MASS-Fragmentation patterns, introduction, principles and applications

MASS-Fragmentation patterns, introduction, principles and applications

1. FRAGMENTATION
PATTERNS
Presented By:
NIVEDITHA G
1
st
M.Pharm
Dept of pharmaceutics
NARGUND COLLEGE OF PHARMACY

2. RULES FOR FRAGMENTATION
Therelativeheightofthemolecularionpeakisgreatestforthe
straightchaincompoundanddecreasesasthedegreeof
branchingincreases.
Therelativeheightofthemolecularionpeakusuallydecreases
withincreasingmolecularweightinahomologousseries.
Cleavageisfavoredatalkylsubstitutedcarbonatoms;themore
substitutedthemorelikelytocleavage.Thisisaconsequence
oftheincreasedstabilityofatertiarycarbonoverasecondary,
whichinturnismorestablethanaprimary.
Doublebonds,cyclicstructuresandespeciallyaromaticor
(heteroaromatic)ringsstabilizethemolecularion,andthus
increasetheprobabilityofitsappearance.
2

3. Double bonds favor allylic cleavage and give
the resonance stabilized allylic carbocation.
β-bond is more susceptible to cleavage in
unsaturated compounds.
3

4. GENERAL MODES OF FRAGMENTATION:
Fragmentationofthemoleculariontakesplacein
followingmodes:
*Simplecleavage
1.Homolyticcleavage
2.Heterolyticcleavage
3.RetroDiels-Alderreaction
*Rearrangementreactionsaccompaniedbytransfer
ofatoms.
1.Scrambling
2.McLaffertyrearrangement
3.Elimination
4

5. FRAGMENTATION PATTERNS OR
FRAGMENTATION TYPES:
A)Simplecleavage
1.Homolyticcleavage:
Fragmentationbymovementofoneelectron:
InaMolecularionwhenthebondsarerupturedby
movingoneelectronandthemovementisgenerally
indicatedbyfishhookarrows.Thebondcleavagemay
involvethebondruptureandthenetresultisformationof
astablecarbocation.
Ex:
R-CH
2
-CH
2
-R’ΞR-CH
2
:CH
2
-R’ R-CH
2
-CH
2
-R’ R-CH
2
+
+
-e
.
CH
2
-R
2. Heterolyticcleavage:
Fragmentation by movement of two electrons
5

6. 3.RETRODIELS-ALDERREACTION:
ELIMINATIONBYMULTIPLEBONDRUPTURE:
Unsaturated rings undergo Retro –Diels –Alder reaction.
Ex: cyclohexene is broken down to Diene and Dienophile.
B)Rearrangementreactionsaccompaniedbytransferof
atoms:
1.Scrambling:
Fragmentationgivingrisetostablecarbocation:
Incertaincasesfragmentationtakesplaceatbond,whichgives
stablecarbocation.
6

7. Ex-Molecular ion from the alkyl benzene undergoes fragmentation at
the benzylic bond and final product is seven membered cyclic ion
known as Tropylium ion.
2. Mc Lafferty rearrangement:
Fragmentation due to rearrangement of Molecular or Parent ion:
Here cleavage of bonds in Molecular ion is due to the intramolecular
atomic rearrangement.
Such rearrangement involves the transfer of hydrogen from one part of
the molecular ion to another via, preferably, a six-membered cyclic
transition state.
7

8. Compoundscontaininghydrogenatomatpositiongammato
carbonylgrouphavebeenfoundtoarelativeintensepeak.This
isprobablyduetorearrangementandfragmentationis
accompaniedbythelossofneutralmolecule.
Therearrangementresults-chargedenolsandaneutralolefins.
ToundergoMcLaffertyrearrangement,amoleculemustposses
1)Unsaturatedcarbonyl,alkylgroup
2)γ-Hydrogen
3)β-cleavage.
8

