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Teory of Mass SpectrometryTaslim Ersam

The Mass Spectrum and Structural Analysis

Taslim Ersam

I. Fragmentation – Chemistry of Ions1. One bond σ-cleavages:

a cleavage of C-Ca. cleavage of C C

C C

b. cleavage of C-heteroatom

C C C C+

g

C Z C Z+

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The Mass Spectrum and Structural AnalysisI. Fragmentation – Chemistry of IonsI. Fragmentation Chemistry of Ions1. One bond σ-cleavages:

c. α-cleavage of C-heteroatom

C C Z C C Z+ C Z

C C Z C C Z+

C C Z C Z+ C

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IV. The Mass Spectrum and Structural AnalysisD Fragmentation Chemistry of IonsD. Fragmentation – Chemistry of Ions

2. Two bond σ-cleavages/rearrangements:a. Elimination of a vicinal H and heteroatom:

C C Z Z+ HC C

b. Retro-Diels-Alder

H

+

Full mechanism

Abbreviated:

+

15/09/20113

The Mass Spectrum and Structural AnalysisFragmentation – Chemistry of Ions2. Two bond σ-cleavages/rearrangements:

c. McLafferty Rearrangementy gFull mechanism

H+

H

Abbreviated:

HH+

H

3. Other types of fragmentation are less common, but in specific cases are dominant processesThese include: fragmentations from rearrangement migrations andThese include: fragmentations from rearrangement, migrations, and fragmentation of fragments

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IV The Mass Spectrum and Structural AnalysisIV. The Mass Spectrum and Structural AnalysisI. Fragmentation – Chemistry of Ions4. When deducing any fragmentation scheme:

– The even-odd electron rule applies: “thermodynamics dictates that even electron ions cannot cleave to a pair of odd electron fragments”

– Mass losses of 14 are rare

– The order of carbocation/radical stability is benzyl/3° > allyl/2° > 1° > methyl > H

* the loss of the longest carbon chain is preferred

– Fragment ion stability is more important than fragment radical g y p gstability

– Fragmentation mechanisms should be in accord with the even-oddFragmentation mechanisms should be in accord with the even odd electron rule

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The Mass Spectrum and Structural AnalysisII. Fragmentation Patterns of GroupsA id S l t th th li itl iti t i l b dAside: Some nomenclature – rather than explicitly writing out single bond

cleavages each time:

H CCH2 + H2CCH3

Fragment obs by MS

Neutral fragment inferred by its lossobs. by MS inferred by its loss – not observed

Is written as:Is written as:

57

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The Mass Spectrum and Structural AnalysisThe Mass Spectrum and Structural AnalysisIII. Fragmentation Patterns of Groups1. Alkanes

a) Very predictable – apply the lessons of the stability of carbocations (or radicals) to predict or explain the observation of the fragments

b) Method of fragmentation is single bond cleavage in most cases

c) This is governed by Stevenson’s Rule the fragment with thec) This is governed by Stevenson s Rule – the fragment with the lowest ionization energy will take on the + charge – the other fragment will still have an unpaired electron

E l i b tExample: iso-butane

CH3+ 3

CH3+

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The Mass Spectrum and Structural Analysis4. Fragmentation Patterns of Groups1. Alkanes

Fragment Ions : n-alkanesg• For straight chain alkanes, a M+ is often observed

Ions observed: clusters of peaks C H apart from the loss of• Ions observed: clusters of peaks CnH2n+1 apart from the loss of –CH3, -C2H5, -C3H7, etc.

• Fragments lost: ·CH3, ·C2H5, ·C3H7, etc.

• In longer chains – peaks at 43 and 57 are the most commong p

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The Mass Spectrum and Structural Analysis5. Fragmentation Patterns of Groups1 Alkanes1. Alkanes

Example MS: n-alkanes – n-heptane

43

57

M+

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The Mass Spectrum and Structural Analysis5. Fragmentation Patterns of Groups

1. AlkanesFragment Ions : branched alkanes

• Where the possibility of forming 2° and 3° carbocations is high,Where the possibility of forming 2 and 3 carbocations is high, the molecule is susceptible to fragmentation

• Whereas in straight chain alkanes a 1° carbocation is always• Whereas in straight chain alkanes, a 1 carbocation is always formed, its appearance is of lowered intensity with branched structures

