Mass Spectrometry: Fragmentation -...

H C C C HH

H

H H

HHH C C C H

H

H

H H

HH

C C CH

H

H H

HHH C C C H

H

H

H

HH

H C C C HH

H

H H

HHm/z = 44 m/z = 29

m/z = 28m/z = 43

m/z = 15

propane

ionization

(molecular ion)

Mass Spectrometry: Fragmentation

Remember ! ! ! ! !

10 20 30 40 50

20

40

60

80

100

00

rela

tive

inte

nsity

m/z

44

29

28

1543

Mass Spectrum

Me M

e

e

*FRAGMENTS


Initial Loss of Electron !

from a non-bonding orbital

from a π orbital

non-bonding orbital > π-orbital > σ-orbital

- eO OO

O - e OO

- e

from a σ orbital

- e


Localization of Charge can Inform Fragmentation

O Ovs O+

- eor

A B C

REMEMBER: ONLY POSITIVELY CHARGED FRAGMENTS OBSERVED BY MS


Basic Fragmentation Processes

heterolytic cleavage

homolytic cleavage

CH3 CH2 O R CH3 H2C O R+

CH3 CH2 CH2 Br CH3 CH2 CH2 Br+

CH3 H2C CH2+CH3 CH2 CH2


One Bond Cleavage !

R, R' = H, alkyl, aryl X = halogen, OR, SR, NR , etc.

R CH3 R CH3+

ROR'

OR'

R +

RX

R' R + XR'


Two Bond Cleavage !

R, R' = H, alkyl, aryl

R R'

H OHC C R'R H OH+

RC CR

C C+


Rearrangements !

retro Diels-Alder

McLafferty

R

R'

R+

R'

Y

XHR

Y

XHR+

X, Y = C, N, O

Mass Spectrometry

Basic Fragmentation Processes!

Electron Count & Fragmentation

odd # electronseven + R

odd + N

even # electronseven + N

odd + R

NOTE: an even electron species will not fragment to give two radicals!

(Here, N = neutral)

rare



Factors that Impact Fragmentation

• Energetic factors

- relative bond strengths (BDE)

bond: C–Cl C–Br C–I BDE: 81 68 51 kcal mol-1

- stability of the resulting cations or radical ions - stability of the resulting radicals or neutrals

radical stability as above

• Kinetic factors

- availability of a favorable cyclic transition state

CH

HH

CR

HH

CR

RH

CR

RR

O< < < < < <

leaststable

moststable



Stevenson’s Rule

• The most probable fragmentation is the one that leaves the positive charge on the fragment with the lowest ionization energy

- fragmentation processes that lead to the formation of more stable ions are favored over processes that lead to less stable ions

• Cleavages that lead to formation of more stable carbocations are favored

- cation stability is more important than radical stability • When loss of more than one radical is possible, the largest alkyl radical will be lost preferentially

+ CH2CH3

+ CH2CH3

Mass Spectrometry


Ease of Fragmentation

aromatics alkenes

unbranched hydrocarbons ketones amines esters ethers

carboxylic acids branched hydrocarbons

alcohols

higher relative abundance of M+

lower relative abundance of M+

less fragmentation

more fragmentation


Alkanes ! ! ! ! !

Straight Chain Alkanes • Molecular ion peak usually present but weak • Clusters of fragments appear spaced by 14 amu (corresponds to loss of CH2) • The largest peak in each cluster corresponds to an alkyl radical cation, CnH2n+1

• A peak for M-CH3 is often weak or absent • The intensity of lower m/z fragments is greater in large molecules; relative intensities decrease smoothly up to M-C2H5 Branched Alkanes • Smaller molecular ion peak; may be absent • More fragmentation at highly branched positions Cycloalkanes • Relatively large molecular ion peak • Significant peak at M-28 (often the base peak) due to loss of ethylene • M-15: from rearrangement


Straight Chain Alkanes

hexane

M (86)

71 M-15

57 M-29

43 M-43

29 M-57

base peak

CH3 CH2 CH2 CH2 CH2 CH3

MW = 86


Straight Chain Alkanes

hexane


MW = 86

secondary fragmentation

CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH CH2

m/z = 42

(etc.)

