Download Guidelines

CH 242: Organic Chemistry
Winter 2008
Maazouz
PCC Sylvania
FTIR Tutorial
Work in pairs to answer the following questions regarding FTIR. Your Name___________________________________
Partner’s Name___________________________________
Organic chemists frequently synthesize new compounds in the laboratory. In order to identify the products they must use
various techniques to confirm the identity. In the CH241 lab you learned how to use physical properties (melting points,
boiling points, refractive index, etc.), gas chromatography, and infrared spectroscopy to confirm the identity of various
compounds. During this tutorial you will learn the theory of FTIR and a more detailed description for how to analyze an
FTIR spectrum.
Use the Ch12 lecture notes posted in the lab folder to answer the following questions.
1. Absorption spectroscopy refers to experimental techniques that measure changes in a molecule when matter
interacts with electromagnetic radiation. For example, high energy x-rays are used to eject electrons from the
molecules to create ions, so the molecular effect is ionization.
a. What molecular effect occurs when microwaves are used?
b. What molecular effect occurs when infrared radiation is used?
c.
2.
What molecular effect occurs when radio waves are used as in Nuclear Magnetic Resonance (NMR)
spectroscopy?
What do the letters in the acronym FTIR represent?
3. What is the fundamental difference between conventional IR and FTIR?
4. Name at least four advantages of using FTIR over conventional IR spectroscopy?
5. What is the requirement for a molecule to be IR active?
To answer the following questions go to the following IR tutorial website:
http://orgchem.colorado.edu/hndbksupport/irtutor/tutorial.html
6. What happens to a molecule when it absorbs infrared radiation?
7. What information about the sample is obtained with infrared spectroscopy?
CH 242: Organic Chemistry
Winter 2008
Click on the link IR Spectroscopy, Theory.
Maazouz
PCC Sylvania
8. What is the wavelength region (in m) of the mid-IR region of the electromagnetic spectrum? Show your
calculations starting with the wavelengths in cm.
9. What is a wavenumber?
10. Starting with the wavelength in cm calculate the wavenumber range in a typical IR Spectrum. (You must show
your calculations.)
11. Convert the minimum and maximum wavenumber values into units of Joules per photon.
12. As the wavenumber increases, the energy of molecular vibration increases or decreases?
Refer to the following IR Spectrum for methanol.
13. The highest energy for the IR Spectrum is on the left or right. Explain.
14. What is the meaning of the % Transmittance label on the y-axis ? What is the difference between 0% T and
100% T?
CH 242: Organic Chemistry
Winter 2008
15. What causes a peak in the IR Spectrum?
Maazouz
PCC Sylvania
16. Why is the spectrum typically split into two regions 4000-1300 cm-1 and 1300 to 400 cm -1? What is the name of
the second region?
17. If you did not take CH241 at PCC-Sylvania in F2007, then complete the table provided by the instructor. You
need to work through the tutorial by clicking on each link on the left menu bar. If you took CH241 last term, then
review the links on the left-hand menu bar before proceeding.
Go to the “How to analyze an IR spectrum” link on the left-hand menu bar.
18. Summarize the steps that must be taken when analyzing a spectrum of an unknown compound.
CH 242: Organic Chemistry
Maazouz
Winter 2008
PCC Sylvania
19. Alkanes, Alkenes, Alkynes. Stronger bonds generally absorb at higher frequencies because of their
greater stiffness. Place the C-C, C=C, and C≡C in order of increasing vibrational frequency without looking at the
expected wavenumbers.
20. Obtain the expected stretching frequencies for C-C, C=C, and C≡C. Do they agree with the order you placed
them in question 19? Why or why not?
21. Alkanes, alkenes, and alkynes have characteristic C-H stretching frequencies. The amount of energy required to
stretch this vibration depends on the amount of s character provided by the hybrid carbon atoms. More energy is
required the larger the s character, due to the stiffness of an s atomic orbital. Place the C-C-H, C=C-H, and C≡CH bonds in order of increasing vibrational energy for C-H without looking at the expected wavenumbers.
22. Obtain the expected stretching frequencies for C-C-H, C=C-H, and C≡C-H. Do they agree with the order you
placed them in question 21? Why or why not?
