Advanced Organic Chemistry: 1H NMR spectrum of 2-bromopropane

The H-1 hydrogen-1 (proton) NMR spectrum of 2-bromopropane

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 2-bromopropane

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H-1 proton NMR spectroscopy - spectra index

C3H7Br CH3CHBrCH3 low and high resolution 1H proton nmr spectrum of 2-bromopropane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for isopropyl bromide explaining spin-spin coupling for line splitting 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 - 2-bromopropane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of 2-bromopropane 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 2-bromopropane molecule.

2-bromopropane, C3H7Br, CH3CHBrCH3,  (c) doc b , (c) doc b

Interpreting the H-1 NMR spectrum of 2-bromopropane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 2-bromopropane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH3-CH.

For relatively simple molecules, the low resolution H-1 NMR spectrum of 2-bromopropane is a good starting point with two peaks in the ratio 6:1.

The hydrogen atoms (protons) of 2-bromopropane 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 2-bromopropane).

CH3CHBrCH3

Note the proton ratio 6:1 of the 2 colours of the protons in the 2 chemically different environments

Chemical shifts (a) to (c) on the H-1 NMR spectrum diagram for 2-bromopropane.

The two methyl groups are equivalent to each other.

Although there are 7 hydrogen atoms in the molecule, there are only 2 possible different chemical environments for the hydrogen atoms in 2-bromopropane molecule.

The integrated signal proton ratio 6:1 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of 2-bromopropane.

The high resolution 1H NMR spectrum of 2-bromopropane

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 2-bromopropane - 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 2-bromopropane below.

So, using the chemical shifts and applying the n+1 rule to 2-bromopropane and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) 1H Chemical shift 1.31 ppm, methyl protons: CH3CHBrCH3

This resonance is split into a doublet by the methyl protons (n+1 = 2).

Evidence for the presence of a -CH- group in the molecule of 2-bromopropane

(b) 1H Chemical shift 3.79 ppm, CH protons :CH3CHBrCH3

This resonance is split into a 1:6:15:20:15:6:1 septet by the methyl protons.

Evidence for the presence of a CH3-C-CH3 grouping in the molecule of 2-bromopropane (n+6 = 7).

Note the decreasing effect on the chemical shift as the hydrogen atom is further from the highly more electronegative bromine atom of 2-bromopropane.

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 2-bromopropane

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

All Advanced Organic Chemistry Notes

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