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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