Lecture 11
IR Theory
Next Class:
• Lecture Problem 4 due
• Thin-Layer Chromatography
This Week In Lab:
Ch 6: Procedures 2 & 3
Procedure 4 (outside of lab)
Next Week in Lab:
• Ch 7: PreLab Due
• Quiz 4
• Ch 5 Final Report Due
Spectroscopy
NMR (Nuclear Magnetic Resonance Spectroscopy):
•Uses radio waves (electromagnetic radiation)
•Interacts with sample’s nuclei in the presence of a magnet
•Effect: nuclei flip and relax (known as resonance)
1H
NMR:
Determine bond connectivities/pieces of a structure, whole structure
IR (Infrared Spectroscopy)
•IR radiation
•Interacts with molecule as a whole
•Effect: bond vibrations within molecule
IR Use:
Determine the functional groups present in a structure:
-OH, C=O, C-O, NH2, C=C, CC, C=N, CN
An IR Spectrum of Hexanol
IR Spectroscopy
Main Use: To detect the presence or absence of a functional group (specific
bonds) in a molecule
How It Works:
1. Bonds vibrate freely at specific wavelengths (wavenumbers)
2. Want to cause the bonds to increase the magnitude of this vibrational
frequency
3. Subject compound to IR radiation, 4000-625 cm-1
cm-1 is the unit for wavenumber (n) (The numbers of waves within 1 cm)
n is directly proportional to energy (unlike wavelength)
4. Bonds absorb energy equal to their natural vibrational energy - it is
quantized. This absorption of energy causes a change in dipole moment
for the bond.
5. Upon absorption, bonds stretch and/or bend; the IR measures this
absorption.
Vibrational Modes of Bonds
Correlation Chart
Specific bonds absorb specific IR radiation and signals will appear within certain
wavenumber ranges (similar to NMR).
Your lab notebook also has an IR correlation chart.
Correlation Chart
Specific bonds absorb specific IR radiation and signals will appear within certain
wavenumber ranges (similar to NMR).
Correlation of Bond St retching and IR A bsorption (See also Correlation Ch art & Table in Lab
Gu ide)
Wavenumber Range (cm-1)
Type of Bond
Group
Family of Compounds
Single Bonds
—C— H
Alkanes
2850-3300
=C— H
Alkenes, aromatics
3000-3100
C—H
Alkynes
3300-3320
O—H
Alcohols
3200-3600
N—H
Amines
3300-3500
C—O
Ethers, Esters, Alcohols
Carboxylic Acids
1330-1000
C=C
Alkenes, aromatics
1600-1680
C=O
Carbonyls
1680-1750
Aldehydes, ketones
1710-1750
Carboxylic acids
1700-1725
Esters, amides
1680-1750
C=N
Imines
1500-1650
CC
Alkynes
2100-2200
CN
Nitriles
2200-2300
Double Bonds
Triple Bonds
IR spectrum of
hexanoic acid
O
OH
Funct ional group r egion:
155 0-4 000 cm- 1
Most use ful porti on o f
IR spect rum
Evaluat e t his p orti on fo r
your spect ra l unknown
Fingerpr int reg ion:
40 0- 1550 cm- 1
More diff icult to i nte r pret ; may
cont ain useful i nf or mati on
Different bond stretches & what their signals look like in IR:
A: O-H stretch (strong, broad)
C: C-H stretch (strong, sharp)
E: CC or CN stretch (sharp)
F: C=O stretch (strong, medium to sharp)
G: C=C stretch (sharp)
J: C-O stretch (strong, medium)
K: C-X stretch (sharp)
Lab Guide Problem 16.1
A sample is known to have the molecular formula C4H10O and to be one of
two constitutional isomers, either t-butyl alcohol or isopropyl methyl
ether. What is the structure of the unknown?
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Lab Guide Problem 16.2(a)
Indicate how the following pairs of compounds could be distinguished using
characteristic IR peaks:
(a) Benzaldehyde (C6H5O) and benzoic acid (C6H5COOH)
1. Consider each structure:
2. Determine the main differences that would be seen in IR.
Lab Guide Problem 21.20
An unknown oxygen-containing compound is suspected of being an alcohol,
a ketone, or a carboxylic acid. Its IR spectrum shows a broad strong peak
at 3100-3400 cm-1 and a strong, sharp peak at 1700 cm-1. What kind of
compound is it?
Consider what type of bonds appear in the ranges given. Refer to
correlation chart.
Broad peak at 3100-3400 cm-1 
Strong, sharp peak at 1700 cm-1 
Lab Guide Problem 21.21
A compound with molecular formula C4H8O shows no absorption in the IR
region near 1700 cm-1 or 3400 cm-1. What can you deduce about its
structure? Propose one possible structure.
What can you deduce about its structure?
Propose one possible structure.
1. Calculate HDI.
2. Give possible structure(s):