• M+ peaks become weak to non-existent as the size and branching of the molecule increase

• Peaks at 43 and 57 are the most common as these are the iso-propyl and tert-butyl cations

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The Mass Spectrum and Structural Analysis5. Fragmentation Patterns of Groups1 Alk1. Alkanes

Example MS: branched alkanes – 2,2-dimethylhexane

M+ 1145757

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Mass Spectrometry

The Mass Spectrum and Structural Analysis5 Fragmentation Patterns of Groups5. Fragmentation Patterns of Groups

1. AlkanesFragment Ions : cycloalkanes

• Molecular ions strong and commonly observed – cleavage of the ring still gives same mass value

• A two-bond cleavage to form ethene (C2H4) is common – loss of 28H2C CH2

+HC C R H C CH+C CH H

H2C CH2

CH2

n

• Side chains are easily fragmented y g

15/09/2011 12Prof Dr. Taslim Ersam

Mass Spectrometry

The Mass Spectrum and Structural Analysis5. Fragmentation Patterns of Groups1 Alkanes1. Alkanes

Example MS: cycloalkanes – cyclohexane

+

M+ 84

M - 28 = 56


Mass Spectrometry

IV The Mass Spectrum and Structural AnalysisIV. The Mass Spectrum and Structural AnalysisE. Fragmentation Patterns of Groups

1. AlkanesExample MS: cycloalkanes – trans-p-menthane

97

M+ 140


Mass Spectrometry


2. Alkenesa) The π-bond of an alkene can absorb substantial energy – molecular

ions are commonly observed

b) After ionization, double bonds can migrate readily – determination of isomers is often not possible

c) Ions observed: clusters of peaks CnH2n-1 apart from -C3H5, -C4H7, -C5H9 etc. at 41, 55, 69, etc.

d) Terminal alkenes readily form the allyl carbocation, m/z 41

HR

H2C +RC

HCH2 H2C C

HCH2



2. AlkenesExample MS: alkenes – cis- 2-pentene

55

M+ 70

55

M+ 70

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2. AlkenesTake home assignment:What is M-42 and m/z 42?

Example MS: alkenes –1-hexene

4156

41

M+ 84

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Mass Spectrometry


2. AlkenesTake home assignment 2:What is m/z 42?

Example MS: alkenes –1-pentene

M+ 700


Mass Spectrometry


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, 29



2. AlkenesFragment Ions : cycloalkenes

• Molecular ions strong and commonly observed – cleavage of the ring still gives same mass valueg g

• Retro-Diels-Alder is significant

+

observed loss of 28

• Side chains are easily fragmented

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2. AlkenesExample MS: cycloalkenes –1-methyl-1-cyclohexene

81

M+ 9668

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E. Fragmentation Patterns of Groups3. Alkynes – Fragment Ions

a) The π-bond of an alkyne can also absorb substantial energy –molecular ions are commonly observed

b) For terminal alkynes, the loss of terminal hydrogen is observed (M-1) – this may occur at such intensity to be the base peak or eliminate the presence of M+e ate t e p ese ce o

c) Terminal alkynes form the propargyl cation, m/z 39 (lower intensity than the allyl cation)than the allyl cation)

RH2C +RC CH H2C C CH

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Mass Spectrometry


3. Alkynes Example MS: alkynes – 1-pentyne

H

67H

M+ 68

39


Mass Spectrometry


3. AlkynesExample MS: alkynes – 2-pentyne

53

M+ 68


Mass Spectrometry


4. Aromatic Hydrocarbons – Fragment Ionsa) Very intense molecular ion peaks and little fragmentation of the

ring system are observed

75 eV e-

b) Where alkyl groups are attached to the ring a favorable mode of

75 eV e

b) Where alkyl groups are attached to the ring, a favorable mode of cleavage is to lose a H-radical to form the C7H7

+ ion (m/z 91)

c) This ion is believed to be the tropylium ion; formed from rearrangement of the benzyl cation

CH2CH3


Mass Spectrometry


4. Aromatic Hydrocarbons – Fragment Ionsd) If a chain from the aromatic ring is sufficiently long, a McLafferty

rearrangement is possible

e) Substitution patterns for aromatic rings are able to be determined by MS – with the exception of groups that have other ion chemistry


