Chem-805Chem-805Identification of organic and inorganic Identification of organic and inorganic

compounds by spectroscopycompounds by spectroscopy

Mass SpectrometryMass Spectrometry•fragmentationfragmentation

NMR

Infrared

Index MS

Fragmentation processFragmentation process

• Cleavage of σσ bondThere are 3 type of fragmentations:

---- C – C -------- C – C ---- ---- C ---- C ++ . . C ----C ----++

---- C – ---- C – ZZ ---- ---- ---- C ---- C ++ . . Z ----Z ----++

At heteroatomAt heteroatom

+ + ..

+ + ..

αα to heteroatomto heteroatom

---- C - C – ---- C - C – ZZ ---- ---- C=Z C=Z ++ ---- C ---- C . . +++ + ..

---- C - C – ---- C - C – ZZ ---- ---- Z Z + .+ . ---- C = C ---- C = C +++ + ..

Fragmentation processFragmentation process

• Cleavage of 22 σσ bond (rearrangements)There are 3 type of fragmentations:

---- HC – C – ---- HC – C – ZZ ---- ---- ---- C=C ---- C=C ++ HZHZ++

Retro Diels-alderRetro Diels-alder

++ ..

++ ..

CH2

CH2

CH2

CH2+++ + .. + + ..

McLaffertyMcLafferty

ZH

Z R

CH2

CH2

ZH

Z R

++ ..

Fragmentation processFragmentation process

• Cleavage of ComplexComplex rearrangementsThere are 3 type of fragmentations:

Fragmentation rules in MSFragmentation rules in MS• Intensity of MM..++ is Larger for linear chainLarger for linear chain than

for branched compound

• Intensity of MM..++ decreasedecrease with IncreasingIncreasing M.W.M.W. (fatty acid is an exception)

• Cleavage is favored at branchingfavored at branching reflecting the Increased stability of the ionIncreased stability of the ion

Stability order: CH3+ < R-CH2

+ < RR

CH+ < C+

R

R

R

RRR”R”

CHR’R’

Loss of Largest Subst. Is most favoredLoss of Largest Subst. Is most favored

Illustration of first 3 rules (large MW)Illustration of first 3 rules (large MW)

CH3CH3

CH3CH3

CH3CH3

CH3

MW=170MW=170

MM..++ is absent with heavy branchingFragmentation occur at branching: largest fragment losslargest fragment loss

Branched alkanesBranched alkanes

Illustration of first 3 rules Illustration of first 3 rules (Linear alkane with Smaller MW)(Linear alkane with Smaller MW)

Molecular ion is stronger than in previous sample

Illustration of first 3 rules Illustration of first 3 rules (Branched alkane with Smaller MW)(Branched alkane with Smaller MW)

Molecular ion smaller than linear alkane

Cleavage at branching is favored

4343

Rule 3Rule 3 AlkanesAlkanes

Cleavage Favored at branching

Loss of Largest substituentFavored

Rule1: intensity of MM..++

is smaller with branching

Fragmentation rules in MSFragmentation rules in MS• Aromatic Rings, Double bond, Cyclic Aromatic Rings, Double bond, Cyclic

structures stabilizestructures stabilize MM..++

• Double bond favor Allylic CleavageDouble bond favor Allylic Cleavage Resonance – Stabilized Cation

CH2+ CH CH2 R

- R.

CH2+

CH CH2

CH2 CH CH2+

Aromatic ring has stableAromatic ring has stable MM.+.+

Cycloalkane Cycloalkane ring has ring has

stablestable MM.+.+

Fragmentation rules in MSFragmentation rules in MS• a) Saturated Rings lose α Alkyl Chain

(case of branching)

• Unsaturated Rings Retro-Diels-AlderCH2

CH2

CH2

CH2+++ + .. + + ..

R

++ .. ++

-R..

Retro Diels-alderRetro Diels-alder

++ .. ++ ..

