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Advanced Organic Chemistry: H-1 NMR spectrum of butanal

The H-1 hydrogen-1 (proton) NMR spectrum of butanal (butyraldehyde)

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 butanal

H-1 nmr spectrum of butanal analysis of chemical shifts ppm interpretation of proton chemical shifts ppm of butyraldehyde 1-H proton nmr 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 - butanal here.

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

butanal (butyraldehyde), C4H8O,  aldehydes and ketones nomenclature (c) doc b, aldehydes and ketones nomenclature (c) doc b, aldehydes and ketones nomenclature (c) doc b

Interpreting the H-1 NMR spectrum of butanal

For relatively simple molecules, the low resolution H-1 NMR spectrum of butanal is a good starting point.

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

CH3CH2CH2CHO

Note the ratio 3:2:2:1 of the four colours of the protons in the four chemically different environments

Although there are 8 hydrogen atoms in the molecule, there only three possible chemical environments for the hydrogen atoms in butanal molecule.

The proton ratio 3:2:2:1 observed, corresponds with the structural molecular formula of butanal.

The very high resolution spectrum of butanal - tricky in places!

CH3CH2CH2CHO

All low and high resolution spectra of butanal show four groups of protons and in the 3:2:2:1 ratio expected from the molecular structure of butanal. The ppm quoted represent the peak of resonance intensity for a particular proton group in the molecule of butanal.

(a) The 0.97 ppm 1H NMR chemical shift: CH3CH2CH2CHO

The adjacent CH2 protons split the CH3 proton resonance into a 1:2:1 triplet (n+1 = 3).

Evidence of a CH2 group in the butanal molecule.

(b) The 1.64 ppm 1H NMR chemical shift: CH3CH2CH2CHO

From the n+1 rule, the 'left-hand' CH2 protons (H2) are split by adjacent CH3 protons (H3) and by the other adjacent CH2 protons (H2), (5 protons in total), into a 1:5:10:10:5:1 sextet of resonance lines (n+1 = 6).

This is pattern of resonances is a good indication of a propyl group (CH3CH2CH2) and the sextet is clearly seen on the very high resolution spectrum diagram of butanal above.

(c) The 2.37 ppm 1H NMR chemical shift: CH3CH2CH2CHO

You might think that the middle right-hand CH2 protons (H2) are split on the left sides by CH2 protons (H2) and on the right by the CH proton (H), (3 protons in total), so you would a 1:3:3:1 quartet of resonance lines, but this is not so!

This CH2 proton resonance is split by the other adjacent CH2 protons into a triplet, which is then split into doublets by the CH (aldehyde proton), giving a series of 1:2:1 doublets - from what I can gather from the internet, its even more complicated than this.

(This is more university level NMR spectroscopy, so don't worry, concentrate on the basic proton ratio of 3:2:2:1 to match the structure of butanal).

(d) The 9.76 ppm 1H NMR chemical shift: CH3CH2CH2CHO

The adjacent CH2 protons split the CH proton (of the aldehyde group) resonance into a 1:2:1 triplet.

Evidence of a 2nd CH2 group in the butanal molecule.

Number of protons 1H causing splitting Splitting pattern produced from the n+1 rule 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

Key words & phrases: Interpreting the proton H-1 NMR spectra of butanal, low resolution & high resolution proton nmr spectra of butanal, H-1 nmr spectrum of butanal, understanding the hydrogen-1 nmr spectrum of butanal, explaining the line splitting patterns in the high resolution H-1 nmr spectra of butanal, revising the H-1 nmr spectrum of butanal, proton nmr of butanal, ppm chemical shifts of the H-1 nmr spectrum of butanal, 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 butanal, how to work out the number of chemically different protons in the structure of the butanal organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of butanal using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of butanal deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of butanal examining the 1H nmr spectrum of  butanal analysing the 1-H nmr spectrum of butanal proton nmr spectrum of butyraldehyde

Molecular structure diagram of the proton NMR diagram for the 1H NMR spectrum of butanal. The proton ratio in the 1H NMR spectrum of butanal. Deducing the number of different chemical environments of the protons in the butanal molecule from the 1H chemical shifts in the hydrogen-1 NMR spectrum of butanal. Analysing the high resolution 1H NMR spectrum of butanal. Analysing the low resolution 1H NMR spectrum of butanal. You may need to know the relative molecular mass of butanal to deduce the molecular formula from the proton ratio of the 1H NMR spectrum of butanal. Revision notes on the proton NMR spectrum of butanal. Matching and deducing the structure of the butanal molecule from its hydrogen-1 NMR spectrum.

Links associated with butanal

The chemistry of ALDEHYDES and KETONES revision notes INDEX

The infrared spectrum of Butanal

The mass spectrum of Butanal

The C-13 NMR spectrum of Butanal

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

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