Mass Spectrometry


4. Aromatic HydrocarbonsExample MS: aromatic hydrocarbons – p-xylene

/ 91

M+ 106CH3CH3

m/z 91

M 106H3C


Mass Spectrometry


4. Aromatic HydrocarbonsExample MS: aromatic hydrocarbons – n -butylbenzene

H H+

92

M+ 13491


Mass Spectrometry


5. Alcohols– Fragment Ionsa) Additional modes of fragmentation will cause lower M+ than for the

corresponding alkanes1° and 2° alcohols have a low M+, 3° may be absent, y

b) The largest alkyl group is usually lost; the mode of cleavage typically is similar for all alcohols:typically is similar for all alcohols:

primary OHH C

O H+

m/z

31

secondary

H2C+

O H+OH 45y

tertiary

+

OH O H+

OH

59

45

tertiary + 59


Mass Spectrometry


5. Alcohols– Fragment Ionsc) Dehydration (M - 18) is a common mode of fragmentation –

importance increases with alkyl chain length (>4 carbons)• 1,2-elimination – occurs from hot surface of ionization chamber,

• 1,4-elimination – occurs from ionization

• both modes give M - 18, with the appearance and possible subsequent fragmentation of the remaining alkene

d) For longer chain alcohols, a McLafferty type rearrangement can produce water and ethylene (M - 18, M - 28)

OHR H O

HR

H

+


Mass Spectrometry


5. Alcohols– Fragment Ionse) Loss of H is not favored for alkanols (M – 1)

f) Cyclic alcohols fragment by similar pathwaysf) Cyclic alcohols fragment by similar pathways• α-cleavage

OHH OHH OHH OHHOHH OHH

H

OHH

H

OHH

+

• dehydration

m/z 57

OHH

, + H2O

M - 1815/09/2011 31Prof Dr. Taslim Ersam

Mass Spectrometry


5. AlcoholsExample MS: alcohols – n -pentanol

OHH OH

H

+

42-H2O70OH

31

42

M+ 88

31

M+ 88


Mass Spectrometry


5. AlcoholsExample MS: alcohols – 2-pentanol

OH

4545

M+ 88


Mass Spectrometry


5. AlcoholsExample MS: alcohols – 2-methyl-2-pentanol

OH

59

87OH

87

M+ 102


Mass Spectrometry


5. AlcoholsExample MS: alcohols – cyclopentanol

OHH OHH

+

57

M+ 86


Mass Spectrometry


6. Phenols– Fragment Ionsa) Do not fully combine observations for aromatic + alcohol; treat as a

unique group

b) For example, loss of H· is observed (M – 1) – charge can be delocalized by ring – most important for rings with EDGs

c) Loss of CO (extrusion) is commonly observed (M – 28); Net loss of the formyl radical (HCO·, M – 29) is also observed from this process

OH

O

HH O

CO

-CO HC CO -H


Mass Spectrometry


5. Example MS: phenols – phenol

-CO 66-HCO 65

M+ 94


Mass Spectrometry


An interesting combination of functionalities: benzyl alcoholsUpon ring expansion to tropylium ions, they become phenols!

M+ 108M 108

M – 1, 107

H H

+“tropyliol” - CO

79

OH

M 1, 107“tropyliol”

HO

79

H

77

+ H2


Mass Spectrometry


7. Ethers– Fragment Ionsa) Slightly more intense M+ than for the corresponding alcohols or

alkanes

b) The largest alkyl group is usually lost to α-cleavage; the mode of cleavage typically is similar to alcohols:

RH2C O R R H2C O R+

c) Cleavage of the C-O bond to give carbocations is observed where favorable

RHC O R R CH O R

RR+

RR


Mass Spectrometry


7. Ethers– Fragment Ionsd) Rearrangement can occur of the following type, if α-carbon is

branched:

HR C O C R C

HH R

CH2

H

HO

H

R+

e) Aromatic ethers, similar to phenols can generate the C6H5O+ ion by loss of the alkyl group rather than H; this can expel C≡O as in the h li d d tiphenolic degradation

