This slide contains fragmentation of important functional groups i.e. Alkane and Carbonyl compounds in mass spectrometry.

1. FRAGMENTATION OF
IMPORTANT FUNCTIONAL
GROUPS –
ALKANES
AND
CARBONYL COMPOUNDS
PREPARED BY
PRAVIN Y. PATIL
FIRST YEAR M. PHARM
(PHARMACEUTICS)
MODERN ANALYTICAL TECHNIQUES
PARUL INSTITUTE OF PHARMACY,LIMDA.
1

2. FRAGMENTATION OF IMPORTANT
FUNCTIONAL GROUPS –
ALKANES
AND
CARBONYL COMPOUNDS
2

3. TABLE OF CONTENT
 Introduction.
 Fragmentation process.
 Fragmentation patterns and fragmentation characteristic
relation to parent structure and functional groups.
 Alkane and it’s example.
 Carbonyl compounds and it’s example.
 References.
3

4. INTRODUCTION
 An extremely useful result of Electron Impact ionization
in particular is a phenomenon known as fragmentation.
 The radical cation that is produced when an electron is
knocked out of a neutral closed-shell molecule in
Electron Impact MS initially possesses a lots of energy.
4

5. CONTI…
 Energy sufficient to break chemical bonds, radical cation
will usually break into a neutral radical and a cation. It
is also possible for a neutral closed shell fragment (such
as water) to fall off.
 Peaks with smaller m/z values called fragment ion peaks
represent positively charged fragments of the molecule.
5

6. BACKGROUND
 Mass spectrometry uses high energy electrons to
break a molecule into fragments.
 Separation and analysis of the fragments provides
information about:
 Molecular weight.
 Structure.
6

7. CONTI………
 The impact of a stream of high energy electrons
causes the molecule to lose an electron forming a
radical cation.
 A species with a positive charge and one unpaired
electron.
+ e
-
C H
H
H
H H
H
H
HC + 2 e
-
Molecular ion (M+)
m/z = 16 7

8. CONTI…….
 The impact of the stream of high energy electrons
can also break the molecule or the radical cation
into fragments.
8

9. FRAGMENTATION PROCESS
 Bombardment of molecules by an electron beam with
energy between 10-15ev usually results in the ionization of
molecules by removal of one electron (Molecular ion
formation).
 When the energy of electron beam is increased between
50-70ev, these molecular ions acquire a high excitation
resulting in their break down into various fragments. This
process is called “Fragmentation process”.
9

10. FRAGMENTATION PATTERNS
AND
FRAGMENTATION CHARACTERISTICS
RELATION TO
PARENT STRUCTURE
AND
FUNCTIONAL GROUPS
10

11. FRAGMENTATION
PATTERNS
 The impact of the stream of high energy electrons
often breaks the molecule into fragments,
commonly a cation and a radical.
 Bonds break to give the most stable cation.
 Stability of the radical is less important.
11

12. SATURATED
HYDROCARBONS
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.
 The Molecular ion peak (although weak) is normally present.
 The spectra generally consist of clusters of peaks separated by
14 mass units corresponding to differences of CH2 groups.
12

13. CONTI….…
 The largest peak in each cluster represents CnH2n+1
fragment. This is accompanied by CnH2n and CnH2n-1
fragment corresponding to the loss of one and two H
atoms respectively.
13

14. CONTI…….
BRANCHED CHAIN HYDROCARBONS:
 Greater the branching in Alkanes less is the appearance
of the molecular ion and if it appears, intensity will be
low.
 Bond cleavage takes place preferably at the site of
branching. Due to such cleavage, more stable secondary
or tertiary carbonium ions results.
14

15. CONTI…….
 Generally, largest substituent at a branch is
eliminated readily as a radical. The radical achieves
stability by the de-localization of lone electron.
 Greater number of fragments results from the
branched chain compound compared to the straight
compound. This is due to greater pathways available
for cleavage.
15

16. ALKANES
The general rules for alkane are as follows:
 The relative height of the Molecular ion peak is greatest for
straight chain molecules and decreases as the branching
increases.
 The relative height of the Molecular ion peak decreases with
chain length for a homologous series.
 Cleavage is favoured at alkyl-substituted carbons, with the
probability of cleavage increasing as the substitution
increases. 16

17. CONTI…
 Fragmentation often splits off simple alkyl groups:
 Loss of methyl M+ - 15
 Loss of ethyl M+ - 29
 Loss of propyl M+ - 43
 Loss of butyl M+ - 57
 Branched alkanes tend to fragment forming the most
stable carbocations.
17

18. EXAMPLES
OF
ALKANES
18

19. THE MASS SPECTRUM OF
2-CHLOROPROPANE
19

20. CONTI…….
 The compound contains a chlorine, because M+2
peak is 1/3 the height of the molecular ion peak.
 The base peak at m/z = 43 results from heterolytic
cleavage of the C–Cl bond.
 The peaks at m/z = 63 and m/z = 65 have a 3:1
ratio, indicating the presence of a chlorine atom.
20

