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Chem-805 Identification of organic and inorganic compounds by spectroscopy. Mass Spectrometry fragmentation NMR Infrared. Index. MS. Fragmentation process. There are 3 type of fragmentations:. Cleavage of s bond. + . ---- C +. ---- C – C ----. +. . C ----. At heteroatom. + .

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chem 805 identification of organic and inorganic compounds by spectroscopy

Chem-805Identification of organic and inorganic compounds by spectroscopy

Mass Spectrometry

fragmentation

NMR

Infrared

Index

MS

fragmentation process
Fragmentation process

There are 3 type of fragmentations:

  • Cleavage of s bond

+ .

---- C +

---- C – C ----

+

. C ----

At heteroatom

+ .

---- C +

---- C – Z ----

+

. Z ----

a to heteroatom

+ .

C=Z +

---- C - C – Z ----

+

---- C .

+ .

Z + .

---- C - C – Z ----

+

---- C = C

fragmentation process3

+ .

+ .

+

Fragmentation process

There are 3 type of fragmentations:

  • Cleavage of 2s bond (rearrangements)

+.

---- C=C +

---- HC – C – Z ----

+

HZ

Retro Diels-alder

McLafferty

+.

+.

fragmentation process4
Fragmentation process

There are 3 type of fragmentations:

  • Cleavage of Complex rearrangements
fragmentation rules in ms
Fragmentation rules in MS
  • Intensity of M.+ is Larger for linear chain than for branched compound
  • Intensity of M.+decrease with IncreasingM.W. (fatty acid is an exception)
  • Cleavage is favored at branching reflecting the Increased stability of the ionStability order: CH3+ < R-CH2+ <

R

R

CH+ < C+

R

R

R

R’

R

CH

R”

Loss of Largest Subst. Is most favored

branched alkanes
Branched alkanes

MW=170

M.+ is absent with heavy branching

Fragmentation occur at branching: largest fragment loss

illustration of first 3 rules linear alkane with smaller mw
Illustration of first 3 rules (Linear alkane with Smaller MW)

Molecular ion is stronger than in previous sample

illustration of first 3 rules branched alkane with smaller mw
Illustration of first 3 rules (Branched alkane with Smaller MW)

43

Molecular ion smaller than linear alkane

Cleavage at branching is favored

rule 3

Alkanes

Rule 3

Cleavage Favored at branching

Loss of Largest substituent

Favored

Rule1: intensity of M.+

is smaller with branching

fragmentation rules in ms11
Fragmentation rules in MS
  • Aromatic Rings, Double bond, Cyclic structures stabilizeM.+
  • Double bond favor Allylic Cleavage Resonance – Stabilized Cation
fragmentation rules in ms14

+ .

+ .

+

Fragmentation rules in MS
  • a) Saturated Rings lose a Alkyl Chain (case of branching)
    • Unsaturated Rings  Retro-Diels-Alder

+.

+

-R.

fragmentation rules in ms16
Fragmentation rules in MS
  • Aromatic Compounds Cleave in b Resonance Stabilized Tropylium

-R.

Tropylium ion

m/z 91

fragmentation rules in ms18
Fragmentation rules in MS
  • C-CNext to Heteroatom cleave leaving the charge on the Heteroatom

- [RCH2]

- [R2]

larger

fragmentation rules in ms19
Fragmentation rules in MS
  • Cleavage of small neutral molecules(CO2, CO, olefins, H2O ….)Result often from rearrangement

McLafferty

- CH2=CH2

Ion Stabilized

by resonance

Y  H, R, OH, NR2

alkenes

+

CH2

CH

CH

Et

CH2

CH

+CH2

CH +

CH

CH

Et

Me

Et

Et

Me

Me

Alkenes

Most intense peaks are often:

m/z 41, 55, 69

Rule 4: Double Bond Stabilize M+

Rule 5: Double Bond favor

Allylic cleavage

-Et

-29

m/z = 83

M+ = 112

hydroxy compounds
Hydroxy compounds

- R3

If R1=H m/z 45, 59, 73 …

Loss of largest group

If R1=alkyl m/z 59, 73, 87 …

– (H2O)

M – (H2O)

- H2O

- CHR=CHR

M – (H2O) – (C1=C2) Alkene

aromatic ether
Aromatic Ether

Molecular ion is prominent

-CO

- R

1)

C5H5

m/z 65

m/z 93

Cleavage in b of aromatic ring

- CH2=CH2

2)

Rearrangement

m/z 94

aliphatic ether

B

Aliphatic Ether

Cleavage of C-C next ot Oxygen

Loss of biggest fragment

• +

CH3—CH2 —O—CH2 —CH2 —CH2 —CH3

CH3—CH2 —O+ =CH2

m/z 59

CH3—CH2 —O—CH2+

ether rearrangement
Ether Rearrangement

m/z 45

B

H— CH3

CH3—CH2—CH —O —CH2 —CH3

CH =O+ —CH2

CH3

CH3

CH =O+H

CH3

m/z 73

1- Cleavage of C-C next to Oxygen

Box rearr.

M·+

m/z 73

2- Cleavage of C-O bond: charge on alkyl

m/z 45

Index

MS

MS-fragmentation-2