Appendix 3: REVIEW OF THE
ANALYSIS OF IR SPECTRA
This is meant to be a quick review, and not a detailed explanation of how IR works. Remember
there are exceptions in all the general, suggested procedures below. There is a good discussion
in Brown, Foote & Iverson, 4th Ed, pp. 435-489. Common IR absorption frequencies are
provided in Appendix 4.
Suggested sequence of steps:
1. First look at the 3600-3200 cm−1 region. If you see an intense, broad band you probably
have an alcohol. This is the absorption of the alcoholic O-H stretch. You should also see the
C-O absorption band near 1300-1000 cm−1. The O-H stretch may also show up as a sharp,
less intense peak around 3600 cm−1 if the sample is a very dilute solution in a non-hydrogenbonding solvent. (If you know the molecular formula of the sample and it does not contain
oxygen, you probably have a contaminant in your sample, such as water or an alcoholic
solvent. Water absorbs at ~3710 and ~1630 cm−1.)
If your molecular formula contains N, you should consider the presence of −NH2 or −NH-R.
The N-H stretch is also in this region and is usually sharper and less intense than the O-H
stretch. Primary amines show 2 peaks and secondary amines show 1 peak. Remember tertiary amines have no N-H. Broadness and location of peaks depend on the extent of Hbonding.
Also in this region is the C−H of C≡C−H (3300 cm−1), sharp (medium-strong), and could be
masked by any O-H stretch that may be there.
2. Next look at the 3000 cm−1 region. These are the absorptions of the C−H stretch. Peaks
immediately to the left (larger than 3000 cm−1) indicate the presence of C=C and/or
benzene. C−H stretch of C=C-H is around 3100-3000 cm−1 (weak) and are referred to as
the “olefinic C−H stretch.” C−H of benzene is around 3030 cm−1 (medium-weak) and are
referred to as “aromatic C−H stretch.” If all absorptions in this region are below 3000 cm−1
(around 2999-2800 cm−1) this means you have only C−C single bonds. We refer to these as
the absorptions of “aliphatic C−H stretch.”
Also in this region, is the “messy” O−H stretch of carboxylic acid, CO2H. This stretch is a
very distinctive strong, broad peak that spans 2500-3300 cm−1. Since the carboxyl group
contains a carbonyl C=O, you should be seeing the C=O stretch as described below.
SUMMARY:
ALKANE:
ALKENE:
ALKYNE:
AROMATIC:
aliphatic C−H stretch
olefinic C−H stretch
olefinic C−H bending
alkynyl C−H stretch
aromatic C−H stretch
aromatic C−H bending
(sp3)
(sp2)
(sp2)
(sp)
(sp2)
(sp2)
2999-2800 cm−1 (strong)
3100-3000 cm−1 (weak)
1650 cm−1 (weak)
3300 cm−1 (sharp, medium-strong)
3030 cm−1 (medium-weak)
1659-1450 cm−1 (strong)
A-7
A-8
APPENDIX 3: REVIEW OF ANALYSIS OF IR SPECTRA
3. Next look at the 1700 cm−1 region. If you see a strong peak anywhere near 1700 cm−1 there
is a carbonyl, C=O, group. (To be specific, the peak is between 1820-1660 cm−1). That
means you have either a ketone, aldehyde, carboxylic acid, ester or amide.
Acid Anhydride ~1870 & 1740 cm−1 (2 peaks), also C−O stretch at 1175−1050 cm−1
Ester
~1735 cm−1 with range of 1800-1735 cm−1
(also two strong C−O stretches at 1300-1000 cm−1, no O-H peak)
Aldehyde
~1725 cm−1 with range of 1750-1660 cm−1
(also aldehyde C−H stretch, 2 weak peaks at 2850 and 2750 cm−1)
Ketone
~1715 cm−1 with range of 1750−1660 cm−1
(and no aldehyde C−H stretch)
Carboxylic Acid ~1710 cm−1 (dimer if solid or conc. soln)
(also ~1860 cm−1 (monomer, if v. dilute solution)
(and O−H stretch, v. broad 3000−2500 cm−1)
Amides 1°
1690 cm−1 (also 2 sharp N-H peaks at 3500−3450 cm−1)
2°
1680 cm−1 (also 1 N-H peak at 3500-3400 cm−1)
3°
1650 cm−1 (no N−H stretch in 3° amides)
4. If N is in the formula and you don’t see the N−H stretching above 3000 cm−1, you should
consider nitriles and look for the C≡N at 2400-2200 cm−1 (medium, narrow band).
