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

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

      CยบC    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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