Fragmentation rules in MSFragmentation rules in MS

• Aromatic Compounds Cleave in Aromatic Compounds Cleave in ββ Resonance Stabilized TropyliumTropylium

C

CH+

R

-R.. CH+

CH2 CH2+

+Tropylium ionTropylium ion

m/z 91m/z 91

+

+

Tropylium ionTropylium ion

• C-CC-C Next to HeteroatomNext to Heteroatom cleave leaving the charge on the Heteroatomcharge on the Heteroatom

Fragmentation rules in MSFragmentation rules in MS

R CH2 CH2 Y Rx CH2 Y R

+

CH2+

Y R

x

R2

C

R1

O

C

R1

O+

C+

R1

O

- [RCH2]]

- [R2]]

largerlarger

Fragmentation rules in MSFragmentation rules in MS

• Cleavage of small neutral moleculesCleavage of small neutral molecules(CO(CO22, CO, olefins, H, CO, olefins, H22O ….)O ….)

Result often from rearrangementResult often from rearrangement

McLaffertyMcLafferty

x

CH2

CH2

H

CH2

O

CY

Y Y H, R, OH, NR2 H, R, OH, NR2Ion Stabilized Ion Stabilized by resonanceby resonance

x

CH2

CH2

H

CH2

O

CY

- CH- CH22=CH=CH22

x

CH2

O

CY

H

AlkenesAlkenesMost intense peaks are often:

m/z 41, 55, 69m/z 41, 55, 69

Rule 4: Double Bond Stabilize MRule 4: Double Bond Stabilize MRR++

Rule 5: Double Bond favor Rule 5: Double Bond favor Allylic cleavageAllylic cleavage

CHCH22 CHCH CHCHC+ EtEt

EtEtMeMe

-Et-

++CHCH22 CHCH CHCH

EtEtMeMe

CHCH22 CHCH CH CH ++

EtEtMeMe

-29-29

MMMM++ = 112 = 112 m/z = 83m/z = 83

AlkenesAlkenes

Aromatic Aromatic compoundcompound

AlcoholsAlcohols

Hydroxy compoundsHydroxy compounds

R2 C

R3

R1

O Hx

Loss of largest groupLoss of largest group

- R33

R2

CR1

O+

HR2

C+

R1O H

If RIf R11=H m/z 45, 59, 73 …=H m/z 45, 59, 73 …

If RIf R11=alkyl m/z 59, 73, 87 …=alkyl m/z 59, 73, 87 …

x

OH

CHR

H

CHR

CHR

CHR

OH+

CHRCHR

CHR

CHR

H

CHR+

CHR

CHR

CHR CHR+

CHR

CHR

CHR

x

OH

CHR

H

CHR

CHR

CHR

CHR

CHR

M – (HM – (H22O) – O) – (C1=C2) Alkene(C1=C2) Alkene

- H2O

- CHR=CHR

M – (HM – (H22O)O)

– – (H(H22O)O)

PhenolPhenol

Aromatic EtherAromatic Ether Molecular ion is prominent Molecular ion is prominent

1)1)

Cleavage in Cleavage in ββ of aromatic ring of aromatic ring

RearrangementRearrangement

2)2)

xO

R

O+

CC55HH55-CO-CO

m/z 93m/z 93 m/z 65m/z 65

O

H

- CH- CH22=CH=CH22 x

O

H

H

x

O

H

H

m/z 94m/z 94

- R- R--

Aliphatic EtherAliphatic Ether

CHCH33 —CH—CH22 —O—CH —O—CH22 —CH —CH22 —CH —CH22 —CH —CH33 CHCH33 —CH—CH22 —O —O+ + =CH=CH22

CHCH33 —CH—CH22 —O —O —CH—CH22++

•• ++

Cleavage of C-C next ot OxygenCleavage of C-C next ot OxygenLoss of biggest fragmentLoss of biggest fragment

m/z 59m/z 59

BB

Ether Ether RearrangementRearrangement

Index MS MS-fragmentation-2MS-fragmentation-2

CHCH33 —CH—CH22—CH —O —CH—CH —O —CH22 —CH —CH33

CHCH33

CH =OCH =O++ —CH —CH22

CHCH33

HH— CH— CH33

Box rearr.Box rearr.

CH =OCH =O++ HH

CHCH33

1- Cleavage of C-C next to Oxygen1- Cleavage of C-C next to Oxygen

m/z 73m/z 73

m/z 45m/z 45

MM·+·+

2- Cleavage of C-O bond: charge on alkyl2- Cleavage of C-O bond: charge on alkyl

m/z 73m/z 73

m/z 45m/z 45BB