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Advanced Organic Chemistry: Carbon-13 NMR spectrum of bromomethane

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The Carbon-13 C13 13CNMR spectrum of bromomethane (methyl bromide)

Doc 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

CH3Br C-13 nmr spectrum of bromomethane analysis of chemical shifts ppm interpretation of 13C chemical shifts ppm of methyl bromide C13 13-C nmr doc brown's advanced organic chemistry revision notes 

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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The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

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