Interpretation – IR spectra – Instrumental Methods of Analysis B. Pharma 7th Semester

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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

       
Structural Unit

      Wave number
Cm-1

                O-H

           
3400-2400

           C=O
stretch

           
1730-1700

                C-O

           
1320-1210

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
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

Stretching
vibrations (multiple bonds)

 

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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