Transcript 1H NMR - INTEC Chemistry Blog

The slides used in this presentation borrow
heavily from the great downloadable:
Chapter 13
Nuclear Magnetic
Resonance Spectroscopy
Organic Chemistry, 5th Edition
L. G. Wade, Jr.
Jo Blackburn
Richland College, Dallas, TX
Dallas County Community College District
2003, Prentice Hall
1 GHz machine
in France!
Wow !
4 Essential points Re: NMR
1. Number of peaks = No of H environments
(a peak may be split into many smaller peaks called a splitting pattern)
2. Position of the peak depends of ‘chemical’ environment
(actually, magnetic environment).
3. Area under the peak = relative number of H in that
chemical environment
4. Splitting patters reveals info about adjacent H’s
The NMR Spectrometer
=>
The NMR output
Two H
environments,
so two peaks
TMS
reference
=>
CH3
H3C
Si CH3
Tetramethylsilane
CH3
• TMS is added to the sample. Functions as a reference
• Since silicon is less electronegative than carbon, TMS
protons are highly shielded. Signal defined as zero.
• Organic protons absorb downfield (to the left) of the
TMS signal.
=>
1) Number of Signals
Equivalent hydrogens have the same chemical
shift. Each H in the methyl group at ‘a’ are
equivalent to each other. (etc)
=>
2) Peak position (location of signals)
shielding / deshielding
• More electronegative
atoms deshield adjacent
protons more and give
larger shift values.
• (Effect decreases with distance)
• Additional electronegative
atoms cause increase in
chemical shift
=>
Delta Scale
 OLD MACHINE
 NEWER MACHINE
This slide basically says peak position is independent of electromagnetic frequency
=>
Typical Values – you will have to
use the ones in your databook
=>
e.g. Ethanol
All
Protons
relatively
shielded.
So,
Peaks
lie
towards
‘upfield’
region
upfield
Take note: The peaks are split.
(discuss later)
Aromatic Protons appear
‘downfield’, 7-8
=>
Vinyl (terminal alkenes) Protons,
5-6
=>
Acetylenic (CC) Protons, 2.5
=>
Aldehyde Proton, 9-10
Electronegative
oxygen atom
=>
Carboxylic Acid proton, 10+
9.3 + 2.5 = 11.8 
=>
Aromatic H’s = 7-8 ppm (downfield)
Notice the splitting:
Aromatic H’s = complicated multiplet,
Aliphatic H’s = splitting more clear
O-H and N-H Signals
• Chemical shift depends on concentration.
• Hydrogen bonding in concentrated solutions
deshield the protons, so signal is around 3.5
for N-H and 4.5 for O-H.
• Proton exchanges between the molecules
broaden the peak.
=>
3) Area intensity of signals
• The area under each peak is
proportional to the number of protons.
• Shown by integral trace.
=>
Summary
• Flv Video: Instrumentation
How many (envionment) types of
hydrogen?
When the molecular
formula is known,
each integral rise can
be assigned to a
particular number of
hydrogens.
Spin-Spin Splitting/Coupling
• Nonequivalent protons on adjacent carbons have
magnetic fields that may align with or oppose the
external field.
• This magnetic coupling causes the proton to
absorb slightly downfield when the external field is
reinforced and slightly upfield when the external
field is opposed.
• All possibilities exist, so signal is split. =>
Range of Magnetic
Coupling
• Protons on adjacent carbons normally will couple.
• Equivalent protons do not split each other.
• Protons separated by four or more bonds will not
couple.
• Protons bonded to the same carbon will split each
other only if they are not equivalent. (e.g CH2
next to a C*-H)
=>
Simple spectra
Stolen from http://www.organicchemistryreview.com/spectroscopy.html
Stolen from http://www.organicchemistryreview.com/spectroscopy.html
Doublet: 1 Adjacent Proton
=>
Triplet: 2 Adjacent Protons
=>
No splitting – no adjacent H
1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
=>
ethanol
The N + 1 Rule
If a signal is split by N equivalent protons,
it is split into N + 1 peaks.