OR

O

R + C O + C5H5+


Mass Spectrometry


7. Example MS: ethers – butyl methyl ether

O

45

M+ 88


Mass Spectrometry


7. Example MS: ethers – anisoleTake home – what is m/z 78?

M+ 108O

M-28 (-CH3, -CO)

77

O93

65


Mass Spectrometry


8. Aldehydes - Fragment Ionsa) Weak M+ for aliphatic, strong M+ for aromatic aldehydes

b) α-cleavage is characteristic and often diagnostic for aldehydes –b) α cleavage is characteristic and often diagnostic for aldehydes can occur on either side of the carbonyl

OR C O + H M 1 peakR HR C O + H

O

M-1 peak

c) β-cleavage is an additional mode of fragmentation

R H

OH C O+R m/z 29

c) β cleavage is an additional mode of fragmentation

H

O+RR O m/z R+

M - 41HH can be R-subs.


Mass Spectrometry


8. Aldehydes - Fragment Ionsd) McLafferty rearrangement observed if γ-Hs present

m/z 44+OHR

OHR

) A i ld h d l f bl b h l

H

e) Aromatic aldehydes – a-cleavages are more favorable, both to lose H· (M - 1) and HCO· (M – 29)

C O H

O

H

m/z R+

C O + HH

O m/z RRemember: aromatic ring can be subs.

H

O+H


Mass Spectrometry


8. Example MS: aldehydes (aliphatic) – pentanal

m/z 44

CO +

O

H

HO

H

HC

29

M+ 86

M-185

M 86


Mass Spectrometry


8. Example MS: aldehydes (aromatic) – m-tolualdehyde

M 1M+ 120

M-1119

O

H

91


Mass Spectrometry


9. Ketones - Fragment Ionsa) Strong M+ for aliphatic and aromatic ketones

b) α-cleavage can occur on either side of the carbonyl – the largerb) α cleavage can occur on either side of the carbonyl the larger alkyl group is lost more often

O M – 15, 29, 43…

R R1R C O + R1

R1 is larger than R

M 15, 29, 43…m/z 43, 58, 72, etc.

c) β-cleavage is not as important of a fragmentation mode for ketones compared to aldehydes – but sometimes observedp y

R1

O+RR

R

O

R1


Mass Spectrometry


9. Ketones - Fragment Ionsd) McLafferty rearrangement observed if γ-H’s present – if both alkyl

chains are sufficiently long – both can be observed

HR HR+O

R1

HRO

HR

R1

e) Aromatic ketones – α-cleavages are favorable primarily to lose R·(M – 15, 29…) to form the C6H5CO+ ion, which can lose C≡O

R b

C O + R

O

R

Remember: aromatic ring can be subs.

+ C O

m/z 105

+ C O

m/z 7715/09/2011 48Prof Dr. Taslim Ersam

Mass Spectrometry


9. Ketones - Fragment Ionsf) cyclic ketones degrade in a similar fashion to cycloalkanes and

cycloalkanols:O O OO

H

O

H

O

+

O

m/z 55

O O O

m/z 55

+

m/z 70

- CO

m/z 4215/09/2011 49Prof Dr. Taslim Ersam

Mass Spectrometry


9. Example MS: ketones (aliphatic) – 2-pentanone

O

43

OH

OH

+

M+ 86M 15

58

M-15


Mass Spectrometry


9. Example MS: ketones (aromatic) – propiophenone

O

C O

O

m/z 105

M+ 134

m/z 77


Mass Spectrometry


10. Esters - Fragment Ionsa) M+ weak in most cases, aromatic esters give a stronger peak

b) Most important α-cleavage reactions involve loss of the alkoxy-b) Most important α cleavage reactions involve loss of the alkoxyradical to leave the acylium ion

O

c) The other α-cleavage (most common with methyl esters, m/z 59)

RR1 R C O + OR1O

involves the loss of the alkyl group

O O

RR1

OR C

O+

OO R1


Mass Spectrometry


10. Esters - Fragment Ionsd) McLafferty occurs with sufficiently long esters

OH O

H

) E h l d l ( lk h i ) d h M L ff

R1

O+

OR1O

e) Ethyl and longer (alkoxy chain) esters can undergo the McLafferty rearrangement

