butan-1-ol low high resolution H-1 proton nmr spectrum of 1-butanol analysis interpretation of chemical shifts ppm spin spin line splitting H1 1-H nmr doc brown's advanced organic chemistry revision notes

Advanced pre-university organic chemistry: ¹H NMR spectrum of butan-1-ol CH₃CH₂CH₂CH₂OH

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Interpreting and explaining the 1H (proton) NMR spectrum of butan-1-ol

(1-butanol, n-butanol, n-butyl alcohol) CH₃CH₂CH₂CH₂OH

[Author © Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses, IB chemistry & US K12 grade 11, grade 12 & AP honors chemistry courses: Molecular spectroscopy of butan-1-ol (1-butanol) [spectrum page updated RE-EDIT]

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H-1 proton NMR spectroscopy - spectra index

See also the Isomers of molecular formula C₄H₁₀O (M_r = 74)

Introductory note on the 1H NMR spectra of butan-1-ol

Students and teachers please note my explanation of the proton NMR spectrum of butan-1-ol is designed for advanced, but pre-university, chemistry courses.

The chemical shift δ splitting pattern effects for butan-1-ol are confined to a proton spin-spin coupling effects analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment for the butan-1-ol molecule).

It is assumed that the integrated intensities of the ¹H NMR δ chemical shifts give the ratio of the protons in the different non-equivalent chemical environments of the butan-1-ol molecule.

The most common solvent used for investigating the 1H NMR spectrum of compounds like butan-1-ol, is CDCl₃ and other deuterated solvents to avoid confusion with a ¹H NMR signal, ²D (²H) has a different NMR chemical shift.

low resolution 1H proton NMR spectrum of butan-1-ol, 1-butanol, analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for 1-butanol 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 - butan-1-ol here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of butan-1-ol 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 butan-1-ol molecule i.e. 3 : 2 : 2 : 2 :1.

Butan-1-ol C₄H₁₀O , .

The molecular structure and naming of aliphatic alcohols and ethers
Interpreting the H-1 NMR spectrum of butan-1-ol (1-butanol, n-butyl alcohol)

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

The hydrogen atoms (protons) of butan-1-ol occupy 5 different ¹H chemical environments so that the low resolution NMR spectra should show 5 peaks of different H-1 NMR chemical shifts (diagram above for butan-1-ol).

CH₃CH₂CH₂CH₂OH

Note the ratio 3:2:2:2:1 of the 5 colours of the protons in the 5 chemically different environments

Although there are 10 hydrogen atoms in the molecule, there only 5 possible chemical environments for the hydrogen atoms in butan-1-ol molecule.

The proton ratio 3:2:2:2:1 observed, corresponds with the structural formula of butan-1-ol.

You need very a high resolution spectrum of butan-1-ol to sort (b) and (c) chemical shifts.

BUT, an important note about the hydroxyl group on butan-2-ol (for pre-university students):

Unless the alcohol is completely free of water (difficult), the hydrogen on the -O-H hydroxyl group and any hydrogens on the adjacent carbon don't interact to produce any spin-spin splitting. Therefore the -OH peak shows up as a singlet and you don't usually have to consider its effect on any hydrogen atoms, if present on the adjacent carbon atom (C-OH), and, neither do you have to consider the splitting effect of adjacent C-H protons on the hydrogen of the OH group.

very high resolution H-1 proton nmr spectrum of butan-1-ol, 1-butanol, analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for 1-butanol doc brown's advanced organic chemistry revision notes

The low and high resolution spectra of butanal show 5 groups of protons and in the ratio expected from the formula of butan-1-ol, namely 3:2:2:2:1, but the high resolution spectrum is very complex.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of butan-1-ol - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin filed splitting effects - see high resolution notes on butan-1-ol below.

