MASS SPECTROSCOPY FRAGMENTATION & INTERPRETATION PDF

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MASS SPECTROSCOPY
FRAGMENTATION

& INTERPRETATION

www.DuloMix.com

 

CONTENTS

 FRAGMENTATION
 PRESENTATION OF MASS SPECTRA
 CHARACTERISTICS OF MASS SPECTRA
 INTERPRETATION OF MASS SPECTRA
 MASS SPECTRA OF CHEMICAL CLASSES
 CONCLUSION
 REFERENCES www.DuloMix.com

 

FRAGMENTATION

 CHEMICAL PROCESS RESULTING IN BOND BREAKING
 INTIATED BY ELECTRON IMPACT
 MOLECULAR ION UNDERGO FRAGMENTATION
 CATION –RADICAL CHARACTER IS THE MAJOR DRIVING

FORCE
EI

[M·]+ A+ + B· (neutral)

or
www.D

B+ uloMix + A·
.com

 

Example: The spectrum of hexane

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13 com

 

TYPES OF FRAGMENTATION

 SIMPLE CLEAVAGE

 RETRO-DIELS ALDER REACTION

 SKELETAL REARRANGEMENTS

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SIMPLE CLEAVAGE

 CLEAVAGE OCCURS ONLY ON SINGLE
COVALENT BOND

 TWO TYPES
– HOMOLYTIC CLEAVAGE
– HETEROLYTIC CLEAVAGE

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HOMOLYTIC CLEAVAGE

 BREAKAGE OF C-C BONDS
 THREE MODES OF CLEAVAGE

MODE I
MODE II
MODE III

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MODE I

 CLEAVAGE OF C-C BOND IN β-POSITION TO
SINGLE BONDED HETERO ATOM

eg:Alcohols,Amines

RO:
RO +

w RO:
Obs. in Mass wSwpec + neutr

.Dulo al
Resonance stabiliMzied

x.com

 

MODE II

 CLEAVAGE OF C-C BOND AT α-
POSITION WHERE HETERO ATOM
ATTACHED BY DOUBLE BOND

eg: ketones,esters,amides

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+

:O . : O + +
neutral

Prominent for +
: CH3C=O+

O
ketones :

m/z=43
Obs.w win mass spec.

w.
AcyliuDmuloM ions are

ix
resonance.-cosmtabilized

 

MODE III

 CLEAVAGE OF C-C BOND β TO
AROMATIC RING

eg: benzylic cleavage

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CH2+
Benzylic
cation
(stabilized C+ CH
including 2

H
“tropylium” +

H C
ion m/z=91

CH2

ww
Good cleavage w.Duto

lo CH+
aromatic rings Mix.com

 

HETEROLYTIC CLEAVAGE

 CLEAVAGE OF C-X BOND
 CHARGE CARRIED BY CARBON ATOM
 ALKYL IODIDES &BROMIDES EASILY

BROKEN

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CLEAVE  TO HETEROATOMS LIKE O, N

neutral
R

+
R • : O. :

:O.
+

+

w Observed in Mass Spec
ww.Dulo provided that a good stabilized

Heterolytic cleavage Mix.comcarbocation can form

 

RETRO-DIELS ALDER
REACTION

 MULTICENTRED FRAGMENTATIONS
eg:cyclic olefins

 CLEAVAGE OF TWO BONDS
 2 STABLE UNSATURATED FRAGMENTS
 CHARGE CARRIED BY EITHER OF 2

FRAGMENTS
 MORE STABLE ONE PREDOMINATES

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THE “RETRO -DIELS-ALDER”
CLEAVAGE

Observe

+ d! +
• •

+

+

Cyclohexenes, with favorable 6- +
membered transition state. Cwan

w
include heteroatoms (N,O, drivw.Deu n

lo
by keto-enol like stability. Mix.co Observe

m
d!

 

SKELETAL REARRANGEMENTS

 THREE TYPES

 ELIMINATION REACTIONS

 ORTHO EFFECT

 McLAFFERTY REARRANGEMENT

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ELIMINATION REACTIONS

ABSTRACTION OF H BY OH ,X OR –OOCCH3
 ELIMINATION OF H2O,HX,CH3COOH

R-CH2-CH-CH2 R-CH2-CH=CH2
H OCOCH3

w +CH3COOH
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ORTHO EFFECT

 OCCURS IN O-SUBSTITUTED
AROMATIC& CIS OLEFINS

 H-ATOM IS IN CLOSE PROXIMITY TO
ELIMINATE A NEUTRAL MOLECULE

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McLAFFERTY REARRANGEMENT

 RADICAL CATIONS LOCALIZED ON
CARBONYL GROUP GIVE β CLEAVAGE

 NEEDS H ATOM ON γ-sp3 CARBON

 OCCURS IN ALDEHYDES
,KETONES,ESTERS

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H R1
+ R2 H

O • + R1 R2
O Loss of

+ neutral

alkene

The new ••

radical OH

cation is
stabilized
by www
resonance .DuloMix.com

 