C≡C stretch is also in this region (at 2140−2100 for monosubstituted, 2260-2190 cm−1 for
disubstituted alkynes).
5. If you think you have an alkene it may be possible to fine-tune the analysis to determine
whether you have a cis- or trans- isomer; or whether you have a monosubstituted methylene
(RCH=CH2) or disubstituted ethylene (R2C=CH2) by examining the C−H bending
absorptions in the 1000-600 cm−1 region (See Appendix 4).
6. If you think you have a benzene ring, it may be possible to fine-tune the analysis to
determine what type of substitutions you have on the benzene ring (monosubstituted, odisubstituted, m-disubstituted or p-disubstituted) (See Appendix 4.)
______________________________________________________________________________
Examples of IR Spectra:
This spectrum shows the difference
in absorptions for the alkene C−H
(above 3000 cm−1) vs. alkane C−H
stretch (below 3000 cm−1).
4000
3000
2000
1400
600 cm−1
APPENDIX 3: REVIEW OF ANALYSIS OF IR SPECTRA
A-9
This spectrum shows the typical
aromatic C−H stretch, above
3000 cm−1.
4000
4000
4000
3000
3000
3000
2000
2000
2000
600 cm−1
1400
1400
1400
1000
600 cm−1
600 cm−1
Note the typical broad, strong
band for the alcoholic O−H
stretch, and the strong peak in
the 1300-1000 cm−1 region.
Note the typical carbonyl
stretch around the 1700
cm−1region.
Note the typical broad,
“messy” OH stretch of the
carboxylic acid, overlapping
with CH stretch absorptions.
4000
3000
2000
1400
600 cm−1
Suggested Sources of Information:
In looking up IR spectra in the literature, often you will find more than one spectrum
provided for a particular compound. It is best to look for the ones marked KBr disc, neat, or
liquid film, which means the spectrum was taken of just the sample, without any solvent. If
marked as nujol mull, it means a solid sample had been mixed with nujol, trade name for
mineral oil, which has strong absorptions at 2900-2800 cm−1 and 1500-1300 cm−1. No
information can be derived about absorptions of a sample in these regions. If marked as in
CCl4 solution, the spectrum was taken with the sample dissolved in CCl4 and probably with a
matched cell containing CCl4 as a blank. However, cells are seldom perfectly matched and
absorptions around 1550 and 850-700 cm−1 will be unreliable.
A-10
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•
•
•
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APPENDIX 3: REVIEW OF ANALYSIS OF IR SPECTRA
Appendix 4 of this manual provides a table of IR absorptions listed by bond type.
UCLA Table of IR Absorptions at http://www.chem.ucla.edu/~webspectra/irtable.html
Merck Index (available in the lab)
CRC Handbook (available in the lab)
Aldrich Catalogue (available in the lab)
Aldrich catalogue of infrared spectra (available in the lab)
From the Internet:
• www.chemfinder.com (VERY USEFUL for physical properties)
• “SDBS” website (Great for obtaining IR, NMR, C-13 NMR spectra)
http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng
There is a link to the above website at student.ccbcmd.edu/~cyau1
• http://www.chemexper.com/
• www.scidiv.bcc.ctc.edu/wv/irsp/eir.html
How to Make Use of IR spectra from the “SDBS” website:
http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng
Provided with each IR spectrum from this website is a table such as the one shown below:
The first column stands for wavenumbers in cm−1. The second column is the % transmittance.
The strongest peak would have the SMALLEST % transmittance. The strongest peak in this
spectrum is at 739 cm−1 (with %T = 4). Thus you can easily get the wavenumbers of the major
peaks in the spectra you obtain from this website.