Tutorials
3-methyl-3-buten-1-ol
IR spectroscopy is all about identifying the functional
groups of a chemical compound. Recall that a functional
group is a particular group of atoms in a compound with
a specific connectivity pattern which is responsible for the
characteristic reactions in that compound. Common
examples of functional groups are alkanes, alkenes,
alkynes, and aromatic rings.
If we know the name of the compound, we can identify
its structure and functional groups. Which of the
following structures identifies 3-methyl-3-buten-1-ol?
3-4 options for the structure of this compound (based on
functional groups, NOT position)
Looking at the name 3-methyl-3-buten-1-ol, we
identify several functional groups:
Buten tells us there is a Carbon double bond, or an
alkene in this compound.
The methyl indicates there is at least one
unsaturated carbon (that is with no multiple bonds)
in the compound, an alkane functional group.
Finally, the –ol indicates that there is an alcohol (an
O-H functional group) present in the compound.
Now, suppose that you were not given the name of
this compound, only an unknown substance. How
could you figure out its functional groups?
Chemists use Infrared Spectroscopy to determine functional groups of unknown substances. Many molecules absorb
infrared light, causing the bonds in the molecule to bend and stretch. The IR spectrum of a sample examines to what
degree a compound absorbs infrared light of different wavelengths. Where the sample absorbs IR light, a peak is
produced in the IR spectrum. Because functional groups in a molecule absorb IR light at specific wavelengths, chemists
can identify the functional groups in a molecule by analyzing the various peaks in a molecule’s IR spectrum. (What was
here originally: Where there is low transmittance (or, from another perspective, high absorbance) the IR spectrum is
said to peak. Examining an IR spectrum’s peaks yields much about the chemical’s structure.)
Spectrum with dashed line
which appears when
button is clicked.
Show Regions button
Consider the IR spectrum of 3-methyl-3-buten1-ol. When looking at the spectrum, chemists
divide it into 2 regions. The first region (for
wave numbers greater than 1450 cm-1) is
called the Functional Group Region and
examines the primary functional groups and
stretching vibrations. The second region (for
wave numbers less than 1450 cm-1) looks at
additional data about the compounds
stretching and bending vibrations. This is
called the Fingerprint Region. We will focus on
the Functional group region.
QUESTION Look in the Functional Group Region. How many peaks do you see?
4, 5, 6, 7
There are 5 peaks. Each peak tells us something different.
(Spectrum with 5 peaks highlighted and numbered)
Firstly, consider any peaks around 3000 cm-1. These examine C-H
stretch bonds.
(zoomed in empty spectrum with shaded regions from 2800-3000,
3000-3100, and 3300, labeled with sketches)
QUESTION: Based on the data above, which type(s) of C-H bonds could
be present in this compound? Check all that apply:
Saturated Carbons
Alkene Carbons
Alkyne Carbons
Aromatic Carbons
Saturated carbons (or alkanes) are present because of peak (1). Peak
(2) tells us there is either an alkene or an aromatic present. However,
there is nothing at 3300, so there is no alkyne in this compound.
(Spectrum with colour-coded, numbered peaks highlighted)
To determine whether or not peak (2) is an alkene or an aromatic, we
must look at secondary peaks in both the functional group and
fingerprint regions. Alkenes will have additional peaks at 1600-1670
cm-1 and 675-990 cm-1, while aromatics have additional peaks at 14501600 cm-1 and 690-885 cm-1. They may also have small peaks the
1600-2000 cm-1 region.
QUESTION: Based on this information, does our compound contain an
alkene or an aromatic?
Alkene
Aromatic
The compound contains an alkene. The peak
around 3000 indicates a stretching in the C-H
bonds, while the 1600-1670 is a stretch in the
double bond, and the early peak is an out of the
plane bend in the C-H bonds.
(graphics of stretching/bending with peaks
highlighted)
The next step is to look for C = O bonds. These
are marked by intense peaks in the 1600-1800
cm-1 region. Are there any C = O bonds in this
compound?
Yes
No
No. While there is a peak in that region, it is not
intense enough to be a C=O bond. It is instead
the C=C bond mentioned earlier.
(graph with lack of bond highlighted)
The final stage is to look for either alcohols (O-H
bonds) or amines (N-H bonds). If either of these
are present, there will be a peak in the 33003650 cm-1 region.
QUESTION: Are there any O-H or N-H bonds
present in the compound?
There is a peak present. Because of this, we must note
the phase of the sample (in this case, the sample is a gas).
To determine if it is an O-H or an N-H stretch, we must
first note the position of the peak.
(graph of peak highlighted)
QUESTION: At approximately what wavenumber is the
peak located?
3300-3500 cm-1
3550-3700 cm-1
(Graph of spectrum
with peak highlighted)
Because the peak is located at a
higher wavenumber in this region,
it is likely an alcohol.
(Graph of empty spectrums and
regions in it, also fingerprint
region.)
However, we can confirm that this
is an O-H bond by looking at the
Fingerprint Region. Alcohols have
an additional peak at 1000-1300
because of the C-O bond. Similarly,
amines have a peak at from the C-N
bond at 1030-1230.
QUESTION: Click on the peak which confirms that there
is an alcohol.
Conclusion
From this IR spectrum, we have determined that
3-methyl-3-buten-1-ol contains at least one of
each of the following functional groups:
Alkane, Alkene, and Alcohol.
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