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A2 Advanced Organic Chemistry - Spectroscopy

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Spectroscopy - Infra-Red

The Infra-red spectra that have to be identified at A2 level are basically the same as with AS.

The groups that have to be identified are -

alcohol groups, O-H   an absorption at 3200 to 3600 cm-1
carbonyl groups, C=O   an absorption at 1650 to 1720 cm-1
ester groups, -COO-   an absorption at 1720 to 1750 cm-1
carboxylic acid groups, -COOH   an absorption at 2500 to 3300 cm-1 (-OH group) and 1700 to 1720 cm-1 (C=O group)

Exemplar spectra -

an (aromatic) alcohol, phenol -

a (aromatic) carbonyl compound, benzophenone, (C6H5)2C=O -

a (aromatic) carboxylic acid, benzoic acid -

an (aromatic) ester, phenyl benzoate -

Spectroscopy - Mass Spectroscopy

With organic chemicals, a mass spectrum can reveal a lot of information. The large molecules break up into smaller fragments and the mass spectrum can reveal the masses of these fragments and their make-up can be determined. This can help piece back together the whole molecule. This is not required at A2.

At A2, the extra piece of information that a mass spectrum can reveal is the molecular mass of the molecule. The peak of highest mass, called the molecular ion peak (M+), which occurs at the far right hand end of the spectrum, is the molecule's mass.

The mass of the molecular ion, coupled with elemental composition data for a molecule, enables the molecular formula of an unknown compound to be worked out. The structural formula requires other spectral information though.

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Spectroscopy - Nuclear Magnetic Resonance

(1) Predicting molecular structure :

An N.M.R. spectrum can reveal all the information needed to draw a structural formula for a molecule.

Example - the 1H-NMR spectrum of styrene oxide

The x-axis of the spectrum uses δ values for protons measured in ppm (parts per million), which indicate how different the resonances of the protons are to each other. This is called the chemical shift of the proton.  The lower the δ value the more saturated the environment of the proton, e.g. CH3CH2CH2CH3 protons at about δ=1-2 ppm. The higher the d value the more unsaturated the environment of the proton, e.g. C6H6 protons at about δ=7 ppm.

The δ values for different types of protons are given on the datasheet and reproduced here :

type of proton : chemical shift, δ
 R-CH3 0.7 - 1.6
 R-CH2-R 1.2 - 1.4
R3CH 1.6 - 2.0
2.0 - 2.9
2.3 - 2.7
 -O-CH3    -O-CH2-R 3.3 - 4.3
 R-OH 3.5 - 5.5 (but very variable)
6.5 - 7.0
7.1 - 7.7
9.5 - 10.0
11.0 - 11.7

The different types of protons in a molecule all give rise to different peaks in the spectrum. For example, in ethanol (CH3CH2OH) there are three different types of protons :

identification of CH3 protons in ethanol
identification of CH2 protons in ethanol
identification of OH protons in ethanol

Therefore, the N.M.R. spectrum shows three groups of peaks, each group caused by a different group of protons.

The area under a group of peaks is directly proportional to the number of protons resonating to cause that peak. So, the relative areas reveal the relative amounts of protons in the molecule. For example, in ethanol there are three groups of peaks with area ratios of 1:2:3; therefore, the ratio of the different protons in ethanol is 1:2:3.

Protons on adjacent carbon atoms interact with one-another. This causes what should be a single peak for each group of protons to be split into a group of peaks.

The so-called splitting patterns depend on the number of neighbouring protons and follow the pattern in the table below :

number of neighbouring protons : splitting pattern (relative peak heights):
0 1
1 1 1
2 1 2 1
3 1 3 3 1

So, if a proton has no neighbours it is not split at all (it is a singlet peak); one neighbouring proton gives rise to a doublet; two neighbouring protons cause a triplet and three neighbouring protons yield a quartet.

The one exception to this rule is that labile protons (see below) do not generally interact with neighbouring protons and so do not cause splitting nor are split themselves.

Putting all this information together enables the structure of the molecule to be deduced.  Even fine structural elements such as structural and E-Z isomerism can be seen.

(2) Predicting spectra :

This process is simply the reverse of the steps above. The table is consulted to find the correct δ values and the structure shows how many neighbouring protons each peak has and therefore what the splitting patterns should be.

(3) Use of D2O in N.M.R. :

Protons attached to oxygen and nitrogen atoms are easily removed and replaced by protons from other sources. This process is continual and generally goes unnoticed.  These protons are called labile.

If an organic molecule containing hydroxyl(-OH), carboxylic acid (-COOH) or amine (-NH2) groups is mixed with deuterium oxide ("heavy" water, D2O), then the protons (1H) are replaced with deuterium atoms (2H). Since the deuterium atom has an even number of particles in its nucleus (a proton and a neutron) it does not show up in proton N.M.R.

So, if spectra are taken of a molecule before and after the use of D2O, a comparison of the two spectra can reveal any labile hydrogens in the molecule.

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written by Dr Richard Clarkson : © Saturday, 1 November 1997

Updated : Saturday, 17th March, 2012

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