MASS FRAGMENTATION.pptx

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 1 st Sem Associate Professor Department of Pharmaceutics Dadasaheb Balpande College Of Pharmacy , Besa , Nagpur – 440037 2022-2023 1

2. Content Basic concepts Fragmentation Process McLafferty Rearrangement Mass spectrum Metastable ion Isotopic peak Nitrogen Rule 2

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 3

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". 4

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.  6

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, 7

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 : 8

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. Ketonisation of the intermediate enol ion by the hydrogen transfer . Hydrogen transfer to enolic oxygen . Consider the McLafferty rearrangement in 4- Heptanone . 9

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. .

11. 11 Metastable Ions Fragment of a parent ion will give rise to a new ion (daughter) plus either a neutral molecule or a radical. M 1 + M 2 + + non charged particle An intermediate situation is possible; M 1 + may decompose to M 2 + while being accelerated. The resultant daughter ion M 2 + will not be recorded at either M 1 or M 2 , but at a position M* as a rather broad, poorly focused peak. Such an ion is called a metastable ion.

12. 12 Nature Of Metastable Ions : Metastable ions have lower kinetic energy than normal ions and metastable peaks are smaller than the M 1 and M 2 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 -5 seconds). 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 .

14. 14 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 . 15

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 ) No 2 + at m/e 46 and ii) No + at m/e 30 . Both these fragment ion appear at even mass number . 16

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 M+ - H i.e. , at m/e 183 and ( 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. 17

18. Reference Y. R. Sharma , Elementary Organic Spectroscopy , principles and chemical application , Revised Edition , Page No. 295-301 18

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