Content: Basic concepts Fragmentation Process McLafferty Rearrangement Mass spectrum Metastable ion Isotopic peak Nitrogen Rule Basic Concept: Mass spectrometry uses high energy electrons to break a molecule into fragmentation. A beam of high-energy electrons breaks the molecule apart. The masses of the fragments and their relative abundance reveal information about the structure of the molecule. Separation and analysis of the fragments provides information about: – Molecular weight – Structure 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". McLafferty Rearrangement: Fragmentation due to rearrangement of Molecular or Parent ion: The 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. 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. This is probably due to rearrangement and fragmentation is accompanied by the loss of neutral molecule. This rearrangement is known as Mc Lafferty rearrangement.  Thus , the molecular formula of the unknown compound can be determined from the various fragment ions and also the parent ion of the mass spectrum . More example of McLafferty reaarangements are : A double McLafferty rearrangement is also reported in ketones . The second hydrogen atom originates exclusively from the γ – position . A secondary hydrogen is preffered to a primary hydrogen atom in this process . The mechanism involves. Ketonisation of the intermediate enol ion by the hydrogen transfer . Hydrogen transfer to enolic oxygen . Consider the McLafferty rearrangement in 4- Heptanone. Mass Spectrum : It is a record of the masses and the relative abundances of the molecular ion and the positively charged fragments formed from it by the electron bombardment. The molecular ion or Parent ion : The electron bombardment with energy 10-15 eV usually removes one electron from the molecule of the organic compoound in the vapour phase it results in the formation of molecular ion The mass of the parent ion gives the molecular mass of the sample. .

1. MASS FRAGMENTATION
Presented By : Guided By :
Tanvi D.Mhashakhetri Dr.Amol Warokar
M.Pharm 1st Sem Associate Professor
Department of
Pharmaceutics
Dadasaheb Balpande College Of Pharmacy ,
Besa , Nagpur – 440037
2022-2023
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2. Content
• Basic concepts
• Fragmentation Process
• McLafferty Rearrangement
• Mass spectrum
• Metastable ion
• Isotopic peak
• Nitrogen Rule
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3. Basic Concept
• Mass spectrometry uses high energy electrons to break a
molecule into fragmentation.
• A beam of high-energy electrons breaks the molecule apart.
• The masses of the fragments and their relative abundance reveal
information about the structure of the molecule.
• Separation and analysis of the fragments provides information
about:
– Molecular weight
– Structure
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4. 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".
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5. McLafferty Rearrangement
Fragmentation due to rearrangement of Molecular or Parent ion:
• The 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.
• Such rearrangement involves the transfer of hydrogen from one
part of the molecular ion to another via, preferably, a six-
membered cyclic transition state. 5

6. • 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.
• This is probably due to rearrangement and fragmentation is
accompanied by the loss of neutral molecule. This rearrangement
is known as Mc Lafferty rearrangement.
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7. Butanal contains a γ- hydrogen atom. The McLafferty ion formed in
this case is shown below :
Similarly , a large number of organic compounds viz. ketones , amines ,
alcohols , esters , acids which contain a γ-hydrogen atom forms as a
McLafferty rearrange ion,
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8. Thus , the molecular formula of the unknown compound can be
determined from the various fragment ions and also the parent ion of the
mass spectrum .
More example of McLafferty reaarangements are :
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9. • A double McLafferty rearrangement is also reported in ketones .
• The second hydrogen atom originates exclusively from the γ – position .
• A secondary hydrogen is preffered to a primary hydrogen atom in this
process . The mechanism involves.
i. Ketonisation of the intermediate enol ion by the hydrogen transfer .
ii. Hydrogen transfer to enolic oxygen . Consider the McLafferty
rearrangement in 4- Heptanone.
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10. 10
Mass Spectrum
It is a record of the masses and the relative abundances of the
molecular ion and the positively charged fragments formed from it by
the electron bombardment.
The molecular ion or Parent ion
• The electron bombardment with energy 10-15 eV usually
removes one electron from the molecule of the organic
compoound in the vapour phase it results in the formation of
molecular ion
• The mass of the parent ion gives the molecular mass of the
sample. .

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Metastable Ions
Fragment of a parent ion will give rise to a new ion (daughter) plus
either a neutral molecule or a radical.
M1
+ M2
+ + non charged particle
An intermediate situation is possible; M1
+ may decompose to M2
+
while being accelerated. The resultant daughter ion M2
+ will not be
recorded at either M1 or M2 , but at a position M* as a rather broad,
poorly focused peak. Such an ion is called a metastable ion.

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Nature Of Metastable Ions:
Metastable ions have lower kinetic energy than normal ions and metastable
peaks are smaller than the M1 and M2 peaks and also broader. These
metastable ions arise from fragmentation that takes place during the flight
down through ion rather than in the ionization chamber.
Molecular ions formed in the ionization chamber do one of the
following things:
1. Either they decompose completely and very rapidly in the ion
source and never reach the collector (as in case of highly branched
molecular ions with life times less than 10-5seconds).
2. Or else they survive long enough to reach the collector and be
recorded there (life times longer than 10-5 seconds).

13. 13
Isotopic Peaks
• Mass spectrum of certain compounds show peaks that occur at one
or two m/e units greater than the parent ion.
• These peaks are attributable to those ions which have same
chemical formula but different isotopic compositions. The size of
the various peaks depends on the relative natural abundance of the
isotopes.
• If the same sample contains two heavy isotopes like Cl, Br, then an
additional smaller peak occurs at M+2.

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• In case of bromo compounds , M+ and ( M+2) peaks are formed
in the intensity ratio1:1
• In case of chloro compounds , M+ and (M+2) peaks are formed in
the intensity ratio 1:3 .
• Isotope peak provides a useful means for determining the
molecular formula of a compound.

15. Nitrogen Rule
• Many signals ( Peaks ) Can be ruled out as possible
molecular ions simply on the grounds of a resonable
structural requirements .
• It states that a molecule of even numbered molecular mass
must contain no nitrogen atom or an even number of nitrogen
atoms.
• An odd numbered molecular mass requires an odd number of
nitrogen atoms.
• The rule holds for all compound containing carbon ,
Hydrogen , Oxygen , Nitrogen , sulphur and halogens .
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16. • An important corollary of this rule states that the fragmentation at a
single bond gives an odd numbered ion fragment from an even
numbered molecular ion .
• Similarly , an even numbered ion fragment results from an odd
numbered molecular ion .
• The fragment ion must contain all the nitrogen atoms of the molecular
ion .
• For Ex.
Nitrobenzene
The signals for molecular ion appears at m/e 123 ; i.e. at odd numbered
molecular mass since the compound contains only one ( odd number )
nitrogen atom .
Two imortant ion fragments which are formed in the mass spectrum of
this compound are
i) No2
+ at m/e 46 and
ii) No+ at m/e 30 .
Both these fragment ion appear at even mass number .
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17. Consider a compound containing two ( even number ) nitrogen atoms ,
2,4 – dinitrophenol .
Its molecular ion ( M+ ) signal appears at m/e 184 . The fragment ion
appear at
i) M+ - H i.e. , at m/e 183 and
ii) ( M+ - H – CO ) i.e. , at m/e 155 .
Thus , the fragment ions containing both the nitrogen atoms appears
at odd mass number . This proves the validity of the nitrogen rule.
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18. Reference
Y. R. Sharma , Elementary Organic Spectroscopy , principles and
chemical application , Revised Edition , Page No.
295-301
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19. THANK YOU
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