R

O+

O R

O

O

HH


Mass Spectrometry


10. Esters - Fragment Ionsf) The most common fragmentation route is to lose the alkyl group by

α-cleavage, to form the C6H5CO+ ion (m/z 105)

RO

ROCO

+

Can lose CO to give m/z 77


Mass Spectrometry


10. Esters - Fragment Ionsg) One interesting fragmentation is shared by both benzyloxy esters

and aromatic esters that have an ortho-alkyl group

OO

H

OH+

CH2CO

benzyloxy ester

fragmentation

ketene

fragmentation

OO

OR

CH2

H

CHO

RO

CH2

+ortho-alkylbenzoate ester


Mass Spectrometry


10. Example MS: esters (aliphatic) – ethyl butyrate

OO

both McLafferty

O

71

O

O

both McLafferty (take home exercise)m/z 88

O

O

29

43

M+ 116


Mass Spectrometry


10. Example MS: esters (benzoic) – methyl ortho-toluate

119

CO

CH2

O

O

O91

M+ 150

CH2

O m/z 118


Mass Spectrometry


11. Carboxylic Acids - Fragment Ionsa) As with esters, M+ weak in most cases, aromatic acids give a

stronger peak

b) Most important α-cleavage reactions involve loss of the alkoxy-radical to leave the acylium ion

O

RH

OR C O + OH

O

c) The other α-cleavage (less common) involves the loss of the alkyl radical. Although less common, the m/z 45 peak is somewhat diagnostic for acids.

RH

OR C

O+

O O H


Mass Spectrometry


11. Carboxylic Acids - Fragment Ionsd) McLafferty occurs with sufficiently long acids

OH O

H

HO

+O

HO

m/z 60

e) aromatic acids degrade by a process similar to esters, loss of the HO· gives the acylium ion which can lose C≡O:

HO

HOCO

+ HO +

+ further loss of CO t / 77CO to m/z 77


Mass Spectrometry


11. Carboxylic Acids - Fragment Ionsf) As with esters, those benzoic acids with an ortho-alkyl group will

lose water to give a ketene radical cation

OH

OC

HOH

O

+ortho-alkylbenzoic acidCH2

H HOCH2

+ortho alkylbenzoic acid


Mass Spectrometry


11. Example MS: carboxylic acids (aliphatic) – pentanoic acid

O

OH

HOH

OH

m/z 60

M+ 102


Mass Spectrometry


11. Example MS: carboxylic acids (aromatic) – p-toluic acid

O

M+ 136OH

OH

O

119

91


Mass Spectrometry


Summary – Carbonyl CompoundsFor carbonyl compounds – there are 4 common modes of fragmentation:

A1 & A2 -- two α-cleavagesO

R

O

G O C G2 + R

O

B -- β-cleavage

OCR + GR

O

G

β g

C McLafferty Rearrangement

O

GR

O

GR +

C – McLafferty Rearrangement

OR H

OR H

+G G


Mass Spectrometry


Summary – Carbonyl CompoundsIn tabular format:

m/z of ion observed

F t ti P thAldehydes

G HKetonesG R

EstersG OR’

AcidsG OH

AmidesG NHFragmentation Path G = H G = R G = OR’ G = OH G = NH2

A1

α-cleavage- R 29 43b 59b 45 44d

A2

α-cleavage- G 43b 43b 43b 43b 43b

B - G 43a 57b 73b 59a 58aBβ-cleavage

G 43 57 73 59 58

CMcLafferty

44a 58b,c 74b,c 60a 59a

McLafferty

b = base, add other mass attached to this chaina = base, if α-carbon branched, add appropriate mass

ffi i l l d i h id f C Oc = sufficiently long structures can undergo on either side of C=Od = if N-substituted, add appropriate mass


Mass Spectrometry


12. Amines - Fragment Ionsa) Follow nitrogen rule – odd M+, odd # of nitrogens; nonetheless, M+

weak in aliphatic amines

b) α-cleavage reactions are the most important fragmentations for amines; for 1° n-aliphatic amines m/z 30 is diagnostic

