Fragmentation Pattern in Mass Spectra

Semester 3 Sakshi Arora

1. Samrat Prithviraj Chauhan Government College Ajmer 2021-2022 Department Of Chemistry Fragmentation Pattern in Mass Spectra Submitted By: Sakshi Arora M.Sc .(Chemistry) Semester III

2. Index

3. When the vaporised organic sample passes into the ionisation chamber of a mass spectrometer, it is bombarded by a stream of electrons of 9 to 15eV energy the molecular ion is produced, by loss of single electron . These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. This ion is called the molecular ion or the parent ion ( . M + e ⁻  + 2e ⁻ Neutral High energy Radical cation molecule electron (Molecular ion) The molecular ions are energetically unstable, and some of them will break up into smaller pieces.   Formation of Molecular Ion

4. It is break-up process of the molecular ion into smaller ions(fragment ions) if the energy of the bombarding electron is around 70eV, the additional energy is consumed in fragmenting the parent ion. It gives further information about the structure of the compound. If the lifetime of the molecular ion is greater than seconds, a peak corresponding to the molecular ion will appear in the mass spectrum. However, molecular ions with lifetimes less than seconds break apart into fragments before they are accelerated within the ionization chamber and enter the mass analyzer. A bond is cleaved in a way that the positive charge remains with one fragment while the odd electron goes with the other. Molecular Ion Cation Radical   Fragmentation

5. The uncharged free radical will not produce a line on the mass spectrum. Only charged particles will be accelerated, deflected and detected by the mass spectrometer. The greater the fragmentation of the molecular ion the greater is the loss of intensity of molecular ion peak. Fragmentation of the molecular ion A• B ⁺ [A-B] ⁺ • Radical Cation Molecular ion (a radical cation) A ⁺ •B Cation Radical

6. Preferred pathway is (I) since positive charge is on the fragment containing the heteroatom(oxygen) whose non bonding electrons provide resonance stabilisation.

7. The Mass spectrum of a compound is a record constructed by plotting mass/charge vs relative abundance. The base peak is the most intense peak in the spectrum. The m/z value of parent ion is equal to the molecular mass of the compound. The more stable an ion is, the more likely it is to form and the higher will be its peak height. The intensity of fragment ions in the mass spectrum reflect on the stability of the ion and the energy relationships of the bonds broken and formed during the reaction leading to the ion. Mass Spectrum

8. Number of fragment peaks The number of abundant ions and their distribution in the mass spectrum gives good information about the type of the molecule. The molecular ion peak in aromatic compounds is relatively much intense due to the presence of π electron system. Example- Aromatic molecules give abundant molecular ion of high stability(few fragment peaks) while aliphatic structure display many fragment ions(increasing in abundance towards low m/z values). Factors Affecting Fragmentation

9. Naphthalene Molecular ion is highly abundant. Due to absence of an easily cleaved group it give abundant molecular ion of high stability . Toluene

10. n-Butane It gives a series of ions separated by 14 mass units. 15( 29( ), 43( )   2,3-Butanedione It shows only two prominent peaks in addition to molecular ion peaks

11. 2. Common fragment ions and structure The presence of an ion at m/z=43 is often an evidence for the presence of a methyl ketone group( CO). The presence of an ion at m/z=91 is characteristic to show the presence of a benzylic residue in a compound. The presence of an ion at m/z=105 is characteristic to show the presence of in mass spectrum of benzophenone. The most intense ion at m/z=30 strongly indicates a primary amine while secondary at m/z=44 and tertiary at m/z=58.  

12. Characteristic m/z values due to functional groups The fragmentation pathway characteristic of a functional groups may provide distinction among isomeric alcohols. Primary and secondary alcohols usually give weak molecular ions and for tertiary alcohols, the molecular ion is either very weak or absent. Prominent base peaks in alcohol is at m/z=31, alcohol at m/z=45 and for alcohol is at m/z=59 due to loss of largest alkyl group.  

13. Fragmentation of Alkanes The relative abundance of saturated hydrocarbons is more than the corresponding branch chain compound with the same number of carbon atoms. Order of stability of carbocations: primary < secondary < tertiary A regular series of fragment peaks separated by 14( ) mass units are observed. There is loss of units in series M-14, M-28, M-42 etc. The intensity of molecular ion is low with increase in branching due to rapid and extensive fragmentation.   Mass Spectral Fragmentation of Organic Compounds

14. n-Pentane The peak with the highest m/z 72 value is called molecular ion peak and it is equal to molecular mass of pentane.

15. 2-Methylbutane Bond cleavage takes place preferably at the site of branching. Loss of radical gives a secondary carbocation.  

16. Fragmentation of 2-Hexanol Absence of molecular ion peak in the mass spectrum means that the compound is highly branched or 3 o alcohols whereas 2 o and 1 o alcohols give very small molecular ion peaks . B ase peak is at m/z=45 due to loss of larger alkyl group. A peak at 18 is due to loss of water.  

17. Fragmentation of 2-Pentanone The larger group on either side of the ketone is preferably lost by an α cleavage. The major fragmentation peak is due to formation of resonance stabilized acylium ion.

18. Fragmentation of 2-Bromopropane Isotopic peaks are also formed along with the molecular ion peak, in case of bromo compounds M + and (M + + 2) peaks are formed in the intensity ratio 1 : 1. (M+2) peak is of same intensity compared to the parent peak.

19. Fragmentation of E thylamine If the is formed at the odd mass number, then the molecule carries an odd number of nitrogen atoms. The base peak is formed at m/z =30 for primary amines due to (iminium ion) which results from α-cleavage of molecular ion. Loss of largest branch from the α-carbon is preferred.  

20. REFERENCE