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CHEM 344Organic Chemistry Lab

September 9th and 10th 2008

Spectroscopy of Organic Compounds

Lecture 3 –Infrared and Mass Spec

Infrared radiation excites vibrational energy levels of a molecule

Bond stretching, bending, twisting, rocking etc.

IR spectroscopy most useful for functional group assignment

IR bands reported in wavenumbers (cm-1)

4000 3000 2000 1600 400

Wavenumber (cm-1)

N – H

O – H

C – H (sp > sp2 > sp3)

C≡C

C≡N

C=C

C=N

C=O

Fingerprint region

Tra

nsm

itta

nce

(%)

100

0

Alkynes

Nitriles

Alkenes

Imines

Carbonyls

C-C

C-O

C-X

C-N

IR correlation tables in lab manual, textbook

Not used in CHEM 344

N-H

3281 cm-1

2900-3000 cm-1

sp3 C-H

2o amine R2NH

asym N-H

3369 cm-1

3291 cm-1

sym N-H

2900-3000 cm-1

sp3 C-H

1o amine RNH2

Sym and asym N-H bands typical of 1o amine RNH2

C≡C

2120 cm-1

sp3 C-H

sp C-H3315 cm-1

Terminal alkyne

2260 cm-1

Nitrile

C≡N

O-H3639 cm-1

C=O1698 cm-1

C-O band in fingerprint region

1685 cm-1

C=O

1619 cm-1

C=C

sp2 sp3 C-

H

C=O usually most intense band in IR spectrum

C=O and C-C in same region (1700 – 1600 cm-1) – which is which?

OC

Et

HO

1716 cm-1

OC

Et

HO

O C

Et

OH

2800 - 2600 cm-

1

Broad band typical of RCO2H

Mass Spectrometry

Uses high energy electron beam (70 eV), sample in gas phase

Ionization potential for most organic molecules 8-15 eV

Gives info on molecular weight of compound by m/z

Gives info on connectivity of molecule by fragmentation pattern

M + e [M] + 2 e

Molecular Fragments

Molecule

Molecular Ion

[M].+ (also called the parent peak) gives the mass of the molecule

Fragments give info on connectivity (i.e. structure) of the molecule

Mass Spectrum of methanol CH3OH

32

31

29

15

Base peak

[M]+.

m/z = mass/charge ratio

In CHEM 344, we only consider singly charged species (i.e. z=1)

The most intense peak is referred to as the base peak

All other peaks measured relative to base peak

[M]

Cation stability more important than radical stability

Molecular Ion

Cation radical

A + B

Cation

Radical

A + B

Cation

Radical

Radicals are not detected by MS

Mass Spectrum of methanol CH3OH

32

31

29

15

[CH2OH]+

Base peak

[CHO]+

[CH3]+

[CH3OH]+.

[M].+

Mass Spectrum of 2,2,4-trimethylpentane

57

43

Me Me

MeMe

Me

MW=114C8H18

Me

Me

Me

C

Me

Me H

3o carbocation

2o carbocation

No [M].+ detected

Mass Spectrum of 1-butanol

5631

43

29

CH2=OH

via loss of H2OCH3CH2

OH

C4H10O MW = 74

No [M].+ detected

OH

MW = 88

C5H12O

Mass Spectrum of 2-methyl-2-butanol

59

73

70

43

Me

OH OH

55

OH OH

Fragmentation pattern of 2-methyl-2-butanol

OH

OH OH

- CH2CH3

- H2O

m/ z = 70

OH OH

m/ z = 73

- CH3

Me

m/ z = 55

- CH3

Mass Spectrum of 2-octanone

OC8H16OMW = 128

113

43 OH

58

O

O

via -cleavage

via -cleavageMcLafferty rearragement

[M].+

O

O

+

CH3+O

m/z = 43

m/z = 113

Molecular ion

m/z = 128

-cleavage at a C=O group

C OC O RR Resonance stabilized acylium cation

OH

m/z = 58

+OH

m/z = 128

McLafferty rearrangement of 1-octanone

1-pentene

Carbonyl compound must possess gamma H-atoms to undergo McLafferty rearrangement

Process always eliminates a neutral alkene, hence McLafferty product is a radical cation

αβ

γ

O

OH

HOH

OH+

m/z = 60m/z = 88

McLafferty rearrangement of a carboxylic acid

ethene

Mass Spectrum of Benzamide

105

77

44

O

O

NH2

O

NH2

C7H7NO

MW=121 121

via -cleavagevia -cleavage

[M].+

Mass Spectrum of Toluene

CH3

MW=92

C7H8

92

91

Loss of H.

Benzyl cation???

Tropylium cation

m/z = 91

CH3

HH

H

H

HH

H-H

Toluene radical cation

m/z = 92m/z = 91 common to all alkyl benzenes