21. CONTI…….
 A cleavage results from the homolytic cleavage of a C–C bond at
the α carbon.
21

22. PENTANE
 The base peak of 43 in the mass spectrum of pentane indicates the
preference for C-2 to C-3 fragmentation.
22

23. CONTI…….
 To identify fragment ions in a spectrum, determine the
difference between the m/z value of a given fragment ion
and that of the molecular ion.
 Carbocations can undergo further fragmentation.
23

24. CONTI…….
 2-methylbutane has the same m/z as pentane.
24

25. 2-METHYLPENTANE
 Mass spectrum of 2-methylpentane(M.W.-86)
25

26. CARBONYL
COMPOUNDS
26

27. McLAFFERTY
REARRANGEMENT
 Fragmentation due to rearrangement of Molecular
or Parent ion: Here cleavage of bonds in
Molecular ion is due to the intramolecular atomic
rearrangement.
 This leads to fragmentation whose origin cannot
be described by simple cleavage of bonds.
 When fragments are accompanied by bond
formation as well as bond for breaking, a
rearrangement process is said to have occurred.
27

28. CONTI…
 Such rearrangement involves the transfer of hydrogen
from one part of the molecular ion to another via
preferably, a six-membered cyclic transition state.
 This process is favoured energetically because as many
bonds are formed as are broken.
 Compounds containing hydrogen atom at position
gamma to carbonyl group have been found to a relative
intense peak. 28

29. CONTI…
 This is probably due to rearrangement and fragmentation
is accompanied by the loss of neutral molecule. This
rearrangement is known as McLafferty rearrangement.
 The rearrangement results in the formation of charged
enols and a neutral olefins.
29

30. CONTI…
 To undergo McLafferty rearrangement, a molecule must
posses.
1. An appropriately located heteroatom (Ex. Oxygen).
2. A double bond.
3. An abstractable Hydrogen atom which is γ (gamma)
to C=O system.
30

31. ALDEHYDES
 The Molecular ion peak of aliphatic Aldehydes is weak.
 Aromatic Aldehydes shows moderate intense peak.
R-CH=O+ R-C≡O+ + H+
 Characteristic feature of Aldehyde (especially Aromatic Aldehyde)
is loss of α hydrogen.
 Second common feature of Aliphatic Aldehyde is β cleavage.
 For ex, Aldehyde with CH2-CHO end group gives rise to
characteristic M-43 peaks. 31

32. EXAMPLE
 Mass spectra of Valeraldehyde.
32

33. KETONES
 Molecular ion peaks are more intense than Aldehyde.
 Most of the abundant ions in the mass spectra of
Ketones can be accounted by α cleavage and
McLafferty rearrangement.
 Strong molecular ion peak.
 Formation of acylium ion (RCO+).
33

34. EXAMPLE
 Mass spectrum of 2-butanone.
34

35. ESTERS
 Weak Molecular ion peak but generally observable.
 Loss of alkyl group attached to the carbonyl group
by α cleavage.
 Formation of acylium ion (RCO+).
 McLafferty rearrangement.
 Acyl portion of ester is OR+.
 Methyl esters: M-31 due to loss of OCH3.
 Highly esters: M-32, M-45, M-46, M-59, M-60,
M-73 etc.
35

36. EXAMPLE
 Mass spectra of Methyl butyrate.
36

37. CARBOXYLIC ACIDS
ALIPHATIC CARBOXYLIC ACID:
 Weak Molecular ion peak but observable.
 A cleavage on either side of C=O.
 M-17 due to loss of –OH.
 M-45 due to loss of –COOH.
 McLafferty rearrangement gives base weak.
37

38. EXAMPLE
 Mass spectra of Butyric acid.
38

39. CONTI…….
AROMATIC CARBOXYLIC ACIDS:
 Strong Molecular ion peak.
 A cleavage on either side of C=O.
 M-17 due to loss of –OH.
 M-18 due to loss of HOH.
 M-45 due to loss of –COOH.
 McLafferty rearrangement gives base peak.
39

40. EXAMPLE
 Mass spectra of para-Anisic acid.
40

41. REFERENCES
 Silverstein R.M., Webster F.X., Kiemle D.J., Spectrometric
Identification of Organic Compounds, 7th edition, John Wiley and
Sons, Inc.; 2005, 19-70.
 Skoog D.A., West D.M., Principle of Instrumental Analysis, 2nd
edition, Sounders Golden Sunburst series, 1980; 476-496.
 Sharma Y.R., Elementary Organic Spectroscopy Principles and
Chemical Applications, 1st edition, New Delhi, S. Chand and
Company Ltd.; 2008, 291-345 .
41

42. CONTI…
 Pavia D. L., Lampman G.M., Kriz G.S., Introduction to
Spectroscopy, 3rd edition, India Edition, Cengage Learning
Publication; 2001, 404-441.
 Patel K.G., Shah P.A., Raval H.G., Shah D.A., Baldania S.L.,
Laboratory Handbook in Instrumental Analysis, 1st edition,
Nirav Prakashan, 2005; 327-350.
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