1800
3000
Great
Interpretive
Power
Wimpy Interpretive Power
Interpreting
IR spectra
All IR bands are equal,
but some are more
equal than others.
Moving down the chart
one at a time, matching
bands as you go, is not
a useful strategy.
This is the first region
you should look at, and
there are 4 different
shapes of bands that
you may see here.
In the C=O region, it’s not
the shape, but the exact
location that reveals
the identity of the fg.
Lastly, You should look at
these features (but only
after analysing the major
bands in the previous
panes)
Saturated and
unsaturated C-H
can be present
at the same time.
Notes
** If you’re not sure what the difference
between an acid and an alcohol is, do the
quick “Functional Group IDentification”
exercise.
** ...if you have one FG. Two 2 amine
would look like a 1 amine.
Instead, let’s arrange
them by their
interpretive power.
A smooth tongue
means an alcohol.
Vampire Fangs Means
a primary amine...
Hairy Beard is a sign for
a carboxylic acid.
-C C H
-N H
R
-COO H
-N H
H
-O H
Anhydrides
Esters
aldehydes
Ketones amides
acids
165017001750
acid chlorides
Unsaturated
C-H stretch
Mono
Mono ortho meta para
gem (1,1-)
di-substituted
cis- trans-
C-H
O
R
3000cm
-1
saturated
C-H stretch
Jon - jkwchui@uvic.ca
one knock-out fang
means a 2 amine,
or a terminal alkyne.
o
o
o
3400 3200
...and if you don’t
see any band here,
You most likely
don’t have any of
these functional
groups (fg)
Note that the exact
position depends on the
surrounding environment:
Conjugated
at C=O
strain, or
Conjugated
at -OIt
must
have rained.
Puddles and
worms...
Inference
Inference
Observation
Observation
At ~2200cm , we
find triple
bonds:
C C, and
C N
-1
Hint: use 3300Cm to
distinguish terminal
and internal alkynes.
usually sharp
(can be
smallish)
C-H
stretch
alkene/aromatic
substitution pattern
can be seen in the
c-H bend regions.
Lastly, a peak at
2700cm , together
with a C=O, is indicative
of an aldehyde.
we looked at these last
because they can be
ambiguous to interpret, or
they have only a narrow
niche of usefulness.
-1
An IR band
of this size
and shape...
Probably That
functional group.
1800
1810
1760
1735
1725
1715
1710
1690
+30cm
-1
-30cm
-1
700,
690
strong
990,
910
~750
~825
2700cm
-1
880,
780,
690
~900
(strong)
~700
(weak)
~970
(strong) ketones have no C-H
in them and can have
no 2700 C-H stretch
peaks.
C=O
stretch
O
O
O
O
O
O
O
O