A2 CHEMISTRY NOTES: SPECTROSCOPY
Infrared in analysis
We studied the use of IR radiation for analytical purposes at AS level. We know that
 Only ___________ molecules absorb infra-red radiation
 Absorbing IR radiation causes ________ stretching and bond __________.
 Different bonds absorb at ____________ wave lengths (wave numbers cm-1)
 Different bonds have characteristic absorption __________ (weak, medium, strong)
 Different bonds have characteristic _________ of absorption (broad, sharp)
Use the IR table from your data sheet to identify
the six IR spectra above.
1. octanoic acid
2. Cyclohexanol
3. squalane, 2,6,10,15,19,23 hexamethyltetracosane, C30H62
4. 4-methylbenzaldehyde
5. diethyl malonate
6. 2-hydroxybenzaldehyde
7. diphenyl methane
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A2 CHEMISTRY NOTES: SPECTROSCOPY
Mass Spectroscopy
The mass spectrometer is used in determining the structure of molecules. In a mass
spectrometer, molecules are converted to _________ ions; the positive _________ ions
formed then break. This is called _______________.
M+• → X+ + Y•
The fragmentation pattern and the abundance of the fragments is characteristic for
individual molecules. Fragmentation occurs to give a greater incidence of the more ______ ions
2°, 3 carbocations and _______, acylium ions.
Match the compounds with the mass spectrum below:
Pentan-2-one
Pentan-3-one
Pentane
Butan-1-ol
Butan-2-ol
Phenylamine
2
Ethylamine
2-methylbutane
A2 CHEMISTRY NOTES: SPECTROSCOPY
What Is Nuclear Magnetic Resonance (Nmr)?
Nuclear magnetic resonance is concerned with the magnetic properties of certain nuclei
such as ___ and ____. These atoms (nuclei) absorb energy in the radio-wave region of the
electromagnetic spectrum. The exact frequency at which they absorb depends on their
____________ (what they are bonded to, what they are near)
Features of an H-1 NMR spectrum
A simple NMR spectrum looks like this:
The peaks
There are two peaks because there are two different ___________ for the hydrogens in the CH3 group and attached to the oxygen in the _____ group. They are in different
places in the spectrum because they resonate at slightly different _________.
The sizes of the two peaks gives important information about the _______ of hydrogen
atoms in each ___________. It isn't the height of the peaks that matters, but the ratio
of the _______ under the peaks. If you could measure the areas under the peaks in the
diagram above, you would find that they were in the ratio of __: __ (__for the larger peak
__ for the smaller one).
That shows a ratio of ____ in the number of hydrogen atoms in the two environments which is exactly what you would expect for CH3COOH.
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A2 CHEMISTRY NOTES: SPECTROSCOPY
The need for a standard for comparison - TMS
Before we can explain what the horizontal scale means, we need to explain
the fact that it has a ______ point - at the right-hand end of the scale.
The zero is where you would find a peak due to the hydrogen atoms in
tetramethylsilane - usually called ____. Everything else is compared with
this.
TMS is chosen as the standard for several reasons. The most important are:




It has 12 hydrogen atoms all of which are in exactly the _____ environment. That
produces a _______, strong peak (because there are lots of __________ atoms)
and so only a trace amount is needed.
It is volatile and can readily be ____________ off to recover the material.
TMS produces a peak on the spectrum at the extreme ________-hand side. Almost
everything else produces peaks to the _________ of it.
It doesn’t usually interact with other molecules and so does not affect the chemical
_______ values.
The chemical shift
The horizontal scale is shown as  (ppm).  is called the ___________ shift and is
measured in parts per million - ____. A peak at a chemical shift of 2.0 is said to be
____________ of TMS. The further to the left a peak is, the more __________ it is.
Solvents for NMR spectroscopy
NMR spectra are usually measured __________ solvents in which any ordinary hydrogen
atoms are replaced by its _________, __________ - for example, CDCl3 instead of
_______. As there is usually only a small amount (milligrams) of substrate dissolved in
1cm3 of solvent if the solvent were not ___________ the signals of the substrate would
completely _________ by the signals of the solvent.