-H-H


abc

ab

C4H9 C2H5 C5H11 CH3

C3H7 C3H7

C5H11 CH3C4H9 C2H5

m/z = 57

m/z = 29

m/z = 43

m/z = 15

m/z = 71

c


Branched Alkanes

2-methylpentane

base peak

M (86)

71 M-15

57 M-29

43 M-43

29 M-57

smaller M+

CH3 CH CH2 CH2 CH3

MW = 86

CH3

larger M-15


Branched Alkanes

2-methylpentane

CH3 CH2 CH2 CH2 CH3

CH3

abcd

ac

C3H7C5H11 CH3

CH3C3H7

m/z = 43m/z = 15

m/z = 71

d

b

C2H5

m/z = 29

C4H9

m/z = 57

m/z = 43

CH3

m/z = 15

CH3

CH3 CH CH2 CH2 CH3

MW = 86

CH3


Branched Alkanes

3-methylpentane

base peak

M (86) 71 M-15

57 M-29

43 M-43

29 M-57

CH3 CH2 CH CH2 CH3

MW = 86

CH3


Branched Alkanes

3-methylpentane

CH3 CH2 CH CH2 CH3

MW = 86

CH3

m/z = 15m/z = 71

CH3 CH2 CH CH2 CH3

CH3

a

b

c

m/z = 29m/z = 57

m/z = 15m/z = 71

where does m/z = 43 come from?

cationicrearrangement

d

m/z = 43

fragmentation pattern


Branched Alkanes

2,2-dimethylbutane

base peak M+ absent

larger M-15

71 M-15

57 M-29

43 M-43

29 M-57

CH3 C CH2 CH3

MW = 86

CH3

CH3


Practice Problem

C10H22 (MW = 142)

match to spectrum

A

C

B


Cycloalkanes

cyclohexane MW = 84

69 M-15

56 M-28

41 M-43

M (84)

strong M+

base peak

even m/z

loss of H2C=CH2


Cycloalkanes

ring fragmentation

[C4H8] + H2C=CH2

[C5H9] + CH3

≡


Cycloalkanes

ring fragmentation

[C4H8] + H2C=CH2

[C5H9] + CH3

≡

H2C CH2+

loss of ethylene

loss of CH3 radical

H

H

H

HH

+ CH3

or rearrangementproducts

H


Cycloalkanes

methylcyclopentane

69 M-15

56 M-28

41 M-43

M (84)

loss of side chain

loss of H2C=CH2

MW = 84


Cycloalkanes

ethylcyclohexane

83 M-29

55 83-28

M (112)

loss of side chain

loss of H2C=CH2

MW = 112


Alkenes ! ! ! ! !

Acyclic Alkenes • Relatively strong M+ ion • Clusters of fragments appear spaced by 14 amu (corresponds to loss of CH2) • Strong peak from fragmentation to form a resonance stabilized allylic cation

(m/z = 41 in terminal double bonds,) • Difficult to identify position of alkene since the double bond migrates easily Cyclic Alkenes • Molecular ions strong and commonly observed – cleavage of the ring still gives same mass value • Double bonds favor allylic cleavage to give the resonance stabilized allylic carbocation • Cyclohexenes can undergo retro-Diels-Alder reactions; may be significant • Side chains are easily fragmented


Alkenes

1- pentene

MW = 70

55 M-15

42 M-28

M (70) 41

M-29

H2C CH2

loss of


Alkenes

cis-2-pentene

MW = 70

55 M-15

42 M-28 M (70) H2C CH2

loss of


Alkenes

trans-2-pentene

MW = 70

55 M-15

42 M-28 M (70) H2C CH2

loss of


Alkenes

fragmentation processes

• McLafferty rearrangement

• allylic α-cleavage

RR'

RR'+

≡

RR'

H H

+CH3

M-15


Alkenes

comparison: alkanes vs. alkenes

Octane (75 eV) M+ 114 m/z 85, 71, 57, 43 (base), 29

Octene (75 eV) M+ 112 (stronger @ 75eV than octane) m/z 83, 69, 55, 41 (base), 29


Cyclic Alkenes

cyclohexene

MW = 82

67 M-15

M (82)

54 M-28

H2C CH2

loss of


Cyclic Alkenes

limonene

68 M-70

M (138)

MW = 138

base peak


Alkynes ! ! ! ! !