9. TABLEFORTHECOMMONMCLAFFERTYPEAKINTHESPECTRAOF
CARBONYLGROUP.
Compound type Substituent (R) McL peak
Aldehyde -H 44
Methyl ketone -CH
3
58
Amide -NH
2
59
Acid -OH 60
Ethyl ketone -CH
2
CH
3
72
Methyl ester -OCH
3
74
9

10. TheC–Cbondsnexttoaheteroatomarefrequentlycleaved,leavingthe
chargeonthefragmentcontainingtheheteroatomwhosenonbonding
electronsprovideresonancestabilization.
Cleavageisoftenassociatedwitheliminationofsmallstableneutral
molecules,suchascarbonmonoxide,olefins,water,ammonia,hydrogen
sulfide,hydrogencyanide,mercaptans,ketonesoralcohol,oftenwith
rearrangement. 10

11. FRAGMENTATION PATTERNSOFSOMECHEMICALGROUPS:
1) Saturated Hydrocarbons-
a.Straight chain compounds:Following are the features of the mass
spectra of Alkanes
The relative height of the parent peak decreases as the molecular mass
increases in the homologous series.
TheMolecularionpeak(althoughweak)isnormallypresent.
Thespectragenerallyconsistofclustersofpeaksseparatedby14mass
unitscorrespondingtodifferencesofCH
2
groups.
ThelargestpeakineachclusterrepresentsC
n
H
2n+1
fragment.Thisis
accompaniedbyC
n
H
2n
andC
n
H
2n-1
fragmentcorrespondingtotheloss
ofoneandHatomsrespectively.
Aseriesofpeaksseparatedby14massunitsduetoCH
2groupincludes
CH
3
+
,C
2H
5
+
,C
3H
7
+
,C
4H
9
+
. 11

12. Example: Dodecane
12

13. b. Branched Chain Hydro Carbons:
GreaterthebranchinginAlkaneslessistheappearanceofthe
molecularionandifitappears,intensitywillbelow.
Bondcleavagetakesplacepreferablyatthesiteofbranching.
Duetosuchcleavage,morestablesecondaryortertiary
carboniumionsresults.
Generally,largestsubstituentatabranchiseliminatedreadilyas
aradical.Theradicalachievesstabilitybythede-localizationof
loneelectron.
Greaternumberoffragmentsresultsfromthebranchedchain
compoundcomparedtothestraightcompound.Thisisdueto
greaterpathwaysavailableforcleavage.
Ex:5-methylpentadecane
13

14. AROMATIC HYDROCARBONS
Anaromatichydrocarbongenerallyshowsaprominentmolecular
ionpeakbecauseofstabilizingeffectofring.
Alkylsubstitutedaromaticcompoundsundergobetacleavagein
thesidechainandgiveresonancestabilizedbenzylcationormore
likelythetropyliumion.Aprominentpeak(m/e=91)indicates
thepresenceofanalkylsubstitutedbenzylring.
m/e=91 m/e=65C
+
CH
3
CH
2
+
HCH
3
CH CH
14

15. Aromaticcompoundswithalkylgroupshavinga
chainofatleastthreecarbonatomscanundergo
ashiftofγ-hydrogenprobablyaccordingto
McLaffertyrearrangement.
Asaresult,aprominentpeakatm/e=92is
obtained.
15

16. CYCLOALKANES:
Incaseofcycloalkanes,themolecularionsaremore
abundantbecausetheringstructuresaremorestable.
Skeletalfissionsoccurwhichinvolvesthefissionof
atleasttwobonds.CH
2
CH
2
+
CH
2
+
CH
2
+
CH
2
CH
2
16

17. ALKENES:
Themolecularionofalkenescontainingonedoublebondtendsto
undergoallyliccleavagei.e.atthebetabondwithoutthedoublebond
andgivesresonancestructure.
Therelativeabundanceofthemolecularionpeakdecreaseswith
increaseinmolecularmass.
Thecyclicolefinalsoshowsgroupofpeakswhichare14massunits
apart.
Thegeneralmodeoffragmentationistheallyliccleavage.
ThefragmentsformedbyMcLaffertyrearrangementareintense.
Ex: 1-pentene
17

