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Infrared Spectroscopy
Infrared Absorbance
IR- Empirical Comparisons
Identifying functional groups in organic molecules
Region of infrared that is most useful lies between2.5-16 mm (4000-625 cm-1)
depends on transitions between vibrational energy states
Stretching: higher energy / higher wave number (cm-
1)
Bending: lower energy / lower wave number (cm-1)
Infrared Spectroscopy
A bond must have a dipole or an induced dipole in order to have an absorbance in the IR spectrum.
When the bond stretches, the increasing distance between the atoms increases the dipole moment.Therefore, the greater the dipole, the more intense the absorption. (i.e., The greater the molar extinction coefficient () in Beer’s law, A = bc.
The energy (IR frequency/ wave number) and the intensity of the absorption band also depends on the concentration of solution from Beer’s law, A = bc.
It is easier to stretch an O–H bond if it is hydrogen bonded
The presence & absence of absorption bands must be considered in identifying a possible structure in IR spectroscopy. Empiricism is critical to successful identification.
NOTE: Bonds which lack dipole moments are notdetected.
Analyzing Structure:Functions & Infrared Spectra
The molecular formula is a critical piece of information, which limits the functional possibilities.
Structural/Functional Components
The peaks are quantized absorption bands corresponding to molecular stretching and
bending vibrations
An Infrared Spectrum
The fingerprint region
The functional groupstretching region
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
O—H (alcohols) 3200-3600
O—H (carboxylic acids) 3000-3100
N—H3350-3500
Infrared Absorption Frequencies
First examine the absorption bands in the vicinity of 4000-3000 cm–1
IR Spectrum of a Primary Amine(1o)
The N–H bending vibration occurs at ~1600 cm–1
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
sp C—H 3310-3320
sp2 C—H 3000-3100
sp3 C—H 2850-2950
sp2 C—O 1200
sp3 C—O 1025-1200
Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
sp C—H 3310-3320
sp2 C—H 3000-3100
sp3 C—H 2850-2950
sp2 C—O 1200
sp3 C—O 1025-1200
Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (multiple bonds)
Infrared Absorption Frequencies
C C 1620-1680
—C N
—C C— 2100-2200
2240-2280
Some hydrocarbon absorption bands
Structural Components & Functional Differences:
Therefore the amide has a longer (weaker) C=O bond (1680-1700 cm-1) and the ester (1730-1750 cm-1) is shorter (stronger).
The nitrogen of an amide is less electronegative than the oxygen of an ester.
Structural unit Frequency, cm-1
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
Infrared Absorption Frequencies
C O
Cyclic aliphatic ketone
Mono substituted aromatic methyl ketone
Mono substituted aromatic ketone
Aliphatic ester I
Aliphatic ester II
Aliphatic ester III
Mono substituted aromatic ester
Mono substituted aromatic conjugated ester
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
sp2 C—O 1200
sp3 C—O 1025-1200
Infrared Absorption Frequencies
Dihexyl Ether
~1100 cm-1
1025-1200 cm-1
~1200 cm-1
Structural unit Frequency, cm-1
Bending vibrations of alkenes
Infrared Absorption Frequencies
CH2RCH
CH2R2C
CHR'cis-RCH
CHR'trans-RCH
CHR'R2C
910-990
890
665-730
960-980
790-840
wavenumber (cm–1) assignment
30752950
1650 and 890
???
cis- 665-730
trans- 960-980
Summary: C–H bond absorption and hybridizationof the carbon atom
Distinctive Stretch of C–H Bond in an Aldehyde (the “waggle” vibration)
Aliphatic aldehyde
Mono-substituted aromatic aldehyde
Mono-substituted aromatic conjugated aldehyde
Mono substituted aromatic ester
Para di-substituted aromatic ether & aldehyde
Infrared Spectroscopy
Common Functional Groups
Handout
http://chemconnections.org/general/chem121/Spectroscopy/IR-handout-11.htm
Structural unit Frequency, cm-1
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
Aromatic Absorption Frequencies
20003500 3000 2500 10001500 500
Wave number, cm-1
Infrared Spectrum of tert-butylbenzene
H—C
Ar—H
Monsubstitutedbenzene
C6H5C(CH3)3
Chemical Communication: Smell / Pheromones
Pheromone Synthesis[20:40-23:51]
http://chemconnections.org/COT/COT-chemcomm-eg.html
http://www.learner.org/resources/series61.html
Infrared Spectroscopy
Common Functional Groups
QUIZ
Question
Is the following IR of cis or trans 2-pentene?
A) cis B) trans
A) benzyl alcoholB) 1,4,6-heptatrien-3-one
C) 2,4,6-heptatrienaldehydeD) benzaldehyde
C7H6O
Question
Identify the compound from the IR above.
Question
C10H12O
Identify the compound from the IR above.
A) 4-phenylbutanaldehydeB) phenylpropyl ketone
C) meta-isopropylbenzaldehydeD) 1-phenyl-2-butanone
Question
C3H4O
A) cyclopropanoneB) propynol
C) 2-cyclopropenolD) 1,2-propadienol
Identify the compound from the IR above.
Question
C3H7NO
Identify the compound from the IR above.
A) N-methylacetamideB) N,N-dimethylformamide
C) 3-aminopropanalD) N-methylamino-ethanal
Match the ortho, meta and para isomers of xylene.
I
III
II
ortho meta para
A) I II III
B) II I III
C) I III II
D) III II I
Question
Question
A) methylbenzoateB) phenylacetateC) p-anisaldehydeD) o-anisaldehyde
Identify the compound from the IR above.
C8H8O2
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