Organic analysis - mass spectrometry and infrared spectroscopy (3.3.6)

The electrophilic addition of bromine to alkenes is used as a test for unsaturation.

is orange and bromoalkanes are colourless. When the substances are mixed they react and as the reaction progresses the orange colour fades.

To complete the test, bromine water is shaken with a test sample, and the results are observed.

Fehling’s solution and Benedict’s solution are used to identify aldehydes.

Both solutions rely upon the oxidation of the aldehyde to a carboxylic acid salt, by a solution of a blue complex of copper(II) ions, which is subsequently reduced to a brick-red precipitate of copper(I) oxide in alkaline conditions.

The ligands on the copper complex in Fehling’s and Benedict’s solutions are different but both exhibit a change from a blue solution to a brick-red precipitate on heating with a reducing agent (e.g. an aldehyde).

Tollens’ reagent contains aqueous silver nitrate, dissolved in ammonia forming the complex ion .

Aldehydes are easily oxidised to carboxylate ions in basic conditions.

Tollens’ reagent acts as an oxidising agent. It is reduced, causing silver ions () to be reduced to metallic silver ( ).

This results in a silver mirror forming on the inside of the test tube, indicating the presence of an aldehyde.

Ketones cannot be oxidised as they lack a hydrogen atom on the carbonyl atom. Therefore ketones do not react with Tollens’ reagent and no silver mirror forms.

Felling’s test and Tollens’ test can be used to distinguish between aldehydes and ketones. Both tests give a positive result for an aldehyde but not for a ketone.

They are also useful for distinguishing primary and secondary alcohols. The alcohols can be oxidised and the products distilled as they are formed then tested. Only primary alcohols produce aldehydes that give a positive result.

Alcohols can be oxidised to carbonyl compounds during redox reactions with oxidising agents.

Acidified potassium dichromate VI () is commonly used in the oxidation of alcohols.

Dichromate ions, , turn from orange to green as they are reduced to . The oxidation state of chromium reduces from +6 to +3.

When oxidised, primary alcohols form aldehydes first before further oxidation to carboxylic acids.

Secondary alcohols are oxidised to form ketones.

Tertiary alcohols CANNOT be oxidised as they do not have a proton on the carbon bearing the group.

Mass spectra of organic molecules will exhibit a tiny peak at 1 mass unit greater than the molecular ion.

This is called the M+1 peak and is caused by the presence of the carbon-13 isotope which comprises of all carbon atoms.

The of propanone, , is 58.

In the spectrum of propanone, the molecular ion peak, , is at 58 and a smaller M+1 peak at 59 shows the proportion of propanone featuring carbon-13.

Molecular ions formed during ionisation are positively charged radicals and are generally unstable.

They split to form a radical and a cation; this is called fragmentation.

The cationic molecular fragment ions can be seen in the mass spectrum.

All fragment ions will have lower molar mass than the molecular ion, .

Some fragment ions may be formed more frequently.

m/z 29 and m/z 43 are commonly used in exams due to the variety of fragments they can be associated with.

The molecular ion peak, , represents the relative formula mass () of the whole molecule.

High-resolution mass spectrometry (HRMS) can measure this peak to a high degree of precision, such as four or more decimal places, providing an exact mass.

By comparing the exact recorded mass to the sum of the masses of the constituent atoms, chemists can calculate the possible combinations of atoms that could make up the molecule. This gives the molecular formula.

Propane () and ethanal () have the same relative formula mass to the nearest whole number, 44.

On a high-resolution mass spectrum the molecules are distinguishable by the peak; gives a molecular ion peak at m/z = 44.0624 while gives a molecular ion peak at m/z = 44.0261.

Vibrational movement of covalent bonds exists as two main types: bond stretching, and bond bending.

Each vibration is associated with a particular wavelength and frequency. When exposed to IR radiation the bonds absorb radiation at their characteristic wavelength and vibrate more vigorously.

Vibrational absorptions are seen where there are inverted peaks; in these regions there is a reduction in the radiation able to pass through the sample.
The deeper the peak the more radiation has been absorbed. These prominent peaks are most distinctive, and therefore most useful in characterisation.

The IR spectrum for propanal is shown below.

It has a prominent peak around 1700 cm^-1 which is characteristic of the carbonyl functional group present in .

Different functional groups present in an organic compound can be identified from their IR stretching peaks.

A basic outline of the main groups and their characteristics absorption frequencies in IR is as shown below:

You will not be required to memorise this data.

The presence of the bond stretching is easy to locate, though of limited use in distinguishing one organic compound from another.

A strong, broad peak is characteristic of bond stretching in alcohols.

This notable peak is located just to the left of the landmark bond peak and is very popular in exam questions.

Be careful as the bond, although a weaker signal, is located in the same region as a bond.

The bond stretching, found in aldehydes, ketones, and other carbonyl-containing compounds is very easy to recognise as a strong narrow peak, almost in the middle of the spectrum around .

More details are required to fully identify the functional group.

The spectrum for the carboxylic acid group contains a stretch at and the very broad and strong peak of the group in carboxylic acids from .

Its breadth is due to hydrogen bonding between carboxylic acid groups causing variation in the vibration.

The carboxylic acid peak overlaps the bond peak, which can often be seen just below the broad carboxylic acid peak.

If there was a narrow peak and a peak this would indicate an alcohol and a carbonyl were present, not a carboxylic acid.

In amine’s, bond stretching appears as a characteristic peak in the IR spectrum, typically between .

This peak is often medium in intensity and can be either sharp or broad, depending on whether the amine is primary or secondary. It is significantly less intense than the of an alcohol, but is in the same region.

Most exam questions present an infrared spectrum and ask for the identity of the compound, or some structural features, deducible from the data.

Using the supplied data sheet spectroscopy table is essential here but a good understanding of the key peaks below will speed up your analysis.

IR spectra are most commonly assessed in combination with other techniques such as nuclear magnetic resonance (NMR) and mass spectrometry.

The region comprising a series of many overlapping peaks, to the right hand side of the IR spectrum, from 1500 – 500 cm^-1, is known as the fingerprint region.

Two similar molecules, such as structural isomers, propan-1-ol and propan-2-ol, will produce a similar spectra in the functional group region, however, there will always be significant differences in the fingerprint region.

The fingerprint region provides a set of peaks that is uniquely characteristic of each compound, meaning it serves as a molecular fingerprint that allows identification of a molecule.

By comparing the fingerprint region of an unknown compound to reference spectra, chemists can accurately identify the compound. The process is called fingerprinting.

The most abundant atmospheric gases, and , show no change in dipole moment and do not interact with infrared radiation.

Some atmospheric gases do absorb IR radiation; these include , , and .

These gases are known as greenhouse gases.

When solar radiation enters the earth’s atmosphere, it is absorbed by the earth’s surface and oceans before being re-emitted at longer wavelengths, mainly as IR radiation.

The increased presence of atmospheric gases that interact with IR radiation cause a greater proportion of this radiation to be held within the atmosphere rather than entering space. This causes heating of the atmosphere, and scientists refer to this as global warming.

Human activity, most notably the extraction and combustion of fossil fuels, have led to much greater levels of atmospheric in recent history.

Increased reliance on renewable energy sources with reduced production such as nuclear, wind, wave, solar, and tidal power, alongside using carbon capture and storage methods should slow greenhouse gas production and global warming.