Interpretation – IR spectra
Objectives
After this session
students will be able to
• Interpret
the IR spectra of certain classes of organic compounds such as hydrocarbons,
alcohols, esters, acid chlorides, amines, nitro compounds etc.
Interpretation – IR spectra
• No
rigid rules for interpreting IR spectrum
• IR
spectra -interpreted from empirical comparison of spectra and extrapolation of
studies of simpler molecules
• Looking
for presence/absence of functional groups
• Polar
bond is usually IR active
• Nonpolar
bond in a symmetrical molecule will absorb weakly or not at all
BOND | TYPE OF VIBRATION | FREQUENCY | INTENSITY |
C-H | Alkanes(stretch) CH3 (bend) CH2 (bend) Alkenes(Stretch) Alkyne(Stretch) Aldehyde | 3000-2850 1450-1375 1465 3100-3000 3300 2860-2800,2760-2700 | S M M M S S |
C-C | Alkanes | 1200(Not useful) | |
C=C | Alkenes Aromatic | 1680-1600 1600 and 1475 | M-w M-w |
C≡C | Alkyne | 2250-2100 | M-w |
C=O | Aldehyde Ketone Carboxylic acid Ester Amide Anhydride Acid chloride | 1740-1720 1725-1705 1725-1700 1750-1730 1680-1630 1810 and 1760 1800 | S S S S S S S |
C-O | Alcohols, ethers, esters | 1300-1000 | S |
O-H | Alcohols, esters, ethers | 1300-1000 | S |
N-H | Primary and secondary amines(stretch) bend | 3500-3100 1640-1550 | M M |
C-N | Amines | 1350-1000 | M-s |
C=N | Imines and oximes | 1690-1640 | W-s |
C≡N | Nitriles | 2260-2240 | M |
X=C=Y | Allenes, isothiocyanate, isocyanates | 2270-1940 | M-S |
N=O | Nitro | 15550 and 1350 | S |
S-H | Mercaptans | 2550 | W |
S=O | Sulphoxides | 1050 | S |
C-X | Fluoride | 1400-1000 | S |
C-X | Chloride | 785-540 | S |
C-X | Bromide, Iodide | 667 | S |
Carbon –
Carbon bond stretching
• Stronger bonds absorb at higher
frequencies:
– C-C 1200 cm-1
– C=C 1660 cm-1
– CºC 2200 cm-1
(weak or absent if internal)
• Conjugation lowers the frequency:
– Isolated
C=C 1640-1680
cm-1
– Conjugated
C=C 1620-1640 cm-1
– Aromatic
C=C approx. 1600 cm-1
IR spectra
of Hydrocarbons: alkanes
Structural unit | Wave number (Cm-1) |
Sp3 C—H | 2850-2950 |
Sp2 C—H | 3000-3100 |
Sp C—H | 3310-3320 |
CH2 | 1465 |
CH3 | 1375 |
IR spectra
of alkenes
Structural unit | Wave number Cm-1 |
=C-H | 3000-3100 |
Isolated C=C | 1620-1680 |
Conjugated C=C | 1620-1640 |
IR spectra
of alkynes
Structural unit | Wave number Cm-1 |
≡C-H | 3300 |
C≡C | 2150 |
IR spectra
of alcohols
IR spectra
of ethers and epoxides
Structural unit | Wave number Cm-1 |
C-O | 1300-1000 |
IR spectra of aldehydes
Structural unit | Wave number Cm-1 |
C=0 | 1745-1725 |
C=C | 1640 |
H-CO | 2860-2800 |
IR spectra
of ketones
Structural Unit | Wave number Cm-1 |
C=O | 1720-1708 |
C=C | 1644-1617 |
IR spectra
of carboxylic acids
| Wave number |
O-H | |
C=O | |
C-O | |
IR spectra
of esters
Structural Unit | Wave Number Cm-1 |
C=O | 1750-1735 |
C-O stretching | 1300-1000 |
IR spectra
of acid chlorides
Structural Unit | Wave number Cm-1 |
C=O | 1810-1775 |
Conjugated | 1780-1760 |
IR spectra
of amines
Structural unit | Wave number cm-1 |
N-H | 3500-3300 |
10 amines | 1640-1560 |
20 amines | 1500 |
30 amines | 1350-100 |
IR spectra
of anhydrides
Structural unit | Wave number Cm-1 |
C=O | 1830-1800 and 1775-1740 |
IR spectra
of nitriles
Structural unit | Wave number Cm-1 |
-C≡N | 2250 |
IR spectra
of isocyanates
Structural unit | Wave number Cm-1 |
N=C=O | 2270 |
IR spectra
of nitro compounds
Structural unit | Wave number Cm-1 |
NO2 | 1550 1350 |
Aliphatic | 1600-1530(asymmetric) 1390-1300(symmetric) |
Aromatic | 1550-1490(asymmetric) 1355-1315(symmetric) |
IR spectra
of sulfur compounds
Structural unit | Wave number Cm-1 |
S-H | 2250 |
IR spectra
of halides
Structural unit | Wave number Cm-1 |
C-F | 1400-1000 |
C- Cl | 785-540 |
C-Br | 650-510 |
C-I | 600-485 |
Summary
Infrared
Spectrum of Hexane
Infrared
Spectrum of 1-Hexene
Infrared
Absorption Frequencies
Structural unit | Frequency, cm-1 |
Stretching | |
sp C— H | 3310-3320 |
sp2 C—H | 3000-3100 |
sp3 C—H | 2850-2950 |
sp2 C—O | 1200 |
sp3 C—O | 1025-1200 |
Stretching | |
C=C | 1620-1680 |
C≡C | 2100-2200 |
C≡N | 2240-2280 |
Stretching vibrations (carbonyl groups) | |
Aldehydes and ketones | 1710-1750 |
Carboxylic acids | 1700-1725 |
Acid anhydrides | 1800-1850 and 1740-1790 |
Esters | 1730-1750 |
Amides | 1680-1700 |
Bending vibrations of alkenes | |
RCH=CH2 | 910-990 |
R2C=CH2 | 890 |
cis-RCH=CHR | 665-730 |
trans-RCH=CHR | 960-980 |
R2C=CHR | 790-840 |
Bending vibrations of derivatives of benzene | |
Monosubstituted | 730-770 and 690-710 |
Ortho-disubstituted | 735-770 |
Meta-disubstituted | 750-810 and 680-730 |
Para-disubstituted | 790-840 |
Stretching vibrations (single bonds) | |
O—H (alcohols) | 3200-3600 |
O—H (carboxylic acids) | 3000-3100 |
N—H | 3350-3500 |
Infrared Spectrum
of tert-butylbenzene
Infrared
Spectrum of 2-Hexanol
Infrared
Spectrum of 2-Hexanone
Summary
• IR
spectra are mainly useful for identification of functional groups
• Characteristic
C- H stretch absorption is observed for alkanes
• -OH
stretch absorption is at 3300 cm-1
• Carbonyl
compounds absorb at 1700 cm-1 which represents –C=O str.
• Samples
in all three physical states can be handled in IR
• IR
spectroscopy is valuable since
(a) It shows what functional groups are
present
(b) It indicates the absence of other
functional groups
(c) By comparison with an authentic sample,
it can confirm the identity of a compound
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