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The scale UNITS: cm-1 unit is the wavenumber and is given by 1 / (wavelength in cm). ṽ = 1/ λ
Infrared (IR) spectroscopy deals spectrum with the interaction between a molecule and radiation from the IR region of the electromagnetic (IR region = 4000 - 400 cm-1).
IR radiation causes the excitation of the vibrations of covalent bonds within that molecule ( motions between nuclei).
These vibrations include the stretching and bending modes
How to can we type a Compound by IR?
Draw four lines on your IR spectrum 3000 cm-1 2500 cm-1
2000 cm-1 1500 cm-1
Get a big picture first!
It is important the frequency of the energy absorbed by the type of bond (E= hC/) and the shape of the
peak corresponding to that absorption
AB
C
DE
F
A. broad and strong, typical of alcohols, centered around 3300cm -1, takes 80 % of the chart
B. broad and medium, typical of carboxylic acids, is not as smooth as alcohols due to other absorptions in the same region, 50% of the chart.
C. Sharp strong, typical of carbonyl group around 1700 cm -1, Usually, 98% of the chart
D. Sharp strong, typical of ΞC-H centered around 3300 cm-1, usually 75% of chart
E. Sharp medium, typical of C=C bond and others in the finger print region F. Sharp weak, typical of amines, and overtones
The IR Chart1800-1600
C=O
C=N
C=N
~3300
H-EN
~2300
CΞC
CΞN
Compound class Bond Frequency (cm-1)
U S
3000 1500
Saturated Sp3 carbonstretching
Saturated Sp3 carbonBending
FINGERPRINT
Above 3000 cm-1 [X-H (X = O, N, sp ΞC, or sp2 =C)]
3000-2500 [C-H (sp3 C –H) stretching]
2500-2000 (CΞC, CΞN triple bond stretching)
2000-1500 [C=O, appr.1700; C=C and N=O, appr. 1600]
Under 1500 (fingerprint region)
Alkanes C-H 2960-2850 (strong, stretching), 1470-1350 (bending)
Single bonds: n-hexane
U S
Double bonds/hydrocarbons
Alkenes, arenes
=C-H
Compound Bond Frequency (cm-1)
U S
3000 1500
Saturated Sp3 C-Hstretching
Saturated Sp3 carbonbending
Unsaturated Sp2 C-Hstretching
~ 1600 cm -1 C=Cstretching
Alkenes 2-hexene C=C, 1680-1640 (medium, stretching)
C-H 3090-3020 (strong, stretching),
1000-675 (bending)
C=C
Double bonds /hydrocarbons
Methyl benzene
Aromatic Rings
C=C 1600-1500 (stretching)
=C-H 3100-3000 (stretching),
2000-1600 (phenyl ring substitution overtones)
U S
overtones
Double bonds /hydrocarbons
Other Double bondsNitro Compounds N=O 1660-1500 (s, asymmetrical s1390-1260 (s, symmetrical stretching)
C=O 1760-1670 (s, stretching)FAMILY A
Aldehydes, Ketones,
FAMILY B
Carboxylic acids, EstersAmides Acyl halidesAcid Anhydrides C=O 1760-1670 (s, stretching)
Acetone: Residues of acetone used during cleaning or in contamination during GC analysis will be observed with a weak peak at 1715 cm-1
Other double bonds
1-HEXYNE
Triple bonds/hydrocarbons
Alkynes
C-H 3333-3267(sharp, strong, stretching),
700-610 (bending)
CΞC 2260-2100 (sharp, medium, stretching)
ΞC-H
CΞC
Internal Alkyne
CΞC
overtones
C=C
Triple bonds
Alkynes
C-H 3333-3267(sharp, stretching),
700-610 (bending)
CΞC 2260-2100 (sharp, strong, stretching)
Nitriles
CΞN 2260-2220 (sharp, strong stretching)
Compound Bond Frequency (cm-1)
Cyano benzene
C=C
CΞN
NitrilesCΞN 2260-2220 (sharp, strong stretching)
Alcohols, Phenols O-H (H-bond formation) 3640-3160 (s & br, stret.)
Carboxylic acids O-H (H-bond formation) 3500-2500 (b, stretching)
AminesN-H (H-bond formation)
(2 peaks for primary amine, one peak for secondary amine)
3500-3300 (m, stretching)
Compound BondFrequency (cm-1)
H-Electronegative atom
1-pentanol
Difference between a carboxylic acid and an alcohol.
Hexanoic acid
Isopropanol
Cyclobutanol
cyclohexanol
2-methyl-2-propen-1-ol, and alkene/alcohol
Benzyl alcohol
Observe C-O fingerprint regionEthyl-vinyl ether/ether- alkene
Diethyl amine
N-H
Butyl amine
N-H
H
Observe a primary amine shows two absorptions, one for each hydrogen bonded to the electronegative atom Nitrogen
Compound Bond Frequency (cm-1)
Ketones and aldehydes C=O 1760-1670 (strong, stretching)
Recall that aldehydes also show: CHO two peaks 2850 and 2780 medium.
Carbonyl family A
3-pentanone / a ketone
2-butanone- /a ketone
(Gas Phase) of Formaldehyde, H(C=O)H
2-methylpropanal: an aldehyde
butyraldhyde
CHO two peaks sharp, medium, around 2850 and 2780cm-1.
Carboxylic acids, Esters
C=O 1760-1670 (strong, stretching)
C-O 1260-1000 ( strong, stret.)
Recall that Carboxylic acids also show: O-H (H-bond formation)3500-2500 (broad, stretching)
Carbonyl/ Family B
Carboxylic acid
Benzoic acid
O-H
C=CC=O
Ester C=O and C-O
C=O C-O
Ethyl acetate
C=OC-O
amides C=O 1760-1670 (strong, stretching)
C-O 1260-1000 ( strong, stret.)
Recall amide also show:
N-H as in amines but strong and broad for H-bond formation)
One peak for secondary amide, two peaks for primary amide
Carbonyl family
Amide/acetamide
C=ON-H
H
Benzamide
C=O C=CN-H
N-H
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