IMPORTANT EXAMPLE OF
MCLAFFERTY REARRANGEMENT

+
OH

OH

m/z = 60

Seen for primary carboxylic acids
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Nitroaromatics
m/z= 93

Loss of
(this can

+ •N=O
O + form

N O
from lots

O• of
Loss of different
CO origins)

Aromatic!
CH+

m/z=65
www

Good test f.DoulorM ix
aryloxy .com

 

Common Mass Spec Fragments
m/z lost Moiety Compounds Exhibiting Loss

1 H aldehydes

15 CH3 branched sites

16 O sulfoxides, nitro compounds

16 NH2 amides, aromatic amines

17 OH acids

w
18 H2O alcohwwo.D ls, aldehydes, ketones, ethers

uloMix.com

 

Common Mass Spec Fragments

m/z lost Moiety Compounds Exhibiting Loss

26 CN alkylcyanides

29 C2H5 or CHO alcohols

31 OCH3 or CH2OH methyl esters, alcohols

35 Cl halide-containing

45 OC2H5 or COOH ethyl esters or carboxylic acids
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FRAGMENTATION RULES

 Peak height of [M]+.↑ for straight chain
compounds

 Peak height ↓ as branching increases
 Cleavage favoured acco: to stability of

carbocation
30 >20 >10

 Aromatic rings & cyclic structures stabilize [M]+.

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Contd..….

 Double bonds favours allylic cleavage
 Unsaturated rings undergo retro-diels

alder reaction
 Alkyl substituted aromatics give

tropylium ion
 Cleavage releases small,stable,neutral

molecules like CO,H2O etc
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FACTORS AFFECTING
FRAGMENTATION

 THERMAL DECOMPOSITION
 BOMBARDMENT ENERGY
 RELATIVE RATES OF COMPETING

FRAGMENTATION ROUTE

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MASS SPECTRUM

PRESENTATION

 REPRESENTED BY BAR GRAPH
 PRODUCES A SPECTRUM OF MASSES BASED ON

STRUCTURE OF MOLECULE

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Fragment Ions

M•+ (EI)
“molecular ion”

base
peak

isotope peak
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mix/.cozm

 

X-AXIS

 REPRESENTS m/z VALUES
 m –unified atomic mass
 z- charge on the ion (positive value)

m/z values give mol.wt of ion

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Y-AXIS

 REPRESENTS SIGNAL INTENSITY
ie, relative abundance of each ion
produced

most intense peak is base peak(100%)

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CHARACTERISTICS

MOLECULAR ION PEAK

 REPRESENTED BY [M]+.

 MOLECULE BOMBARDED WITH
ELECTRON BEAM PRODUCES [M]+.

 GIVES MOL.WT OF COMPOUND
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ISOTOPIC ABUNDANCE
3 Classes of Isotopes

 A – only a single isotope
 EX: F, P, I

 A+1 – two isotopes with significant relative abundance
differing by 1 mass unit
 EX: H, C, N

 A+2 – two isotopes with significant relative abundance
ww

differing by 2 mass uniwt.DusloMi
 EX: Cl, S, O x.com

 

Natural Isotopic Abundance of
Common Elements in Organic
Compounds

Element Isotope Relative Isotope Relative Isotope Relative
abundan abundan abundan

ce ce ce

Carbon 12C 100 13C 1.11

Hydrogen 1H 100 2H .016

Nitrogen 14N 100 15N .38

Oxygen 16O 100 17O .04 18O .20

Sulfur 32S 100 33S .78 34S 4.40

Chlorine 35Cl 100 w 37
w Cl 32.5
w.Du

Bromine 79Br 100 loM 81
i Br 98.0
x.com

 

BASE PEAK

 MOST INTENSE PEAK
 ASSIGNED A VALUE OF 100%
 OTHER PEAKS RELATIVE TO BASE

PEAK

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INTERPRETATION
OF MASS SPECTRA

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GENERAL RULES FOR INTERPRETATION

1.EXACT MOLECULAR WEIGHT
2.ISOTOPE EFFECT
3.NITROGEN RULE
4.RING RULE

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1.EXACT MOLECULAR WEIGHT

 IDENTIFICATION OF THE PARENT
PEAK

 DETERMINES THE MOLECULAR
WEIGHT

 DETERMINES MOLECULAR FORMULA
FROM MOLECULAR WEIGHT

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2. ISOTOPE EFFECT
 CARBON 12 HAS AN ISOTOPE, CARBON 13.

12C= 100%,
13C= 1.1%.

 IF A COMPOUND CONTAINS 6 CARBONS, EACH
ATOM HAS A 1.1% ABUNDANCE OF (13)C.

 IF THE MOLECULAR ION PEAK IS 100%, THEN
THE ISOTOPE PEwAK (1 MASS UNIT HIGHER)

ww
WOULD BE 6X1.1%.Dulo=6.6%.