RC

N RC

N+

c) McLafferty not often observed with amines, even with sufficiently long alkyl chains

d) Loss of ammonia (M – 17) is not typically observed


Mass Spectrometry


12. Amines - Fragment Ionse) Mass spectra of cyclic amines is complex and varies with ring size

f) Aromatic amines have intense M+f) Aromatic amines have intense M

g) Loss of a hydrogen atom, followed by the expulsion of HCN is typical for anilinestyp ca o a es

NH2 NH H H H

+ H + HCN + H

h) Pyridines have similar stability (strong M+, simple MS) to aromatics, expulsion of HCN is similar to anilines


Mass Spectrometry


12. Example MS: amines, 1° – pentylamine

NH2

30

M+ 87


Mass Spectrometry


12. Example MS: amines, 2° – dipropylamine

NH

72NH

H

72H

M+ 101


Mass Spectrometry


12. Example MS: amines, 3° – tripropylamine

N

114

M+ 143


Mass Spectrometry


13. Amides - Fragment Ionsa) Follow nitrogen rule – odd M+, odd # of nitrogens; observable M+

b) α-cleavage reactions afford a specific fragment of m/z 44 forb) α cleavage reactions afford a specific fragment of m/z 44 for primary amides

O

RC

NH2R + O C NH2

m/z 44

c) McLafferty observed where γ-hydrogens are present

OH

OH

NH2

O

NH2

+


Mass Spectrometry


13. Example MS: amides – butyramide

OC

NH2

O

44 H44O

NH2

H

5959

M+ 87


Mass Spectrometry


13. Example MS: amides (aromatic) – benzamide

CNH2O

M+ 121

C

77

NH2OC

NH2O

105


Mass Spectrometry


14. Nitriles - Fragment Ionsa) Follow nitrogen rule – odd M+, odd # of nitrogens; weak M+

b) Principle degradation is the loss of an H-atom (M – 1) from α-b) Principle degradation is the loss of an H atom (M 1) from αcarbon:

H +RH2C C N R C

HC N

c) Loss of HCN observed (M – 27)H

d) McLafferty observed where γ-hydrogens are present

NH

NH

) l h k l f

CH2C

CN

+

m/z 41

e) Aromatic nitriles give a strong M+ as the strongest peak, loss of HCN is common (m/z 76) as opposed to loss of CN (m/z 77)


Mass Spectrometry


14. Example MS: nitriles – propionitrile

M-1 54

- HC≡N

M+ 55


Mass Spectrometry


14. Example MS: nitriles – valeronitrile (pentanenitrile)

N

H CCN

H

C

43H2C

m/z 41CN

54

M+ 83


Mass Spectrometry


15. Nitro - Fragment Ionsa) Follow nitrogen rule – odd M+, odd # of nitrogens; M+ almost never

observed, unless aromatic

b) Principle degradation is loss of NO+ (m/z 30) and NO2+ (m/z 46)

O OR N

O

OR N

O

O+

m/z 46

+m/z 30

R NO

OR N

O

OR O N O R O N O

m/z 30O


Mass Spectrometry


15. Nitro - Fragment Ionsc) Aromatic nitro groups show these peaks as well as the fragments of

the loss of all or parts of the nitro group

NO2 O

+ NO + CO

NO2

m/z 93 m/z 65

+ NO2 + HC CHC4H3

m/z 77 m/z 51


Mass Spectrometry


15. Example MS: nitro – 1-nitropropane

NO2

43

M+ 89NO2+ 46NO+ 30


Mass Spectrometry


15. Example MS: nitro (aromatic) – p-nitrotoluene

NO2

M+ 13791

2

OC5H5

+

m/z 107


Mass Spectrometry


16. Halogens - Fragment Ionsa) Halogenated compounds often give good M+

b) Fluoro- and iodo-compounds do not have appreciable contributionb) Fluoro and iodo compounds do not have appreciable contribution from isotopes

c) Chloro and bromo compounds are unique in that they will showc) Chloro- and bromo-compounds are unique in that they will show strong M+2 peaks for the contribution of higher isotopes

d) For chlorinated compounds, the ratio of M+ to M+2 is about 3:1

e) For brominated compounds, the ratio of M+ to M+2 is 1:1) p ,

f) An appreciable M+4, 6, … peak is indicative of a combination of these two halogens – use appropriate guide to discern number ofthese two halogens use appropriate guide to discern number of each


