC with a
record result of 1.4525. An IR spectra was produced on a Perkin Elmer 1420 Infrared
Spectrophotometer with the neat sample held between Sodium Chloride (NaCl) plates (Figure 2).
Experimental Procedure:
This experiment followed the procedures outlined in Ford & Yau (Ref. 1).
Results:
Sample #621 was translucent with a slight yellow color and a tested boiling point of 142143OC. Index of refraction was tested on two separate refractometers with one reading 1.4525
and the other reading 1.425. These reading were confirmed by a second testing with a different
individual arriving at the same results. After discussion with Dr. Evon G. Ford, it was
determined that one of the instruments must be providing a false reading and or be out of
calibration. The IR spectra obtained (see Figure 2 attached) contained the following significant
peaks:
Table 1. IR Spectra Absorption.
Significant Absorption (cm-1)
Likely Source
2960 – 2920 (strong, sharp)
C-C Single Bonds
C-H Stretch (Alkane)
2850 (strong, sharp)
C-H Stretch
Aldehyde Secondary Peak
2760 (weak, sharp)
C-H Stretch
Aldehyde Secondary Peak
1740-1700 (strong, sharp)
C=O Stretch, Aldehyde
Discussion:
Based off the experimental data Cyclohexanone (C6H10O) and Cyclopentanone (C5H8O)
where selected from the table on page 47 (Ref. 1) for comparison, both being aldehydes with
characteristics similar to the unknown sample, Table 2.
Table 2. Comparison of Unknown Sample with Similar Compounds.
Reagents or
Products
Sample #621
Physical Properties
Experimental Values
Literature Values (Ref 2.)
Boiling Point = 142-143oC
Index of Refraction = 1.4525 / 1.425
Cyclohexanone
C6H10O
O
Boiling Point = 154-156oC
Index of Refraction = 1.4507
Cyclopentanone
C5H8O
O
Boiling Point = 130-131oC
Index of Refraction = 1.437
While the boiling point of the unknown sample tested as in between cyclohexanone
(C6H10O) and cyclopentanone (C5H8O), it is more closely approximates that of cyclohexanone.
Also while two different index of refraction were obtained for the unknown the one that is most
similar to one of the above compounds is 1.4525 that corresponding to the 1.4507 for
cyclohexanone. For these reason the recorded index of refraction of 1.425 on the unknown is
being considered erroneous, as the refractometer it was recorded on is likely in need of
calibration.
IR Analysis: The IR spectrum (Figure 2) is consistent with that of Cyclohexanone. The
strong, sharp peaks at 2960 – 2920cm-1 is due to the C-C single bond stretch and the C-H alkene
stretch. The strong, sharp 1740-1700cm-1 peak is consistent with the C=O stretch of an aldehyde
and is confirmed by the secondary aldehyde peaks at 2850cm-1 and 2760cm-1. The lack of any
intense, broad peaks in the 3600-3200 cm-1 region indicate that the sample does not contain the
O-H stretch of an alcohol. Additionally, the N-H stretch associated with this same region is not
present nor is the C=N stretch at 2400-2200cm-1 indicating the absence of N. Also as the first
primary peak is below 3000cm-1 both the C-H stretch of C=C-H and C=C are absent.
An IR Spectra for Cyclohexanone (Figure 1) was obtained from (Ref. 3). It also shows a
strong, sharp absorption in the 2960 – 2920cm-1 region along with the C=O stretch of aldehyde at
1717cm-1 and the secondary peaks at 2867cm-1 and 2750cm-1.
Figure 1. IR Spectra of Cyclohexanone.
Conclusion:
Based on the experiment boiling point, index of refraction and a comparison of the IR spectra
unknown sample #621 is cyclohexanone.
References:
(1) Ford, E. G.; Yau, C. L. CHEM 201 Experiments in Organic Chemistry I Laboratory
8th Ed; Academx: Bel Air, MD, 2014; pp 27-34.
(2) http://www.chemspider.com (accessed 9-14-14)
(3) http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi (accessed 9-19-14)
Answers to Post Lab Questions:
1. Based on the formula C3H8O, the IHD for the compound would be zero, indicating that
there are only single bonds present. Based off that, there are three possible
configurations for the compound, see below:
OH
H3C
H3C
H3C
OH
CH3
O
CH3
In the case of the first two compounds, 1-propanol and 2-propanol (Isopropanol) there is the
presence of an OH alcohol group. In the third case ethyl-methyl-ether, the OH alcohol group is
not present. This is relevant in that according to the IR spectra there is a strong, broad band at
3340cm-1 and a strong band at 1080cm-1 both of which point to the presence of an alcohol.
From here the boiling points need to be examined. In the first case 1-propanol has a listed BP of
97OC, while 2-propanol is listed at 82-83OC. Based off this the compound in question would
H
have to be 2-propanol, with a full chemical structure of:
H
C
H
H
C
O
H
H
H
C
H