Advanced Organic Chemistry: H-1 NMR spectrum of propylbenzene

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

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 propylbenzene

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H-1 proton NMR spectroscopy - spectra index

C9H12 low and high resolution H-1 proton nmr spectrum of propylbenzene analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for 1-phenylpropane 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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - propylbenzene here.

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

propylbenzene, C9H12 , (c) doc b  , (c) doc b 

Interpreting the H-1 NMR spectrum of propylbenzene

For relatively simple molecules, the low resolution H-1 NMR spectrum of propylbenzene is a NOT a good starting point.

The hydrogen atoms (protons) of propylbenzene actually occupy 6 different chemical environments so that the very high resolution NMR spectra should show 6 peaks of different H-1 NMR chemical shifts (diagram above for propylbenzene).

Although there are 12 hydrogen atoms in the molecule, there are only 6 possible different chemical environments for the hydrogen atoms in propylbenzene molecule.

The integrated signal proton ratio 5:2:2:3 is observed in the above spectrum, but the true 'very high resolution' ratio is 1:2:2:2:2:3 for the 5 aryl and 7 alkyl protons in propylbenzene.

The high resolution H-1 NMR spectrum of propylbenzene

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

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

(a) to (c) 1H Chemical shift 7.02 to 7.40 ppm, aromatic ring proton resonances

There are three proton resonances close together originating from the benzene ring protons - see diagram above.

Very high resolution would produce three peaks in the ratio (a) 1 : (b) 2 : (c) 2 and no doubt splitting will be observed.

1H resonances (a) to (c) and indicative of benzene ring protons and a monosubstituted benzene compound too.

(d) 1H Chemical shift 2.57 ppm side-chain CH2 nearest the benzene ring

This is split into a triplet by the adjacent CH2 protons.

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

(e) 1H Chemical shift 1.64 ppm resonance from the 2nd side-chain CH2

This is split into a 1:5:10:10:5:1 sextet by the CH2 and CH3 protons from each side (n+1 = 6).

Evidence for the presence of an alkyl group with adjacent CH2 and CH3 group either side in the molecule of propylbenzene

(f) 1H Chemical shift 0.94 ppm resonance from the side-chain methyl group.

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

Evidence for another CH2 group in the molecule - see (d).

(d) to (f) provide evidence of a propyl group in the molecule.

What you can get from the spectrum is a ratio of 5:7 for aryl:alkyl protons in 1(1-methylethyl)benzene (2-phenylpropane) and evidence of a propyl group in the alkyl side-chain.

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 propylbenzene

The chemistry of AROMATIC COMPOUNDS 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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