Transcript Hints on Column Chromatography

Lecture 4
• 13C NMR: DEPT
• IR Spectroscopy:
- How it works
- Interpretation of spectra
Due:
Lecture Problem 2
Determine the structure of this unknown (MF is C8H9Cl)
13C
NMR Correlation Chart
N-H
1H
O-H
NMR Correlation Chart
X-CH
O-CH
COCH
CHO
CH3
CO2H
C CH
C=CH
ArH
CH, CH2
12.0
11.0
10.0
9.0
8.0
7.0
6.0
5.0
4.0
Chemical Shift,  (ppm)
3.0
2.0
1.0
0.0
DEPT-NMR
(Distortionless Enhancement by Polarization Transfer)
• Distinguishes between CH, CH2, and CH3 carbons
13C
NMR:
broadband
decoupled
(normal)
13C
NMR:
DEPT-90
13C
NMR:
DEPT-135
MRI: A Medicinal Application of NMR
Magnetic Resonance Imaging:
• MRI Scanner: large magnet; coils to excite
nuclei, modify magnetic field, and receive
Signals
• Different tissues yield different signals
• Signals are separated into components by
Fourier transform analysis
• Each component is a specific site of origin
in the patient  a cross-sectional image of
the patient’s body
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
MRI showing a vertical
Cross section through a
Human head.
How it works:
http://en.wikipedia.org/wiki/Magnetic_resonance_imaging
• Most signals originate from hydrogens of
Water molecules
• Water is bound to different organs in different way  variation of signal among
organs & variation between healthy and diseased tissue
MRI: A Medicinal Application of NMR
Some Magnetic Resonance Imaging Uses:
• Detailed images of blood vessels
• Examine the vascular tree
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
• Differentiate intracelluar and
extracelluar edema  stroke patients
• Detecting cancer, inflammation, tumors
Current research:
MRI showing a vertical
Cross section through a
Human head.
http://en.wikipedia.org/wiki/Magnetic_resonance_imaging
• 31P nuclei analysis: investigate celluar
metabolism (ATP and ADP)
Spectroscopy
1H
NMR:
Determine bond connectivities/pieces of a structure, whole structure
13C NMR:
Types of carbons (DEPT)
IR:
Determine the functional groups present in a structure:
-OH, C=O, C-O, NH2, C=C, CC, C=N, CN
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)
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
Stretches are more noted than
bends
Correlation Chart
Specific bonds absorb specific IR radiation and signals will appear within certain
wavenumber ranges (similar to NMR).
Note: O-H stretches are broader than N-H stretches
N-H Stretches:
1° Amines (RNH2) has two peaks
2° Amines (RNHR) has one peak
3° Amines (NR3) has no peaks
IR Correlation Chart
Specific bonds absorb specific IR radiation and signals will appear within certain
wavenumber ranges (similar to NMR).
Correlation of B ond Stretching and IR Absorption (See also Correlation Chart & Table in Lab
G uide)
Wavenumber Range (cm-1)
Type of B ond
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
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)
IR spectrum of
hexanoic acid
Functional Group Region:
1550-4000 cm-1
Most useful portion
Fingerprint Region:
400-1550 cm-1
More difficult to interpret
An IR Spectrum
O-H stretches are broad due to H-bonding.
Sample Problem 1
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:
O
O
H
benzaldehyde
OH
Benzoic acid
2. Determine the main differences that would be seen in IR. Use correlation
chart.
Sample Problem 2
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 