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Advanced Organic Chemistry: 1H NMR spectrum of 1-bromobutane

Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of 1-bromobutane

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 1-bromobutane

See also comparing infrared, mass, 1H NMR & 13C NMR spectra of 4 halogenoalkane isomers of C4H9Br

1H proton nmr spectrum of 1-bromobutane low/high resolution diagrams C4H9Br CH3CH2CH2CH2Br analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for n-butyl iodide 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 - 1-bromobutane here.

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

1-bromobutane, C4H9Br, CH3CH2CH2CH2Br, CH3-CH2-CH2-CH2-Br

Interpreting the H-1 NMR spectrum of 1-bromobutane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 1-bromobutane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3, -CH2-CH2-, protons etc.

For relatively simple molecules, the low resolution H-1 NMR spectrum of 1-bromobutane is a good starting point (low resolution diagram above).

The 9 hydrogen atoms (protons) of 1-bromobutane occupy 4 different chemical environments so that the low resolution NMR spectra should show 4 principal peaks of different H-1 NMR chemical shifts (diagram above for 1-bromobutane).

CH3CH2CH2CH2Br 

Note the proton ratio 3:2:2:2 of the 4 colours of the 9 protons of 1-bromobutane in the 4 chemically different proton environments

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

Although there are 9 hydrogen atoms in the molecule, the proton NMR spectrum shows there are only 4 possible different chemical environments for the hydrogen atoms in 1-bromobutane molecule.

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

The high resolution 1H NMR spectrum of 1-bromobutane

The high resolution spectra of 1-bromobutane also shows 4 groups of proton resonances and in the 3:2:2:2 ratio expected from the structural formula of 1-bromobutane, but we can now consider the splitting of resonance lines from the spin-spin coupling in the molecule of 1-bromobutane.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 1-bromobutane - 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 1-bromobutane below.

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

1H NMR resonance (a) 1H Chemical shift 0.90: CH3-CH2-CH2-CH2-Br

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

Evidence for the presence of a CH2 group in the molecule of 1-bromobutane

1H NMR resonance (b) 1H Chemical shift 1.31 ppm: CH3-CH2-CH2-CH2-Br

This resonance is split into 1:5:10:10:5:1 sextet by the CH3 and CH2 protons on either side (n+1 = 6).

Evidence for the presence of a CH3-CHx-CH2 grouping in the molecule of 1-bromobutane (x can be 1 or 2, as in this case).

1H NMR resonance (c) 1H Chemical shift 1.82 ppm: CH3-CH2-CH2-CH2-Br

This resonance is split into 1:4:6:4:1 quintet  by the CH2 and CH2 protons on either side (n+1 = 6).

Evidence for the presence of a CH2-CHx-CH2 grouping in the molecule of 1-bromobutane (x can be 1 or 2, as in this case).

1H NMR resonance (d) 1H Chemical shift 3.51 ppm : CH3-CH2-CH2-CH2-Br

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

Evidence for the presence of another CH2 group in the molecule of 1-bromobutane.

Note the decreasing effect on the 1H chemical shift as the proton is further from the more electronegative bromine atom in 1-bromobutane.


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
Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C4H9Br

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original 1-bromobutane, 2-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane image sizes.  These four molecules are structural isomers of molecular formula C4H9Br and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, bromoalkanes, alkyl bromides).

INFRARED SPECTRA (above): Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, there are no other great striking differences, but each could be identified from its infrared spectrum.

MASS SPECTRA (above): All four give the parent molecular ions of m/z 136 and 138, but it is only a relatively tiny peak for 2-bromobutane and 2-bromo-2-methylpropane. All four give the base ion peak of m/z 57. All four give prominent peaks for m/z ions 27, 29, 39 and 41 and all give a tiny peak from an ionised iodine atom at m/z 127. They look quite similar to me and lack a clear fingerprint fragmentation pattern. There are small differences in the relative abundances (peak heights) for pairs of ions involving 79Br/81Br isotopes e.g. m/z 93/95, 107/109 and 121/123. 1-bromo-2-methylpropane is the only one of the four to have a prominent peak for the m/z 43 ion.

1H NMR SPECTRA (above): The 1H NMR spectra of all four molecules give different integrated proton ratios i.e.1-bromobutane four peaks of ratio 3:2:2:2; 2-bromobutane four peaks of ratio 3:3:2:1, 1-bromo-2-methylpropane three peaks of ratio 6:2:1 and 2-bromo-2-methylpropane gives just one peak '1' (effectively no ratio involved), so all four molecular structures can be distinguished from each other by their 1H NMR spectra proton ratios, numbers of peaks and (n+1) rule splitting patterns.

13C NMR SPECTRA (above): The 13C NMR spectra of the four molecules show various numbers of carbon-13 chemical environments i.e 1-bromobutane and 2-bromobutane show four 13C NMR resonances, 1-bromo-2-methylpropane three 13C NMR resonances and 2-bromo-2-methylpropane only two 13C resonances. Therefore 1-bromo-2-methylpropane and 2-bromo-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-bromobutane and 2-bromobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

Key words & phrases: C4H9Br CH3CH2CH2CH2Br Interpreting the proton H-1 NMR spectra of 1-bromobutane, low resolution & high resolution proton nmr spectra of 1-bromobutane, H-1 nmr spectrum of 1-bromobutane, understanding the hydrogen-1 nmr spectrum of 1-bromobutane, explaining the line splitting patterns from spin-spin coupling  in the high resolution H-1 nmr spectra of 1-bromobutane, revising the H-1 nmr spectrum of 1-bromobutane, proton nmr of 1-bromobutane, ppm chemical shifts of the H-1 nmr spectrum of 1-bromobutane, explaining and analyzing spin spin line splitting in the H-1 nmr spectrum, how to construct the diagram of the H-1 nmr spectrum of 1-bromobutane, how to work out the number of chemically different protons in the structure of the 1-bromobutane organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of 1-bromobutane using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of 1-bromobutane deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of 1-bromobutane examining the 1H nmr spectrum of  1-bromobutane analysing the 1-H nmr spectrum of 1-bromobutane how do you sketch and interpret the H-1 NMR spectrum of 1-bromobutane interpreting interpretation of the 1H proton spin-spin coupling causing line splitting in the NMR spectrum of 1-bromobutane  assignment of chemical shifts in the proton 1H NMR spectrum of 1-bromobutane formula explaining spin-spin coupling for line splitting for 1-bromobutane  n-butyl iodide alkyl halide functional group haloalkane halogenoalkane bromoalkane

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 1-bromobutane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 1-bromobutane. The m/e m/z value of the molecular ion peak in the mass spectrum of 1-bromobutane.  The m/e m/z value of the base ion peak in the mass spectrum of 1-bromobutane. Possible examples of equations showing the formation of the ionised fragments in 1-bromobutane. Revision notes on the mass spectrum of 1-bromobutane. Matching and deducing the structure of the 1-bromobutane molecule from its mass spectrum. Mass spectroscopy of aliphatic halogenoalkanes bromoalkanes alkyl bromides, mass spectra of 1-bromobutane, an isomer of molecular formula C4H9Br


Links associated with 1-bromobutane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 1-bromobutane (n-butyl bromide)

The mass spectrum of 1-bromobutane (n-butyl bromide)

The C-13 NMR spectrum of 1-bromobutane (n-butyl bromide)

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

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