CH₃CH₂CH₂CH₂OH (below I will refer to the 1st, 2nd and 3rd CH₂ groups from left to right)

So, using the chemical shifts and applying the n+1 rule to the 1H NMR spectrum of butan-1-ol

Chemical shift 0.94 ppm CH₃ protons

The end methyl group proton resonance is split into a 1:2:1 triplet by the first CH₂ group (2 protons, n+1 = 3 = triplet).

Evidence for the presence of a CH₂ group in the molecule of butan-1-ol.

Chemical shift 1.39 ppm 1st CH₂ protons

The first CH₂ proton resonance is split by the CH₃ protons AND the 2nd CH₂ protons into a 1:5:10:10:5:1 sextet (5 protons, n+1 = 6 = sextet).

Evidence for the presence of a CH₃CH₂CH₂ group in the molecule of butan-1-ol

Chemical shift 1.53 ppm 2nd CH₂ protons

The 2nd CH₂ proton resonance is split on either side by the 1st and 3rd CH₂ protons into a 1:4:6:4:1 quintet (4 protons, n+1 =5 = quintet).

Evidence for the presence of a CH₂CH₂CH₂ group in the molecule of butan-1-ol.

Chemical shift 2.24 ppm OH proton

The OH proton resonance is often seen and 'portrayed' as a singlet.

Normally the O-H proton resonance is not split by adjacent carbon atom protons and neither does it, in turn, split the resonance of the same adjacent protons (see extra note below).

This hydroxyl proton of the hydroxyl group can be distinguished from other protons in the butan-1-ol molecule by the addition of D₂O to the CDCl₃ (or other) solvent, and the ensuing rapid proton exchange reduces the ¹H NMR shift signal, which is different to the ²D (²H) chemical shift.

Chemical shift 2.24 ppm 3rd CH₂ protons

You can consider that the 3rd CH₂ proton resonance is split by the 2nd CH₂ protons into a 1:2:1 triplet (2 protons, n+1 = 3 = triplet, as on diagram).

Note that no splitting due to OH proton is detected.

Note the increase in chemical shift as the alkyl protons are closer to the electronegative oxygen of the OH group.

Extra note on the OH proton resonance

If the alcohol is impure, containing water or any source of labile protons, they exchange protons e.g.

R-O-H + H-O-H R-O-H + H-O-H

This means the CH₂ protons no longer experience a 'simple' local field from one singlet proton from two possible orientations, but, over a finite period, experience the averaging effect of exchanging protons.

This removes the spin - spin coupling effect and the OH proton resonance just shows up as a singlet if the butan-1-ol contains even a trace of water (or acid).

This sort of exchange cannot happen with the alkyl protons, but is common with molecules containing a hydroxyl (OH) hydrogen atom like alcohols and carboxylic acids.

Not only that, you also get proton transfer between the alcohol molecules i.e.

R-O-H + H-O-R R-O-H + H-O-R

which gives the same effect as traces of water of acid.

So, in butan-1-ol, all you usually see in the H-1 NMR spectrum is the mutual splitting of the CH₂ and CH₃ proton resonances plus a singlet line for the OH proton resonance.

EXTRA NOTE on why the OH proton chemical shift is usually observed as a singlet in alcohols like butan-1-ol and how deuterium oxide can be used to identify the peak caused by the hydroxyl proton

Although extremely weak acids, there is constant exchanging of protons between alcohol molecules (R = alkyl groups of butan-1-ol).

R-O-H + H-O-R R-O-H + H-O-R

The rate of proton transfer is increased by traces of water.

R-O-H + H-O-H R-O-H + H-O-H

This cannot happen with the non-acidic C-H protons of alkyl groups in alcohols like butan-1-ol.

This rapid proton transfer interferes with the field splitting effects of the hydroxyl O-H protons and carbon C-H protons and the spin-spin coupling effects disappear.

This phenomena can be used to identify the O-H proton resonance from other C-H proton resonances in hydroxyl molecules like butan-1-ol.

If deuterium oxide (D₂O, where D = ²H) is added to the NMR sample, the ¹H protons are rapidly replaced by ²H protons in the butan-1-ol molecule.

