Advanced Organic Chemistry: 1H NMR spectrum of benzaldehyde C6H5CHO

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Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of benzaldehyde C6H5CHO

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 benzaldehyde

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H-1 proton NMR spectroscopy - spectra index

C7H6O C6H5CHO low and high resolution 1H proton nmr spectrum of benzaldehyde analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for benzaldehyde 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 - benzaldehyde here.

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

 benzaldehyde C7H6O, C6H5CHO , (c) doc b , (c) doc b 

Interpreting the H-1 NMR spectrum of benzaldehyde

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

At low resolution you would see a peak ratio of 5 : 1 for the aryl : aldehyde protons.

However, very high resolution can show the 6 hydrogen atoms (protons) of benzaldehyde actually occupy 4 different chemical environments in the ratio 1:2:2:1 (diagram above).

The high resolution 1H NMR spectrum of benzaldehyde

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

(a) to (c) 1H Chemical shifts 7.51 to 7.87 ppm

The three benzene ring proton resonances are close together.

1H resonance (a) 7.61 ppm for the proton attached to benzene ring carbon 4.

1H resonance (b) 7.51 ppm for the protons attached to benzene ring carbon atoms 3 and 5.

1H resonance (c) 7.87 ppm for the protons attached to benzene ring carbon atoms 2 and 6.

Typical of a proton resonance group of a monosubstituted benzene compound e.g. benzaldehyde

(d) 1H Chemical shift of 10.0 ppm

The resonance of the aldehyde group proton is significantly shifted to a higher value by the field effect of the electronegative oxygen atom.

This clearly distinguishes the CHO aldehyde group proton from the benzene ring protons.

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 benzaldehyde

The infrared spectrum of Benzaldehyde

The mass spectrum of Benzaldehyde

The C-13 NMR spectrum of Benzaldehyde

Index of notes on the chemistry of aromatic compounds

Index of notes on the chemistry of aldehydes and ketones

H-1 proton NMR spectroscopy index

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

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