Chem XXXXXXXXXX/653: Lab 8: Investigation of Functional Groups by
Infrared Spectroscopy
Procedure
Background Reading
Background information on the theory and techniques used in this experiment can be found in
your textbook. Please read the following from Making the Connections: A How-to Guide for
Organic Chemistry Lab Techniques prior to coming to lab:
- Chapter 2: Spectroscopy Intro and Infrared Spectroscopy; page 65 – 76
In addition, please read the handout on myCourses on infrared spectroscopy.
Introduction
This experiment is designed to introduce you to the laboratory instrumentation technique
infrared spectroscopy. You will split into teams of 2 people to complete this lab; each person
should run at least one sample and the data should be recorded in your lab notebook for all
samples recorded by the group. Note that the report for this lab is not completed as a
team. You will need all data to write your individual report after lab.
During this experiment, you and your team will obtain an IR spectrum of 6 different
compounds (2 will be taken and 4 provided to you): benzophenone, salicylic acid, ethyl
utyrate, cinnamyl alcohol, isopropanol, and hexanal (Figure 1), but they are labeled as
unknowns. Each one of these compounds contains a different functional group with
characteristic bonds. Because IR spectroscopy reads the energy required to stretch or bend
a bond (see IR reading and handout for more info), the energy needed to stretch these
different bonds will result in a peak on the IR spectrum. The frequency in which this bond
vi
ated is proportional to the energy required to stretch the bond, so, the higher the energy,
the higher the vi
ational frequency displayed in the IR spectrum.
Figure 1. Compounds Investigated by IR Spectroscopy
When it comes to ca
onyl functional group (C=O) as in ca
oxylic acid, ketone, or ester,
you can reason the vi
ational frequencies by estimating the strength of that double bond
using resonance theory. Every compound containing a ca
onyl ca
on has at least one
esonance structure (Figure 2). The actual compound is a composite of both of these
structures. You can think of this C=O as a partial double and a partial single bond; the
strength of this bond depends on the importance of both of these structures. If both are of
equal importance, the ca
on-oxygen bond would be halfway between a single bond and a
double bond (it has 50% single bond character and 50% double bond character). If a C=O
has more double bond character, it will take more energy to stretch that bond.
XXXXXXXXXXFigure 2. Resonance structures of a ca
onyl ca
on
Depending on what else is su
ounding the C=O, more resonance structures can exist
(Figure 3). If more resonance structure exists where the ca
on-oxygen bond is a single
ond, this increases the contribution of those ca
on-oxygen single bond resonance
structures and the overall structure will have less double bond character.
Figure 3. Resonance structures of an amide
Each of the unknowns will display a different combination of peaks, from that you can
determine which unknown number co
esponded to which compound.
Procedure:
Working in a team of 2 people, obtain an infrared spectrum of unknown number 1 and
number 2. Print two copies of each. Make sure to record any notes on how to run the IR in
your lab notebook afterward (past tense!!).
In your notebook, write down a list of all-important peaks present in each IR spectrum for
unknown 1 and 2 (wavenumbers? sharp or
oad? strong or weak? bond assignment?).
Keep the printouts of the IR spectrum you obtained – you’ll need to label them and upload
them with your report.
Obtain an IR spectrum handout for the remaining unknown 3 to 6 to work on and
identify the important peaks (wavenumbers? sharp or
oad? strong or weak? bond
assignment?) and match each spectrum to the co
esponding unknown you identified (see
eport form).
Reminder-Check-out today
Please follow your TA’s instructions regarding check-out.
Chem XXXXXXXXXX/653: Lab 8: Investigation of Functional Groups by
Infrared Spectroscopy
Laboratory Report
Laboratory Notebook Setup
Add the following information to your laboratory notebook prior to coming to lab:
(1 point) Descriptive title of experiment
(1 point) Objectives
(1 point) Techniques
(3 points) Structures
Provide the structures and names of the six compounds you will investigate by IR
spectroscopy.
(6 points) Table of Reagents
Compound Functional Group(s)
Benzophenone
Salicylic acid
Ethyl Butyrate
Cinammyl alcohol
Isopropanol
Hexanal
(5 points) Laboratory Notebook
Follow the requirements in the notebook guidelines. Notebook pages should contain enough
information that you or a peer could easily repeat the experiment. Remember to record
observations. Remember to include data for all samples – not just the ones you ran.
Chem XXXXXXXXXX/653 Report Form
Lab #8 – Infrared Spectroscopy Name
Date TA
(12 points) Relevant Peak Information
List relevant peaks from each IR (example: 2987 cm-1), describe them (sharp or
oad, weak
or strong: for example, sharp/strong), and assign the bond type responsible (example: C-H).
Do not try to list every single IR peak. Choose those that are most relevant to the
identification of your compound.
Unknown 1:
Important IR Peaks Description Assignment
Unknown 2:
Important IR Peaks Description Assignment
Unknown 3:
Important IR Peaks Description Assignment
Unknown 4:
Important IR Peaks Description Assignment
Unknown 5:
Important IR Peaks Description Assignment
Unknown 6:
Important IR Peaks Description Assignment
(6 points) Unknown Identification
Write the compound name to identify each compound:
Unknown 1: Unknown 2:
Unknown 3: Unknown 4:
Unknown 5: Unknown 6:
Formal Report Sections
These sections must be typed in a double-spaced document. Points listed below are an
outline of what to include in each section and should not be answered in a question/answer or
ullet point format. Instead, formulate answers in complete sentences that flow together, as
you are writing a paper. Please, see the handouts on myCourses for additional information on
writing proper laboratory report sections.
Introduction
(6 points)
a) Write a
ief introduction on the theory behind IR spectroscopy- Explain what the
instrumentation technique is measuring and the theory behind it (how does it work?).
What are the peaks telling you? What is transmittance? What does 100%
transmittance mean? Your textbook and the handout on myCourses are great sources
for this information. You can also use notes from lab lecture or any other reputable
sources.
(2 points)
) Write a
ief introduction of the purpose of this experiment; what are you trying to
accomplish.
Results and Discussion
(5 points)
An infrared spectrum shows energy abso
ed by a molecule that co
esponds to the energy
needed for a bond in that molecule to vi
ate. Explain why the ca
onyl stretch (C=O) varied for
the molecules (benzophenone, salicylic acid, ethyl butyrate, hexanal) used in this part of the
experiment. Why do certain compounds show a higher vi
ational frequency for C=O, while
others show a lower vi
ational frequency? Clearly explain your reasoning.
(5 points) List of References
List all references used for your introduction using the ACS reference style. (See the handout
on myCourses for more information).
(4 points) Laboratory Technique
Points for good lab technique. You are not required to complete anything here; the TA will
give your score on the ru
ic.
(5 points) Formatting/Grammar
Points for proper writing style and formatting. You are not required to complete anything here;
the TA will give your score on the ru
ic.
(6 points) IR Spectrum
Upload the two IR spectrum you obtained for unknown #1 and unknown #2 in lab with all
elevant peaks labeled with their bond assignments.
Post-Lab Questions
(12 points)
1. Draw the structure of the compound from the list below that would give each IR (next to
the spectrum) and label the important (numbered) peaks with the type of bond that
co
espond to them. Note that only 4 out of 5 of those structures match the IR spectra,
a.
.
c.
d.
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