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A2 CHEMISTRY NOTES: SPECTROSCOPY
Low Resolution Nmr Spectra
In a low resolution spectrum the numbers
against the peaks represent the relative
_____ under each peak.
Interpreting a low resolution spectrum
Using the total number of peaks
Each peak represents a different ____________ for __________ atoms in the molecule.
In the methyl propanoate, CH3CH2COOCH3, spectrum above, there are _______ peaks
because there are _______ different ____________ for the hydrogens.
Using the areas under the peaks
The ratio of the areas under the peaks tell you the ratio of the numbers of __________
in the various ____________. In the methyl propanoate case, the areas were in the ratio
of ______, which is exactly what you want for the two differently placed CH3 groups and
the CH2 group.
Using chemical shifts
The position of the peaks tells you useful things about what groups the various ________
atoms are in. In any exam, you will be given a table of __________ shifts if you need
them. The important shifts for the groups present in methyl propanoate are:
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A2 CHEMISTRY NOTES: SPECTROSCOPY
Some sample questions
Example 1
An organic compound was known to be one of
the following. Use its low resolution NMR
spectrum to decide which it is.
Notice that there are ________ peaks
showing ____ different ___________ for
the hydrogens. That eliminates _________ as
a possibility because that would only give
____ peaks - due to the ___ differently
situated CH3 group hydrogens.
Does the ratio of the areas under the peaks help? Not in this case - both the other
compounds would have three peaks in the ratio of ______.
Now you need to look at the chemical shifts:
Checking the positions of the various hydrogens in
the two possible compounds against the chemical shift
table gives you this pattern of shifts:
Comparing these with the spectrum
means that the substance was
______________________,.
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A2 CHEMISTRY NOTES: SPECTROSCOPY
Example 2
How would you use low resolution NMR to distinguish
between the isomers propanone and propanal?
The propanone would only give ______ peak in its NMR spectrum because both CH3 groups
are in an _________ environments - both are attached to -COCH3.
The propanal would give ______ peaks with the areas underneath in the ratio ______.
You could refer to the chemical shift table above to decide where the peaks are likely to
be found, but it isn't necessary.
Example 3
How many peaks would there be in the low resolution NMR spectrum of
the following compound, and what would be the ratio of the areas under
the peaks?
All the CH3 groups are in __________ so would produce ___ peak. There would also be
peaks for the hydrogens in the CH2 group and the COOH group.
There would be _____ peaks in total with areas in the ratio _____.
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A2 CHEMISTRY NOTES: SPECTROSCOPY
HIGH RESOLUTION NMR SPECTRA
What a low resolution NMR spectrum tells you
Remember:



The number of peaks tells you the number of different __________ the hydrogen
atoms are in.
The _____ of the areas under the peaks tells you the ratio of the numbers of
hydrogen atoms in each of these environments.
The ___________ shifts give you important information about the sort of
environment the hydrogen atoms are in.
High resolution NMR spectra
In a high resolution spectrum, you find that many of what looked like single peaks in the
low resolution spectrum are ______ into clusters of peaks.
You will only need to consider these possibilities:
1 peak
a singlet
2 peaks in the cluster
a doublet
3 peaks in the cluster
a triplet
4 peaks in the cluster
a quartet
You can get exactly the same information from a high resolution spectrum as from a low
resolution one - you simply treat each cluster of peaks as if it were a single one in a low
resolution spectrum. But in addition, the amount of __________ of the peaks gives you
important extra information.
Interpreting a high resolution spectrum
The n+1 rule
The amount of ________ tells you about the number of _________ attached to the
carbon atom or atoms next door to the one you are currently interested in.
The number of sub-peaks in a cluster is one more than the number of ___________
attached to the next door carbon(s).