=>
Pascal's triangle
Helps you interpret the
splitting Pattern.
OR gives the
splitting pattern
(prediction)
http://www.lincoln.k12.nv.us/alamo/high/Departments/Math/Pascal/Pascal's_Triangle_Webquest.html
Splitting for Ethyl Groups
=>
Splitting for
Isopropyl Groups
=>
http://www.absoluteastronomy.com/topics/Proton_NMR
Double bond equivalents
• Satd organic molecule has 2n+2 H’s for every C.
• Each double bond ‘takes 2 H away’ No H’s =2n e.g.
ethene C2H4
• A ring also ‘takes 2 H’s away’ cyclehexane C6H12
From molecular formula…
•
•
•
•
•
•
For every halogen, add 1 H
For every N take away 1 H
For every O, do nothing.
Ethanol = C2H5OH =>C2H6
Compare it to satd formula for 2C’s No difference therefore molecule
C2H5OH has no double bond equivalents.
Ethanal = ? Methylbenzene = ?
•
1. NMR spectroscopy - Introduction to proton nuclear magnetic resonance James Mungall - flv.avi
•
2. NMR spectroscopy - Integration - James Mungall - flv.avi
•
3. NMR spectroscopy - Chemical shift and regions of the spectrum - James
Mungall - flv.avi
•
4i. NMR spectroscopy - Coupling - James Mungall - flv.avi
•
4ii. NMR spectroscopy - Coupling - James Mungall - flv.avi
•
5i. NMR spectroscopy - Examples of NMR spectra - James Mungall - flv.avi
•
5ii. NMR spectroscopy - Examples of NMR spectra - James Mungall - flv.avi
•
•
MRI
Dangers of MRI
Functional group region >1400 cm-1
Stolen from http://www.organicchemistryreview.com/spectroscopy.html
Answer?
Sure?
2-methylpropan-1-ol
http://science.widener.edu/svb/nmr/seminar/isobutanol.html
Quiz
High Performance (High Pressure)
Liquid Chromatography (HPLC)
HPLC schematic
http://www.idex-hs.com/support/upchurch/i/hplcDiagram.gif
http://www.goehler-hplc.de/images/parts.jpg
The
column
High Performance Liquid
Chromatography (HPLC)
(High Pressure)
High pressure gives separation much faster than
say gravity based chromatography (paper or
column chromatography)
Detector uses UV light to detect presence of
chromophores
High Performance Liquid
Chromatography (HPLC)
(High Pressure)
• Stationary phase in the form of a steel
encased column (size of a straw) = silica
gel, Al2O3 or C18 hydrocarbon – giving
different polarities.
• Mobile phase = solvent
• Area under peak = measure of the relative
abundance of the compound
• Add a standard of known concentration to
get it’s peak area.
• The same species have the same
retention time (if all other factors are kept
constant e.g. pressure, solvent, column
type)
• Can separate chiral compounds 
Amino acids are poor at absorbing UV so they
Have a UV chromophore bonded to them for
the purpose of being HPLC’d
UV detector set at λ=338nm
http://www.biocompare.com/Articles/ApplicationNote/1508/ANALYSIS-OF-AMINO-ACIDS-BY-REVERSED-PHASECHROMATOGRAPHY-WITH-PRECOLUMN-DERIVATISATION-AND-UV-VISIBLE-DETECTION.html
http://www.aldbot.com/HPLC-Hawaii.gif
Sample will pass quickly through
the column IF:
• It has similar polarity to mobile phase
• Stationary phase in the column has a large
particle size.
• Pressure of HPLC is increased
• Temp not really a factor as HPLC tends not to
have a heater added
• Column is short.
• IN GENERAL LONGER COLUMN TIMES ARE
BETTER (GIVE BETTER SEPARATION IN
MISTURES SO REVERSE THE ABOVE
CONDITIONS.