FRAGMENTATION PATTERNS & FRAGMENTATION CHARACTERISTICS IN RELATION TO PARENT STRUCTURE AND FUNCTIONAL GROUPS GENERAL RULES FOR FRAGMENTATION: A number of general rules for predicting prominent peaks in Electronic Impact spectra are as follows: 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. (Fatty esters appear to be an exception). 3. Cleavage is favored 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. CH 3 + < RCH 2 + < R 2 CH + < R 3 C + STEVENSONS RULE: When an ion fragments, the positive charge will remain on the fragment of lowest ionization potential.
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FRAGMENTATION PATTERNS & FRAGMENTATION
CHARACTERISTICS IN RELATION TO PARENT STRUCTURE AND
FUNCTIONAL GROUPS
GENERAL RULES FOR FRAGMENTATION:
A number of general rules for predicting prominent peaks in Electronic
Impact spectra are as follows:
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. (Fatty esters appear to be an exception).
3. Cleavage is favored 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+
STEVENSONS RULE:
When an ion fragments, the positive charge will remain on the fragment of lowest
ionization potential.
Generally the largest substituent at a branch is eliminated most readily as a radical,
presumably 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
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.
5. Double bonds favor allylic cleavage and give the resonance stabilized allylic
carbocation.
Ex: Mass spectrum of 1-butene
6. Saturated rings tend to lose alkyl side chains at the carbon atom. This positive
charge tends to stay with the ring fragment.
Ex: Mass spectrum of n-propyl cyclohexene
Unsaturated rings can under go retro-Diels-Alder reaction:
7. In alkyl substituted aromatic compounds, cleavage is very probable at the bond
to the ring, giving the resonance stabilized benzyl ion or more likely, the
tropylium ion: Ex: mass spectra of n-butyl benzene.
8. The C-C bond next to the hetero atom are frequently cleaved, leaving the charge
on the fragment containing the hetero atom whose non bonding electrons provide
resonance stabilization.
Ex:
9. Cleavage is often associated with elimination of small, stable, neutral molecules
such as carbon monoxide, olefins, water, ammonia, hydrogen sulphide, hydrogen
cyanide, mercaptans, ketene, or alcohols, often with rearrangement.
Factors influencing Fragmentation process:
1. Bombardment energies
2. Functional groups
3. Thermal decomposition
4. Relative rates for computing fragmentation routes
1. Bombardment energies: Bombardment with low energy voltage (10-15 eV)
results in the formation of Molecular ion only. As the bombardment energy is
increased to 50-70eV, formation of Fragment ion is favored. The relative
abundance of ions in a spectrum is only reproducible when bombardment energies
are constant.
2. Functional groups: Functional groups like –OH, -COOH, -CHO, -CO, -NH2 etc
will influence the fragmentation process. They favor the fragmentation at their
vicinity. So characteristic fragments are produced. Depending on the electron
releasing and electron withdrawing nature they produce typical fragments.
Ex: Alcohol produce CH2=OH fragment, favor the loss of H2O.
3. Thermal decomposition: Thermal decomposition of thermo labile compounds
may occur in the ion source, and commonly leads to difficulty in interpreting the
mass spectrum of alcohol which may be dehydrated before ionization. In case of
Alcohols, thermal decomposition may be wrongly interpreted for loss of water
molecule, whether the loss occur before or after ionization.
If thermal decomposition is suspected, the compound can be ionized in a cooled
ion source. So the electron bombardment of whole molecule takes place.
4. Relative rates for competing fragmentation routes: When two reactions are
occurring at time, fragments are produced from that reaction whose reaction time is
less.
Ex: A+ --------------- B+ + C (fast) ----------1
A+ --------------- B+ + C (slow) ----------2
So the fragments produced are of reaction -------1
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
FRAGMENTATION PATTERNS OR FRAGMENTATION TYPES:
* Simple cleavage
1. Homolytic cleavage:
Fragmentation by movement of one electron:
In a Molecular ion when the bonds are ruptured by moving one electron and the
movement is generally indicated by fishhook arrows ( ). The bond
cleavage may involve the bond rupture and the net result is formation of a stable