R-O-H + D-O-D R-O-D + H-O-D

The ²H chemical shift frequency is different to the ¹H chemical shift frequency, so the effect of D₂O is to remove the chemical shift for the OH proton from the ¹H NMR spectrum of butan-1-ol, thereby identifying the original ¹H chemical shift as belonging to the hydroxyl group O-H proton and not a C-H proton of the butan-1-ol molecule.

The splitting pattern from proton spin-spin coupling effects is analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment).

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: 1-butanol n-butyl alcohol Interpreting the proton H-1 NMR spectra of butan-1-ol, low resolution & high resolution proton nmr spectra of butan-1-ol, H-1 nmr spectrum of butan-1-ol, understanding the hydrogen-1 nmr spectrum of butan-1-ol, explaining the line splitting patterns in the high resolution H-1 nmr spectra of butan-1-ol, revising the H-1 nmr spectrum of butan-1-ol, proton nmr of butan-1-ol, ppm chemical shifts of the H-1 nmr spectrum of butan-1-ol, 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 butan-1-ol, how to work out the number of chemically different protons in the structure of the butan-1-ol organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of butan-1-ol using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of butan-1-ol deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of butan-1-ol examining the 1H nmr spectrum of butan-1-ol analysing the 1-H nmr spectrum of butan-1-ol how do you sketch and interpret the H-1 NMR spectrum of butan-1-ol 1-butanol n-butyl alcohol How do you interpret the H-1 NMR spectrum of 1-butanol butan-1-ol How to interpret the H-1 NMR spectrum of 1-butanol butan-1-ol Explanatory diagram of the chemical shifts of the 1H H-1 proton NMR spectrum of the 1-butanol butan-1-ol molecule in terms of its molecular structure. Listing data of all the chemical shift peaks in ppm in the proton NMR spectrum of 1-butanol butan-1-ol. How to explain the H-1 NMR spectrum of 1-butanol butan-1-ol. The chemical shifts and integrated values of the proton ratios in the 1-H NMR spectrum of the 1-butanol butan-1-ol molecule. How to work out the molecular structure of the 1-butanol butan-1-ol molecule from its proton NMR spectrum. The uses and distinctive features of the proton NMR spectrum of the 1-butanol butan-1-ol molecule explained. What does the H-1 proton NMR spectrum chemical shifts tell us about the structure and properties of the 1-butanol butan-1-ol molecule? How do you interpret the H-1 NMR spectrum of butan-1-ol 1-butanol How to interpret the H-1 NMR spectrum of butan-1-ol 1-butanol Explanatory diagram of the chemical shifts of the 1H H-1 proton NMR spectrum of the butan-1-ol 1-butanol molecule in terms of its molecular structure. Listing data of all the chemical shift peaks in ppm in the proton NMR spectrum of butan-1-ol 1-butanol. How to explain the H-1 NMR spectrum of butan-1-ol 1-butanol. The chemical shifts and integrated values of the proton ratios in the 1-H NMR spectrum of the butan-1-ol 1-butanol molecule. How to work out the molecular structure of the butan-1-ol 1-butanol molecule from its proton NMR spectrum. The uses and distinctive features of the proton NMR spectrum of the butan-1-ol 1-butanol molecule explained. What does the H-1 proton NMR spectrum chemical shifts tell us about the structure and properties of the butan-1-ol 1-butanol molecule? explaining the spin-spin proton coupling effects in the 1H NMR spectrum of butan-1-ol 1-butanol. interpretation diagram explaining the proton splitting pattern produced from the n+1 rule and the theoretical ratio of chemical shift and values of intensities for the proton NMR spectrum lines of butan-1-ol 1-butanol

Links associated with butan-1-ol

The infrared spectrum of butan-1-ol (1-butanol)

The mass spectrum of butan-1-ol (1-butanol,)

The C-13 NMR spectrum of butan-1-ol (1-butanol)

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