Alkynes • Relatively strong M+ ion • Strong M-1 peak is observed in terminal alkynes • Strong peak from fragmentation to give resonance stabilized propargyl

cation (m/z = 39 in terminal alkynes)


Alkynes

1-pentyne

67 M-1

M (68)

53 M-15

39 M-29

HMW = 68

base peak


Alkynes

2-pentyne MW = 68

M (68) 53 M-15


Aromatic Hydrocarbons

Aromatic Hydrocarbons • Strong M+ ion • Strong M-1 tropylium ion • Substituted benzenes can undergo McLafferty rearrangement

(substitutent = propyl or larger)



benzene MW = 78

M (78)



toluene MW = 92

CH3

91 M-1 M (92)

base peak

m/z = 65 m/z = 39



formation of tropylium ions

further fragmentation

H

Hm/z = 65 m/z = 39

H

H

+ +

CH2CH2

≡

R

- R

benzylcation

tropyliumion

m/z = 91



ethylbenzene

CH2CH3

MW = 106

91 M-15

M (106)

65 m/z 91-26

39 m/z 65-26



butylbenzene

91 M-43

M (134)

MW = 134

92 M-42

McLafferty

RH

HH +

R

m/z = 92



o-xylene

105 M-1

M (106)

CH3

MW = 106

CH3

91 M-15

CH3



tropylium formation

CH2

≡m/z = 91

CH3

CH3 CH2CH3H

CH3

CH3

- CH3

- H

m/z = 105



xylenes

CH3

CH3

CH3

CH3


Alcohols ! ! ! ! !

Acyclic Alcohols • Weak M+ peak; may be absent • Dehydration (M-18), sometimes with loss of CH2=CH2 • α-Cleavage of an alkyl radical (1° alcohols show m/z = 31)

Largest substituent lost first • Loss of H radical adjacent to OH (M-1) may occur; usually minor Cyclic Alcohols • M+ weak • Dehydration by complex mechanism • α-cleavage of ring, with subsequent fragmentation to give protonated acrolein Benzyl Alcohols • Strong M+ peak • formation of tropyliol ions; then fragmentation to C6H5

+


Alcohols

dehydration processes

1,2-elimination

1,4-elimination

δ-H abstraction

rearrangement

H3C

OH3CH

H H3C

H3CΔ + H O He-

R R'

H OH R'R

n n

+ H O H

OHR H OHR H R+ H O H

OHR H R+ H O H+


Alcohols

α-cleavage

OH

OH

OH

+

+

+

O HH

H

O HCH3

H

O HCH3

H3C

m/z

31

45

59

primary

secondary

tertiary


Acyclic Alcohols

1-pentanol

70 M-18

M (88)

O HH

H

m/z = 31

base peak

m/z = 42

loss of H2O andH2C=CH2

OH

MW = 88

m/z = 55

loss of H2O and CH3


Acyclic Alcohols

2-pentanol

73 M-15

M (88)

base peak

m/z = 45

MW = 88

OH

O HCH3

H

m/z = 55 loss of H2O and CH3

O HH

CH3CH2CH2


Acyclic Alcohols

2-methyl-2-butanol MW = 88

OH

73 M-15

m/z = 59

m/z = 55

O HCH3

CH3CH2CH2

O HCH3

H3C


Cyclic Alcohols

cyclohexanol

OH

MW = 100

82 M-18

M (100)

m/z = 57

99 M-1

O H

H α-cleavage andfragmentation

OH

OH OHH

OHH

OHH

+

m/z = 57


Benzyl Alcohols

benzyl alcohol

OH

MW = 108

M (108)

m/z = 77

107 M-1

OH

m/z = 79

fragmentation oftropyliol ion


Benzyl Alcohols

fragmentation of tropyliol

OH

OH

m/z = 107

- HH

- - H2

HH

H

C O

m/z = 77m/z = 79

dehydration

OHH

+ H O H


Benzyl Alcohols

α-methylbenzyl alcohol

M (122)

104 M-18

OH

MW = 122CH3

m/z = 77

m/z = 107


Phenols ! ! ! ! !

Phenols • Strong M+ peak • May show strong [M-1] • Loss of C≡O (M-28) and net loss of formyl radical (M-29) Thiols • M+ more intense than that of corresponding alcohol • Show significant M+2 peak • Fragmentation patterns similar to alcohols • May lose hydrogen sulfide (M-34)


Phenols

phenol m/z = 94

OH

M (94)

93 M-1

66 M-28

65 M-29

strong M+

loss of C Oloss of HC=O


Phenols

o-cresol m/z = 108

OHCH3

M (108)

107 M-1

80 M-28

79 M-29


Phenols

Loss of C≡O and H-C=O radical (net)

M-28 M-29

≡ M -O

H

O H O

HH

O

O C O- - H


Thiols

2-propanethiol

SH

MW = 76

M (76)

78 M+2

61 M-15

strong M+

42 M-34

Mass Spectrometry: Fragmentation -...

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