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Advanced Organic Chemistry: H-1 NMR spectrum of (1-methylethyl)benzene (cumene)

The H-1 hydrogen-1 (proton) NMR spectrum of (1-methylethyl)benzene (cumene)

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-methylethyl)benzene (cumene)

C9H12 low and high resolution H-1 proton nmr spectrum of (1-methylethyl)benzene (cumene) analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for 2-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 - (1-methylethyl)benzene (cumene) here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of (1-methylethyl)benzene (cumene) 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-methylethyl)benzene (cumene) molecule.

 (1-methylethyl)benzene, 2-phenylpropane, C9H12, , (cumene)

Interpreting the H-1 NMR spectrum of (1-methylethyl)benzene (2-phenylpropane, cumene)

For relatively simple molecules, the low resolution H-1 NMR spectrum of (1-methylethyl)benzene (cumene) is NOT a good starting point (diagram above).

The 1H resonances for the benzene ring protons are very close together, but you can get the ratio of aryl:alkyl protons.

From high resolution spectra, you can observe the hydrogen atoms (protons) of (1-methylethyl)benzene (cumene) occupy 5 different chemical environments but that the low resolution NMR spectra only shows three (diagram above for (1-methylethyl)benzene (cumene)).

Although there are 12 hydrogen atoms in the molecule, there are only 5 possible different chemical environments for the hydrogen atoms in (1-methylethyl)benzene (cumene) molecule.

The high resolution H-1 NMR spectrum of (1-methylethyl)benzene (cumene)

The very high resolution spectra of (1-methylethyl)benzene (cumene) should show 5 groups of proton resonances and in the 1:2:2:1:6 ratio for aryl:alkyl expected from the formula of (1-methylethyl)benzene (cumene).

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of (1-methylethyl)benzene (cumene) - 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-methylethyl)benzene (cumene) below.

So, using the chemical shifts and applying the n+1 rule to (1-methylethyl)benzene (cumene) 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 7.34 ppm for benzene ring protons

These 1H resonances are close together

You need very high resolution to sort (a) to (c) and they are indicative of benzene ring protons and a monosubstituted benzene compound too.

(d) 1H Chemical shift 2.88 ppm side-chain CH proton

This resonance is split into a 1:6:15:20:15:6:1 septet by the six methyl protons of the two adjacent carbon atoms (n+1 = 7).

Evidence for the presence of a CH3CHxCH3 alkyl group in the molecule of (1-methylethyl)benzene (cumene)

(e) 1H Chemical shift 1.24 ppm for the side-chain 2 x CH3 protons.

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

Evidence for the presence of a CH group in the molecule of (1-methylethyl)benzene (cumene)

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


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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