Mix.com

 

 LOOK FOR M+2 PEAK
eg:Cl,S,Br

 LOOK FOR M+1 PEAK
eg:C,H,N

PEAK INTENSITY RATIO OF
Cl – 1:3www.D
Br – 1:1 ulo M i x.com

 

3.NITROGEN RULE
 COMPOUND CONTAINING C, H, O, AND AN

EVEN NUMBER OF NITROGENS (OR NO
NITROGENS) WILL HAVE AN EVEN
MOLECULAR WEIGHT
C6H7BrN2

 COMPOUND CONTAINING C, H, O, AND AN
ODD NUMBER OF NITROGENS WILL HAVE
AN ODD MOLECULAR WEIGHT
C2H7NS

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4. RING RULE

 NO: OF UNSATURATED SITES
 R=NO:OF RINGS+NO:OF DOUBLE

BONDS+2*NO:OF TRIPLE BONDS
 IF CWHXNYOZ,

R=W+1+Y/2+X/2
Eg:C6H6,

R=6+1w+6/2=4(1RING+3DB)
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Major Steps in Analysis of
Mass Spectral Data
 IDENTIFICATION OF MOLECULAR ION

 BASE PEAK
 EXAMINATION OF ISOTOPIC DISTRIBUTION

PATTERN
 NEGATIVE INFORMATION
 DETERMINE ELEMENTAL COMPOSITION

 ANALYSIS OF FRAGMENTATION PATTERN
 PROPOSE POSSIBLE STRUCTURES
 COMPARE POSwTULATED SPECIES TO AVAILABLE

ww
REFERENCE SPEC.DulTo RA

Mix.com

 

Lets assume that we have obtain
following mass spectral data

Mass Abundance Mass Abundance

24 2.4 48 2.0

25 10.3 49 1.7

26 24.2 59 2.0

27 76.4 60 6.5

28 2.3 61 8.4

35 7.3 62 100.0 M
36 2.7 63 5.0

37 2.6 64 30.8 M+2
47 4.8 www.DuloMix.com

 

ASSUMPTIONS

 HERE, BASE PEAK=MOLECULAR ION
PEAK ie, AT 62

 ISOTOPIC PEAK AT M+2 SHOWS
PRESENCE OF Cl,Br,S ie,AT 64

 RATIO OF ISOTOPIC PEAK 1:3 ie,Cl IS
PRESENT

 AS BASE PEAK=PARENT PEAK DB IS
PRESENT www.DuloMix.com

 

 Identify M as 62 and (M+2) as 64

 62-35(Mol.wt of Cl)=27

 27-(2*12) = 3

 so CH2=CHCl w(wwv. inyl chloride)
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MASS SPECTRA OF SOME
CHEMICAL CLASSES

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ALKANES
Hexane

Molecular ion peaks
are present, possibly
with low intensity.

The fragmentation
pattern contains
clusters of peaks 14

www. mass units apart
DuloMix (which represent loss

.com of (CH2).

 

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ALCOHOL
3-Pentanol
C5H12O
MW = 88.15

An alcohol’s molecular
ion is small or non-
existent.
Cleavage of the C-C
bond next to the
oxygen usually occurs.
A loss of H2O may

www.D occur as in the spectra
uloMix.c below.

om

 

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ALDEHYDE
3-PHENYL-2-PROPENAL
M.W-132.18

Cleavage of bonds
next to the carboxyl
group results in the
loss of hydrogen
(molecular ion less
1) or the loss of
CHO (molecular ion

www less 29).
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KETONE
4-Heptanone
C7H14O
MW = 114.19

Major fragmentation
peaks result from
cleavage of the C-C
bonds adjacent to the
carbonyl.

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CARBOXYLIC ACID
2-Butenoicacid
C4H6O2
MW = 86.09

In short chain acids,
peaks due to the loss
of OH (molecular ion
less 17) and COOH
(molecular ion less 45)
are prominent due to

www.D cleavage of bonds next
uloMix. to C=O.

com

 

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ESTER

Ethyl acetate
C4H8O2
MW = 88.11

Fragments appear
due to bond
cleavage next to
C=O (alkoxy group
loss, -OR) and
hydrogen

www.D rearrangements.
uloMix.com

 

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HALIDE
1-Bromopropane
C3H7Br
MW = 123.00

1.The presence of chlorine or
bromine atoms is usually
recognizable from isotopic peaks.

2.Iodides and bromides are easily
www released

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CONCLUSION

 MASS SPECTRUM PROVIDES INFORMATION
ABOUT

-MOLECULAR WEIGHT OF ORGANIC
& INORGANIC COMPOUNDS
-MOLECULAR FORMULA OF
COMPOUNDS

-ANALYTICAL TOOL FOR
CHARACTERIZING ORGANIC MOLECULES

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REFERENCES

 Gurdeep R Chatwal Instrumental Methods Of
Chemical Analysis Fifth Edition :Himalaya
publishing house. Mumbai 2002.

 William Kemp Organic Spectroscopy third
Edition ELBS:Hong Kong 1993

 Silverstein Spectrometric Identification Of
Organic Compounds Sixth Edition
:Wiley&sons,Inc. Newyork1998

 Y.R.Sharma,Elementary Organic
Spectroscopy,20w0ww6 Edition

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