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The Carbon-13 C13 13CNMR spectrum of bromomethane
(methyl bromide)
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Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US
K12 grade 11 grade 12 organic chemistry courses involving molecular
spectroscopy analysing C-13 NMR spectra of bromomethane
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C-13
NMR spectroscopy - spectra index
TMS is the acronym for tetramethylsilane, formula Si(CH3)4,
whose 13C atoms are arbitrarily given a chemical shift of 0.0
ppm. This is the 'standard' in 13C NMR spectroscopy and all other
13C resonances, called chemical shifts, are measured with
respect to the TMS, and depend on the
individual (electronic) chemical environment of the 13C atoms in
an organic molecule - bromomethane here.
Bromomethane, CH3Br
(methyl bromide)
Interpreting the C-13 NMR spectrum of bromomethane
As you can see from the diagram above there is
only one chemical shift line in the C-13 NMR spectrum of
bromomethane
indicating just one chemical environments of the carbon atoms.
The carbon-13 NMR spectra a provides direct evidence of
only one carbon atom environments for the single carbon atom in the
bromomethane molecule,
deduced from the presence of only one 13C chemical
shift (ppm).
Comparing the 13C NMR chemical shift of bromomethane with
other halogen compounds
(1)
Comparing monosubstituted halogen derivatives of methane
Comparing the effect of a mono-substituent in
the methane molecule
| Compound |
fluoromethane |
chloromethane |
bromomethane |
iodomethane |
methane |
| Formula |
CH3F |
CH3Cl |
CH3Br |
CH3I |
CH4 |
| 13C
chemical shift/ppm |
74.7 |
28.7 |
10.2 |
-24.0 |
-2.3 |
| Pauling
electronegativity |
F
4.0 |
Cl
3.0 |
Br
2.8 |
I
2.5 |
H
2.1 |
With the increase in electronegativity of the halogen, the
13C chemical shift for these molecules steadily increases.
(2) Comparing the
effects of increasing chlorine substitution of methane
Comparing the effect of polysubstitution of methane with
chlorine.
| Compound |
methane |
chloromethane |
dichloromethane |
trichloromethane |
tetrachloromethane |
| Formula |
CH4 |
CH3Cl |
CH2Cl2 |
CHCl3 |
CCl4 |
| 1H chemical shift/ppm |
-2.3 |
28.7 |
53.5 |
77.2 |
97.2 |
With increase in chlorine substitution, the 1H NMR chemical
shift is increased.
Increasing the number of atoms more electronegative than
carbon or hydrogen, increases the 1H NMR chemical shift.
You would expect a similar trend for the equivalent bromine
compounds.
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NMR spectrum of bromomethane methyl bromide
Links associated
with bromomethane
The chemistry of HALOGENOALKANES (haloalkanes)
revision notes INDEX
C-13
NMR spectroscopy index
ALL SPECTROSCOPY INDEXES
All Advanced Organic
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