nitrogen rule malaffertey rearrangement mass spectroscopy mass fragmentation

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PRESENTED BY
Mr. Darshan N U B Pharm ( M Pharm)
M Pharmacy II Semester
Dept. of Pharmaceutical Chemistry
SACCP
PRESENTING TO
Dr. T. Yunus Pasha
Professor and Head
Dept. of Pharmaceutical Chemistry
SACCP
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Nitrogen rule and McLafferty rearrangement

2. McLafferty rearrangement
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3. Nitrogen rule
•The nitrogen rule states that a molecule that
has no or even number of nitrogen atoms has
an even nominal mass, whereas a molecule
that has an odd number of nitrogen atoms has
an odd nominal mass.
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6. Molecule: Ammonia Hydrazine TNT Caffeine
Formula: NH3 N2H4
C7H5N3O
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C8H10N4O2
m/z (M):
17
32 227 194
One nitrogen
m/z (M)
is odd
Two nitrogen
s
m/z (M)
is even
Three nitrog
ens
m/z (M)
is odd
Four nitroge
ns
m/z (M)
is even
•
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7. The McLafferty rearrangement is a reaction observed in mass
spectrometry during the fragmentation or dissociation of organic
molecules. It is sometimes found that a molecule containing a keto-
group undergoes β-cleavage, with the gain of the γ-hydrogen atom.
When hydrogen is available at the gamma position on carbonyl
compounds, these compounds go through a specific rearrangement
due to the cleavage of multiple bonds, including the bonds at alpha and
gamma positions. This reaction is known as the McLafferty
rearrangement.
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8. • The McLafferty rearrangement is a characteristic fragmentation of
the molecular ion of a carbonyl compound containing at least one
gamma hydrogen ., e.g.,
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9. Hexan-2-one
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10. • The fragmentation of the molecule ions of hexan-2-one by α-
cleavage at the carbonyl group .
• The α-cleavage at a carbonyl group gives rise to the
stable acetyl cation CH3CO+, m/z 43, and to the •C3H7 radical,
• while transfer of an H atom from the γ-CH2 group to the CO group
accompanied by cleavage of the C(α)C(β) bond produces the radical
cation of the acetone enol and a propene molecule.
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13. • The rearrangement involves a six-membered ring transition state in
which the carbonyl group pulls off the gamma proton, splitting the
molecule into two pieces.
• These pieces consist of an enol radical cation and a neutral alkene
fragment. The enol radical cation is observed in the mass spectrum,
while the neutral alkene fragment is not observed.
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14. • Any carbonyl compound that has hydrogen in the gamma
position is likely to have a peak in the mass spectrum
corresponding to the enol radical cation formed by the
McLafferty rearrangement.
• Both ketones and aldehydes give prominent molecular ion
peaks though the [M+] peak is more prominent in ketones.
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15. •Only ketones and aldehydes that do not undergo
this rearrangement are those which lack a side
chain that can donate a hydrogen atom to the pi
system .
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