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Advanced Organic Chemistry: 1H NMR spectrum of benzoic acid

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

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 benzoic acid

1H proton nmr spectrum of benzoic acid low/high resolution diagrams C7H6O2 C6H5COOH analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for benzoic acid 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 - benzoic acid here.

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

Benzoic acid (benenecarboxylic acid)C7H6O2C6H5COOH  ,  (c) doc b , (c) doc b , (c) doc b

Interpreting the H-1 NMR spectrum of benzoic acid

In terms of spin-spin coupling from the possible proton magnetic orientations, for benzoic acid I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH-CH, - protons etc. but the proton resonance of the hydroxyl group O-H is not, nor does it split any C-H protons.

For relatively simple molecules, the low resolution H-1 NMR spectrum of benzoic acid is NOT a good starting point (diagram above) because 1H proton resonances of the benzene ring in aromatic compounds are often close together.

However, from high resolution spectra (higher than above diagram), you can show the 6 hydrogen atoms (protons) of benzoic acid occupy 4 different chemical environments .

C6H5COOH

Note the proton ratio 5:1 of the 2 colours of the 6 protons of benzoic acid in the benzene ring and carboxylic acid group chemically different proton environments

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

Although there are 6 hydrogen atoms in the molecule, there are only 4 possible different chemical environments for the hydrogen atoms in benzoic acid molecule.

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

The high resolution 1H NMR spectrum of benzoic acid

The high resolution spectra of benzoic acid shows 4 groups of proton resonances and in the 1:2:2:1 ratio expected from the structural formula of benzoic acid.

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

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

Resonance (a) 1H Chemical shift 12.1 ppm

The hydroxyl/carboxylic acid group proton O-H resonance appears a singlet.

There are no adjacent protons on the C1 carbon atom (see diagram below).

Evidence for the presence of an isolated proton containing group in the molecule of benzoic acid

aromatic benzene ring carbon atom positions in phenol 1H proton NMR spectroscopy Ring positions in monosubstituted benzene compounds.

Resonance (b) 1H Chemical shift 8.12 ppm, CH ring protons

The C2 and C6 proton resonance is split by the C3 and C5 protons respectively, into a 1:1 doublet (n+1 = 2).

Note there is no proton on the ring carbon atom designated C1.

Evidence for the presence of a CH group in the molecule of benzoic acid

Resonance (c) 1H Chemical shift 7.45 ppm, CH ring protons

The C3 and C5 proton resonance is split, either side, by the C3 or C5 protons and by the C4 proton into a 1:2:1 triplet (n+2 = 3).

Resonance (d) 1H Chemical shift 7.62 ppm, CH ring proton.

The C4 proton resonance is split, either side, by the C3 and C5 protons into a 1:2:1 triplet (n+2 = 3).


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 benzoic acid

INDEX of all AROMATIC COMPOUND chemistry revision notes

The physical and chemical properties of benzoic acid and selected derivatives

H-1 proton NMR spectroscopy index

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