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A2 CHEMISTRY NOTES: SPECTROSCOPY
So - on the assumption that there is only
one carbon atom with hydrogens on next
door to the carbon we're interested in:
singlet
next door to carbon with __ hydrogens attached
doublet
next door to a ___ group
triplet
next door to a ____ group
quartet
next door to a ____ group
Using the n+1 rule
What information can you get from this NMR spectrum?
Assume that you know that the compound above has the molecular formula C4H8O2.
Treating this as a low resolution spectrum to start
with, there are ______ clusters of peaks and so
_________ different environments for the
hydrogens. The hydrogens in those ____
environments are in the ratio ______. Since there
are __ hydrogens altogether, this represents a
___ group and two ___ groups.
What about the splitting?
The CH2 group at about 4.1 ppm is a _________.
That tells you that it is next door to a carbon with __ hydrogens attached - a ___ group.
The CH3 group at about 1.3 ppm is a ________. That must be next door to a ____ group.
This combination of these two clusters of peaks - one a quartet and the other a triplet - is
typical of an ethyl group, _______. It is very common. Get to recognise it!
Finally, the CH3 group at about 2.0 ppm is a _______. That means that the carbon next
door doesn't have any _________ attached.
So what is this compound? You would also use chemical shift data to help to identify the
environment each group was in, and eventually you would come up with:
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A2 CHEMISTRY NOTES: SPECTROSCOPY
A special case: Alcohols
Where is the -O-H peak?
This is very confusing! The problem is that the
position of the -OH peak varies dramatically
depending on the conditions - for example,
what solvent is used, the concentration, and
the purity of the alcohol - especially on
whether or not it is totally _____.
A clever way of picking out the -OH peak
If you measure an NMR spectrum for an alcohol like ethanol, and then add a few drops of
deuterium oxide, D2O, to the solution, allow it to settle and then re-measure the spectrum,
the -OH peak ___________! By comparing the two spectra, you can tell immediately which
peak was due to the -OH group.
The lack of splitting with -OH groups
Unless the alcohol is absolutely free of any water, the hydrogen on the -OH group and any
hydrogens on the next door carbon don't interact to produce any splitting. The -OH peak is
a _________ and you don't have to worry about its effect on the next door hydrogens.
Equivalent hydrogen atoms
Hydrogen atoms attached to the same carbon atom are said to
be _________. Equivalent hydrogen atoms don't cause any
____________ in the spectrum of the other one. But hydrogen
atoms on neighbouring carbon atoms can also be __________ if
they are in exactly the same environment. For example: These
___ hydrogens are all exactly equivalent. You would get a
___________ with no splitting at all.
The molecule now contains different atoms at each end, the
hydrogens are no longer all in the same ___________. This
compound would give ____ separate peaks on a NMR
spectrum. The high resolution spectrum would show that
both peaks subdivided into _________ - because each is
next door to a differently placed _____ group.
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A2 CHEMISTRY NOTES: SPECTROSCOPY
For each of the compounds A through F indicate the number of distinct groups of
equivalent hydrogens.
A Number of distinct hydrogen groups:
B Number of distinct hydrogen groups:
C Number of distinct hydrogen groups:
D Number of distinct hydrogen groups:
E Number of distinct hydrogen groups:
F Number of distinct hydrogen groups:
Nmr Spectra - The Integrator Trace
What an integrator trace looks like
An integrator trace is a computer generated
line which is superimposed on an NMR
spectrum. In the diagram, the integrator
trace is shown in red.
What an integrator trace shows
An integrator trace measures the relative ______ under the various peaks in the
spectrum. The height gained is proportional to the ____ under the peak or group of peaks.
You measure the height gained at each peak or group of peaks by measuring the distances
shown in green in the diagram above - and then find their _______.
For example, if the heights were 0.7 cm, 1.4 cm and 2.1 cm, the ratio of the peak areas
would be ______. That in turn shows that the ratio of the _____ atoms in the ___
different environments is ______.
