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Advanced Organic Chemistry: 1H NMR spectrum of bromoethane

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The H-1 hydrogen-1 (proton) NMR spectrum of bromoethane

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 H-1 NMR spectra of bromoethane

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H-1 proton NMR spectroscopy - spectra index

C2H5Br CH3CH2Br low and high resolution 1H proton nmr spectrum of bromoethane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for bromoethane explaining spin-spin coupling for line splitting ethyl bromide doc brown's advanced organic chemistry revision notes

TMS is the acronym for tetramethylsilane, formula Si(CH3)4, whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in 1H NMR spectroscopy and all other proton resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - bromoethane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of bromoethane represent the peaks of the intensity of the chemical shifts of (which are often groups of split lines at high resolution) AND the relative integrated areas under the peaks gives you the ratio of protons in the different chemical environments of the bromoethane molecule.

Bromoethane  (c) doc b  (c) doc b  (c) doc b  (c) doc b  (c) doc b 

Interpreting the H-1 NMR spectrum of bromoethane

In terms of spin-spin coupling from the possible proton magnetic orientations, for bromoethane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3.

For relatively simple molecules, the low resolution H-1 NMR spectrum of bromoethane is a good starting point with two obvious proton resonances.

The hydrogen atoms (protons) of bromoethane occupy 2 different chemical environments so that the low resolution NMR spectra should show 2 principal peaks of different H-1 NMR chemical shifts (diagram above for bromoethane).

CH3CH2Br

Note the proton ratio 3:2 of the two colours of the protons in the two chemically different environments

Chemical shifts (a) to (c) on the H-1 NMR spectrum diagram for bromoethane.

Although there are 5 hydrogen atoms in the molecule, there are only 2 possible different chemical environments for the hydrogen atoms in bromoethane molecule.

The integrated signal proton ratio 3:2 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of bromoethane.

The high resolution 1H NMR spectrum of bromoethane

All low and high resolution spectra of bromoethane show 2 groups of proton resonances and in the 3:2 ratio expected from the formula of bromoethane.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of bromoethane - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin coupling field splitting effects - see high resolution notes on bromoethane below.

So, using the chemical shifts and applying the n+1 rule to bromoethane and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) 1H Chemical shift 1.68 ppm, CH3 proton resonance: CH3CH2Br

This resonance is split into a 1:2:1 triplet by the CH2 protons (n+1 = 3).

Evidence for the presence of a CH2 group in the molecule of bromoethane

(b) 1H Chemical shift 3.43 ppm, CH2 proton resonance: CH3CH2Br

This resonance is split into a 1:3:3:1 quartet by the CH3 protons (n+1 = 4).

Evidence for the presence of a CH3 group in the molecule of bromoethane

Note the decreasing effect on the chemical shift as the hydrogen atom is further from the more electronegative bromine atom of bromoethane.


Number of directly adjacent protons 1H causing splitting Splitting pattern produced from the n+1 rule on spin-spin coupling and the theoretical ratio of line intensities
0 means no splitting             1            
1 creates a doublet           1   1          
2 creates a triplet         1   2   1        
3 creates a quartet       1   3   3   1      
4 creates a quintet     1   4   6   4   1    
5 creates a sextet   1   5   10   10   5   1  
6 creates a septet 1   6   15   20   15   6   1

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Links associated with bromoethane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

H-1 proton NMR spectroscopy index  (Please read 8 points at the top of the 1H NMR index page)

ALL SPECTROSCOPY INDEXES

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