23. Assign each of the following spectra as an alkane, alkene, or alkyne. In grammatically correct English sentences,
explain how you assigned these spectra.
24. On each spectrum above label the peaks that correspond to the following stretching modes: C-H, =C-H, ≡C-H,
C=C, and C≡C.
CH 242: Organic Chemistry
Maazouz
Winter 2008
PCC Sylvania
25. The following spectrum is for 4-octyne. Draw the chemical structure of 4-octyne. Is the spectrum consistent with
an alkyne? Explain why or why not.
Alcohols and Amines
26. The O-H bonds of alcohols and the N-H bonds of amines are strong and stiff. Do you expect the vibrational
frequencies to be lower or higher compared to C-H bonds? Explain.
27. The O-H and N-H bonds absorb IR over large energy range, making the peaks broad. Why do you think this
occurs? (Hint: think about intermolecular forces).
28. What is the difference between a primary, secondary, and tertiary amine? How can these be distinguished in an
FTIR spectrum?
29. Identify each of the following as an alcohol or an amine. If the spectrum is an amine, determine if it is a primary,
secondary, or tertiary amine.
30. On each spectrum above label the peaks that correspond to the following stretching modes: O-H, N-H, C-H, C-O,
and C-N.
CH 242: Organic Chemistry
Maazouz
Winter 2008
PCC Sylvania
Carbonyl Compounds
31. What is a carbonyl compound? Which functional groups contain the carbonyl group? Give the corresponding
wavenumber range for the carbonyl stretch for each functional group.
32. How does the carbonyl group stretching frequency compare to the C=C stretching frequency? Why?
33. Go to the Carboxylic Acid link on the left-hand menu bar. Then click on the carbonyl stretching frequencies link.
a.) What is the difference between saturated and unsaturated as used for esters, aldehydes, and ketones?
b.) Does the C=O stretch occur at higher energies for the saturated or the corresponding unsaturated
compounds? Why?
c.) What is the difference between an α and a β carbonyl?
34. Identify each of the following as ketone, aldehyde, carboxylic acid, or ester.
35. On each spectrum above label the peaks that correspond to the following stretching modes: O-H, C-H, C=O, and
C-O.
CH 242: Organic Chemistry
Maazouz
Winter 2008
PCC Sylvania
36. Compare the spectra for carboxylic acids, alcohols, and amines. What differences do you see?
37. Why does the carboxylic acid have a very broad peak in the 3300-2500 cm-1 range?
38. Based on your IR knowledge, place the nitrile bonds C-N, C=N, and C≡N in order of increasing energy.
39. (Circle one) The carbon-nitrogen frequencies are larger than or smaller than the corresponding carbon-carbon
vibrational frequencies. Explain your answer.
40. The following spectrum is for isobutyronitrile (CH3)2CHC≡N. Assign the C-H and C≡N peaks.
41. The last functional group that you will examine today is the amide, another carbonyl compound that is bonded to
nitrogen: O=C-NH2, O=C-NHR, and O=C-NR2. The IR absorption frequency of the amide N-H group is sensitive
to the strength of the hydrogen bonding. Therefore, IR spectroscopy provides structural information about peptide
and protein conformations, which are stabilized by the hydrogen bonding of amide groups. The carbonyl groups
of amides absorb at particularly low IR frequencies; about 1640 to 1680 cm -1. The very low frequency of the
amide carbonyl group might be mistaken for an alkene C=C stretch. Compare the two spectra below, the one on
the left is butyramide (CH3CH2CH2C(=O)-NH2) and the one on the right is 1-methylcyclopentene. Assign the
following peaks in each spectrum C-H, N-H, C=O, C=C.
42. Find at least three differences in the functional group region when comparing the two spectra.
CH 242: Organic Chemistry
Maazouz
Winter 2008
PCC Sylvania
43. For each spectrum below, write down and assign the important frequencies and your proposed functional groups.
Each spectrum may contain more than one functional group. Briefly explain how you assigned the functional
groups.
a.)
b.)
c.)
CH 242: Organic Chemistry
Winter 2008
Maazouz
PCC Sylvania
d.)
e.)
44. Did this exercise help you to learn more about FTIR and how to analyze a spectrum?
45. How would you improve this for the future?