Mass Spectrometry


16. Halogens - Fragment Ionsg) Principle fragmentation mode is to lose halogen atom, leaving a

carbocation – the intensity of the peak will increase with cation stability

h) Leaving group ability contributes to the loss of halogen most

R + XR X

h) Leaving group ability contributes to the loss of halogen most strongly for -I and -Br less so for -Cl, and least for –F

i) L f HX i th d t d f f t tii) Loss of HX is the second most common mode of fragmentation –here the conjugate basicity of the halogen contributes (HF > HCl > HBr > HI)

R +C XCRH

H H

HCH

CH2 H X


Mass Spectrometry


16. Halogens - Fragment Ionsj) Less often, α-cleavage will occur:

R +C XH

H2C XR

k) For longer chain halides, the expulsion of a >δ carbon chain as the radical is observed

R +C XH

H2C XR

radical is observed

RX X

l) Aromatic halides give stronger M+ and typically lose the halogen

R+X

l) Aromatic halides give stronger M , and typically lose the halogen atom to form C6H5

+


Mass Spectrometry


16. Example MS: chlorine – 1-chloropropane

Cl

43

Cl

H2C ClM+2

M+ 78m/z 49, 51H2C Cl


Mass Spectrometry


16. Example MS: chlorine – p-chlorotoluene

91

Cl

M+ 1266

M 2M+2


Mass Spectrometry


16. Example MS: bromine – 1-bromobutane

B

57

Br

M+2

M+ 136H2C Br


Mass Spectrometry


16. Example MS: bromine – p-bromotoluene

91

Br

M+ 170

M+291


Mass Spectrometry


16. Example MS: multiple bromines – 3,4-dibromotoluene

B

M+4

M+2169,171

BrBr

90

BrBr

M+ 248


Mass Spectrometry


16. Example MS: iodine – iodobenzene

M+ 204M+ 204

I

77


Mass Spectrometry

IV The Mass Spectrum and Structural AnalysisIV. The Mass Spectrum and Structural AnalysisF. Approach to analyzing a mass spectrum

1. As with IR, get a general feel for the spectrum before you analyze anything – is it simple, complex, groups of peaks, etc.

2. Squeeze everything you can out of the M+ peak that you can (once you q y g y p y ( yhave confirmed it is the M+)– Strong or Weak?– Isotopes? M+1? M+2 4Isotopes? M+1? M+2, 4, …– Apply the Nitrogen rule– Apply the Rule of Thirteen to generate possible formulas (you can

i kl di f ibiliti b d th b f i t iquickly dispose of possibilities based on the absence of isotopic peaks or the inference of the nitrogen rule)

– Use the HDI from the Rule of Thirteen to further reduce the ibilitipossibilities

– Is there an M-1 peak?


Mass Spectrometry

IV The Mass Spectrum and Structural AnalysisIV. The Mass Spectrum and Structural AnalysisF. Approach to analyzing a mass spectrum

3. Squeeze everything you can out of the base peak– What ions could give this peak? (m/z 43 doesn’t help much)– What was lost from M+ to give this peak?– When considering the base peak initially only think of the mostWhen considering the base peak initially, only think of the most

common cleavages for each group

4 Look for the loss of small neutral molecules from M+4. Look for the loss of small neutral molecules from M+

– H2C=CH2, HC≡CH, H2O, HOR, HCN, HX

5 N id th ibl di ti k th t ( 295. Now consider the possible diagnostic peaks on the spectrum (e.g.: 29, 30, 31, 45, 59, 77, 91, 105 etc.)

6. Lastly, once you have a hypothetical molecule that explains the data, see if you can verify it by use of other less intense peaks on the spectrum – not 100% necessary (or accurate) but if this step works it

dd h f d l lcan add to the confidence level


Mass Spectrometry

End of materialEnd of material

Schedule:

Workshops: Friday Oct. 28th, Monday Oct. 31st, Wednesday Nov. 2nd (if needed).

Exam: Monday, November 7th (5 PM?); take home portion given out Friday, November 4th, due Wednesday, November 9th.

NMR material will begin Friday November 4th.

We will have lecture on Monday, November 7th (NMR)!

What’s left: Two NMR examsWhat s left: Two NMR exams– one in class, one take-home + Final are left 100, 100 and 125 pts.


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