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A2 CHEMISTRY NOTES: SPECTROSCOPY
Nmr Spectra Problems
Draw display formula for the following compounds and identify the NMR spectrum for each
of them:
2-heptanone
Methyl propanoate
2-pentanone
3-methyl propan-1-ol
Ethyl propanoate
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Pentanal
4-methyl propan-2-ol
A2 CHEMISTRY NOTES: SPECTROSCOPY
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A2 CHEMISTRY NOTES: SPECTROSCOPY
Features of a C-13 NMR spectrum
The C-13 NMR spectra are simpler than H-1 spectra. Each C in a different environment
give rise to a peak in the C-13 spectrum. If there is more than 1 carbon in an exactly
equivalent environment than they will be represented by a single line.
The C-13 NMR spectrum for ethanol
There are ___ peaks because there are ___
different environments for the carbons. The
carbon in the CH3 group is attached to 3
hydrogens and a carbon. The carbon in the CH2
group is attached to 2 hydrogens, a carbon and
an oxygen.
The C-13 NMR spectrum for a more complicated compound
This is the C-13 NMR spectrum for 1-methylethyl propanoate.
This time there are ___ lines in the spectrum.
That means that there must be ___ different
environments for the carbon atoms in the
compound. Is that reasonable from the
structure?
If two (or more) carbon atoms in a compound have exactly the same ________, only a
single peak appears in the C-13 spectrum.
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A2 CHEMISTRY NOTES: SPECTROSCOPY
COLUMN CHROMATOGRAPHY
Column chromatography is often used to purify compounds made in the lab. It is a process
developed by a Russian botanist to separate the ________ mixtures in plants.
In thin layer chromatography, the ___________ phase is a thin layer of silica gel or
alumina on a glass, metal or plastic plate. Column chromatography works on a much larger
scale by packing the same materials into a vertical glass column.
A mixture looks green but is
in fact a mixture of two
_______ compounds - one
_____, one _____..
This is loaded onto a column.
The tap is opened so that the
__________ (the
________ phase) can flow
through the column,
collecting it in a flask at the
bottom. As the solvent runs
through, you keep adding fresh
________ to the top so that the
column never dries out.
This diagram shows what might
happen over time.
Explaining what is happening
The blue compound is more
_______ than the yellow one - you
can tell this because the blue
compound doesn't travel through
the column very ________. That means that it must ________ more strongly to the silica
gel or alumina than the yellow one. The less ______ yellow one spends more of its time in
the solvent and therefore washes through the column much ___________.
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A2 CHEMISTRY NOTES: SPECTROSCOPY
Gas-Liquid Chromatography
As in all forms of chromatography gasliquid chromatography involves a
___________ phase (in this gas a high
boiling point liquids absorbed onto a solid)
and a ________ phase, a gas such as
helium. How fast a particular compound
travels through the machine will depend
on how much of its time is spent moving
with the gas as opposed to being attached
to the _______ in some way.
Very small quantities of the sample that you are trying to analyse are injected into the
machine using a small syringe. The injector is contained in an oven whose temperature can
be controlled. It is hot enough so that all the sample boils and is carried into the column as
a gas by the helium (or other carrier gas).
How separation works on the column
The column is a long thin tube packed with the _________ phase usually a waxy polymer.
The stationery phase is coated with a high boiling _______. Some molecules may dissolve
in the liquid ____________ phase. The more soluble molecules will spend more of their
time absorbed into the _____________ phase; the less soluble ones will spend more of
their time in the ____.
Retention time
The _____ taken for a particular compound to travel through the column to the detector
is known as its ____________ time. This time is measured from the time at which the
sample is ___________ to the point at which the display shows a ___________ peak
height for that compound. Different compounds have different retention times. High
solubility in the liquid phase means a _____ retention time.
The detector
There are several different types of detector in use. The _______ ionisation detector
and ______ spectrometers are both commonly used.
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