18. CYCLOALKENES:
Cycloalkenesusuallyshowadistinctmolecularionpeak.A
uniquemodeofcleavageisatypeofRetroDiels-Alder
reaction.Thefragmentationmodeinvolvesthecleavageof
twobondsofacyclicsystemresultingintheformationof
twostableunsaturatedfragmentsinwhichtwonewbonds
areformed.
18

19. ALCOHOLS
The molecular ion peak of 1
º
and 2
º
alcohol is usually of low
abundance. It is not detected in 3
º
alcohols.
The fragmentation modes in alcohols depend upon the fact
whether it is 1
º
, 2
º
or 3
º
alcohols.
The fragmentation of C-C bond adjacent to oxygen atom is
the preferred fragmentation mode i.e. α cleavage.
1
º
alcohols shows M-18 peaks, corresponding to the loss of
water.
19

20. Long chain members may show peaks corresponding to
successive loss of H radicals at M-1, M-2 and M-3.
The CH
2
=OH is the most significant peak in the spectra
of 1
º
alcohols.
Secondary alcohols cleave to give prominent peaks due
R-CH=OH at m/z=45,59,73.
Tertiary alcohols fragment to give prominent peaks due
to RR
’
C=OH
+
at m/z 45,59,73.
20

21. AROMATICALCOHOLS:
The relative abundance of the parent ion of aromatic alcohols
is large.
Some of the fragment modes of benzyl alcohol are loss of
one, two or three hydrogen atoms.
M+H fragment of Benzyl alcohol also rearranges to form
hydroxy tropylium ion.
The OH group in the Benzyl positions fragments in a way,
which favors charge retention on the aryl group.
21

22. PHENOLS:
Themolecularionpeakisintense.
Thepeakduetothelossofhydrogenradical,M
+
-Hissmall.
Thefragmentionduetothelossofcarbonmonoxideis
significantM-28(LossofCO)&M-29(LossofCHO)peaks.OH O
H
H
H
H
+
OH
H
H
+
M-28
M-29
m/e=65
22

23. NITROCOMPOUNDS:
Aliphatic Nitro compounds fragment by loss of NO
2
to
give strong carbonium ion.
R-NO
2
+
R
+
+ NO
2
An Mc Lafferty rearrangement occurs but such type of
peaks are weak.
Ex: Nitrobenzene.
23

24. ALIPHATICACIDS:
Themolecularionpeakinaliphaticacidsislessintenseas
comparedtothatofaromaticacids.
Carboxylgroupisdirectlyeliminatedbyαcleavageanda
signalisformedatm/e45.
Ifαcarbonatomisnotsubstitutedinaliphaticacids
containingagammahydrogenatom,McLafferty
rearrangementionisformedatm/e60.Itisoftenthebase
peak.
Inshortchainacids,M-OHandM-COOHpeaksare
prominent. 24

25. ETHERS
Aliphaticethersundergofacilefragmentationandexhibita
weakmolecularionpeakbecausetheresultantionishighly
stabilizedbyresonance.
Themajorfragmentationmodesoccurthroughαandβ
cleavages
αcleavage:
Cleavageofabondαtooxygenoccurs,thepositivecharge
beingretainedonthecarbonratherthanonoxygen.
βcleavage:
Aheteroatominducescleavageatabondβtoit.Sucha
cleavagefavorsthelossoflargergroupasaradical.The
ionundergoesfurtherfragmentationtogivetheseriesof
ionsatm/z45,59,73,87.
Ex:Ethylsec-butylether.
25

26. AROMATICACIDS
Benzoic acid:
26

27. AROMATICAMIDE:
Ex: Benzamide
27

28. AROMATICAMINES:
Ex: m-toluidine
28

29. REFERENCES:
Elementary organic spectroscopy By:
Y.R.Sharma
Spectrophotometric Identification of Organic
Compounds By: R.M.Silverstein & Webster.
Organic Spectroscopy By: William Kemp
Internet sources
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