Mass fragmentation & rules
- 1. Mass Fragmentation & Its Rules Advanced Instrumental Analysis M. Pharmacy II-Sem Mehul H Jain Pharmaceutical Analysis
- 2. Introduction • Mass spectroscopy is one of the most generally applicable tools providing both qualitative and quantitative information about the atomic and molecular materials. • Here the compound under the investigation is bombarded with a beam of electrons which produce an ionic molecule or ionic fragments of the original species. The resulting charging particle is then separated according their masses. Definition It is instrumental technique in which sample is converted to rapidly moving positive ions by which electron bombardment and charged particles are separated according to their masses. Mass spectra It is a plot of relative abundance against the ratio of mass/charge (m/e)
- 3. Working Principle • In this technique, molecules are bombarded with a beam of energetic electrons. • The molecules are ionized and broken up into many fragments, some of which are positive ions. • Each kind of ion has a particular ratio of mass to charge, i.e., m/e ratio. • For most ions, the charge is one and thus, m/e ratio is simply the molecular mass of the ion. • The ion pass through magnetic and electric fields to reach detector where they are detected and signals are recorded to give a mass spectra.
- 5. 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”.
- 6. General Rules For Fragmentation 1. The relative height of the molecular ion peak is greatest for the straight chain compound and decreases as the degree of branching increases. 2. The relative height of the Molecular ion peak usually decreases with increasing molecular weight in a homologous series. 3. Cleavage is favoured at alkyl substituted carbon atoms; the more substituted, the more likely is cleavage. This is a consequence of the increased stability of a tertiary carbon atom over a secondary, which in turn is more stable than a primary. CH3 + < RCH2 + < R2CH+ < R3C+
- 7. Stevensons Rule: 1. When an ion fragments, the positive charge will remain on the fragment of lowest ionization potential. 2. Generally the largest substituent at a branch is eliminated most readily as a radical because a long chain radical can achieve some stability by delocalization of the lone electron. Ex- cleavage of 1-methyl pentane 1-methyl pentane largest fragment 3. In this fragmentation, positive charge remains on the more high substituted fragments, i.e. the one with lower ionization potential. 4. Double bonds, cyclic structures and especially aromatic or hetero aromatic rings stabilize the Molecular ion and thus increase the probability of its appearance.
- 8. 5. Double bonds favour allylic cleavage and give the resonance stabilized allylic carbonium. Ex: Mass spectrum of 1-butene 6. Saturated rings tend to lose alkyl side chains at the a carbon atom. This positive charge tends to stay with the ring fragment. Ex: Mass spectrum of n-propyl cyclohexene
- 9. 7. In alkyl substituted aromatic compounds, cleavage is very probable at the bond β to the ring, giving the resonance stabilized benzyl ion or more likely, the tropylium ion. Ex: mass spectra of n-butyl benzene. 8. Cleavage is often associated with elimination of small, stable, neutral molecules such as carbon monoxide, olefins, water, ammonia, hydrogen sulphide, hydrogen cyanide, mercaptans, ketone, or alcohols, often with rearrangement.
- 10. General Modes Of Fragmentation Fragmentation of the molecular ion takes place in following modes: Simple cleavage 1. Homolytic cleavage 2. Heterolytic cleavage 3. Retro Diels-Alder reaction Rearrangement reactions accompanied by transfer of atoms. 1. Scrambling 2. Mc Lafferty rearrangement 3. Elimination
- 11. 1. Homolytic cleavage • Here fragmentation is due to electron redistribution between bonds.
- 12. 2. Heterolytic cleavage: Fragmentation by movement of two electrons: • In this type of cleavage both the electrons of the bond are taken over by one of the atoms; the fragments are an even electron cation and a radical with the positive charge residing on the alkyl group. It is designated by a conventional arrow (↶ or ↷) to signify the transfer of a pair of electrons in the direction of the charged site. 3. Retro Diels-Alder reaction: Elimination by multiple bond rupture: Cyclohexene is broken down to Diene and Dienophile. It can be explained by one electron mechanism.
- 13. One electron mechanism: Rearrangement reactions accompanied by transfer of atoms: 1. Scrambling: Fragmentation giving rise to stable carbocation: In certain cases fragmentation takes place at bond, which gives stable carbocation. 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.
- 14. 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. 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. The rearrangement results in the formation of charged enols and a neutral olefins.
- 15. To undergo Mc Lafferty rearrangement, a molecule must posses a. An appropriately located heteroatom (ex. oxygen) b. A double bond c. An abstractable Hydrogen atom which is γ (gamma) to C=O system. 3. Elimination Fragmentation due to loss of small molecule: Loss of small stable molecules such as H2O, CO2, CO, C2H4 from molecular ion during fragmentation. Ex- An alcohol readily looses H2O molecule and shows a peak 18 mass units less than the peak of molecular ion.
- 16. Fragmentation adjacent to the branching point: In case of branched alkanes, bond fission takes place adjacent to the branching point. Hence this leads to the formation of more stable carbocation Ex: 3-methyl pentane
- 17. 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. 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.
- 18. 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). Significance of Metastable ions: • Metastable ions are useful in helping to establish fragments routes. • Metastable ion peak can also be used to distinguish between fragmentation Processes, which occur in few microseconds.
- 19. 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. • In organic compounds, there is generally a small peak appearing at one mass unit higher than the parent peak (M+1) due to small but observable abundance of C, H, O, N, S isotopes. If the same sample contains two heavy isotopes like Cl, Br, then an additional smaller peak occurs at M+2. Isotope peak provides a useful means for determining the molecular formula of a compound.
- 20. Applications Mass spectrometry has both qualitative and quantitative uses. 1. Structure elucidation 2. Detection of impurities 3. Quantitative analysis 4. Drug metabolism studies 5. Clinical, toxicological and forensic applications 6. GC-MS-MS is now in very common use in analytical laboratories that study physical, chemical, or biological properties of a great variety of compounds.
- 21. Qualitative Applications 1. Determination of molecular weight 2. Determination of molecular formula 3. Determination of partial molecular formula 4. Determination of structure of compounds Quantitative applications 1. Determination of isotope abundance 2. Determination of isotope ratio 3. Differentiation between Cis and Trans isomers 4. Mass spectrometry in thermodynamics a. Determination of heat of vaporization b. Determination of heat of sublimation 5. Measurement of ionization potential 6. Determination of ion-molecule reactions 7. Detection of impurity 8. Identification of unknown compounds 9. Identification of proteins
- 22. References 1. Sharma Y.R. Elementary organic spectroscopy principles and chemical applications. 1st ed. S. Chand and Company ltd; New Delhi: 2008. 2. Chatwal G.R, Anand S.K. Instrumental methods of chemical analysis. 1st ed. Himalaya Publishing house; Mumbai: 2004. 3. S. Ravi Shankar. Text book of pharmaceutical analysis. 3rd ed. Rx publication; Tirunelveli: 2006. 4. Skoog DA, West DM. principle of instrumental analysis. 2nd edition.