Organic Analysis: Functional Group Tests & Spectroscopy (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about identifying unknown organic compounds using test-tube reactions, mass spectrometry and infrared spectroscopy.

The three are covered together because they answer the same question in different ways: a test-tube reaction identifies a functional group from a visible change, infrared spectroscopy identifies the bonds present from the radiation they absorb, and mass spectrometry gives the molecular mass and the fragments that pin down the structure.

Chemists often need to identify the functional groups present in an organic compound, and this topic gives three ways of doing it. Test-tube reactions are simple chemical tests that produce a visible result for a particular functional group. Mass spectrometry breaks the molecule into charged fragments and separates them, giving the molecular mass and clues to the structure. Infrared spectroscopy measures how the covalent bonds absorb infrared radiation, so the spectrum shows which bonds are present.

We'll start with the test-tube reactions, then take mass spectrometry in two parts, reading a spectrum and then interpreting its fragments, then infrared spectroscopy, and finish with the link between infrared absorption and global warming.

Before reaching for an instrument, simple observations narrow things down: whether the compound is acidic, what state it is in, and whether it dissolves in water.

Then come the test-tube reactions, simple but highly effective tests that each pair one functional group with one visible result. Bromine water going from orange to colourless identifies an alkene. Acidified potassium dichromate turns from orange to green with primary and secondary alcohols and stays orange with a tertiary one, so that test also gives you the class of alcohol. The halogenoalkane test is the longest: warm with aqueous sodium hydroxide, acidify with nitric acid, then add silver nitrate, and the colour of the silver halide precipitate names the halogen. Tollens' reagent and Fehling's solution both pick out aldehydes, and sodium hydrogencarbonate gives carbon dioxide with a carboxylic acid. Each result names a functional group, which is what the two spectroscopic techniques build on.

Mass spectrometry identifies unknown compounds by weighing them. The sample is bombarded with high-energy electrons, which knock an electron out of the molecule and leave a positively charged molecular ion. An electric field accelerates the ions, and they are separated by their mass-to-charge ratio, the m/z value, before reaching the detector.

Each fragment gives a peak at its own m/z value. The peak with the highest m/z is the molecular ion peak, and that gives the molecular mass of the compound. The base peak is the one corresponding to the most abundant ion, and the heights of the peaks show relative abundance. Where a test-tube reaction told you which functional group is present, this tells you how heavy the whole molecule is.

Molecular mass on its own is not enough, because different compounds can share the same molecular mass. Fragmentation is what separates them. The molecular ion breaks up into new ions, molecules and radicals, and fragments appear either as characteristic pieces of the molecule or through the loss of small molecules such as water, carbon monoxide and carbon dioxide.

Simple alkanes fragment by breaking carbon-carbon bonds, and the common fragments sit at predictable values: 15, 29 and 43 for the first three. Alcohols tend to lose a water molecule, giving a peak eighteen units below the molecular ion. Halogenoalkanes show several peaks around the molecular ion because the halogens have more than one isotope, so bromoethane gives two molecular ion peaks rather than one. Reading the fragments is how a mass spectrum moves from a molecular mass to a structure.

Infrared spectroscopy works on the bonds rather than the mass. Every covalent bond behaves like a tiny spring rather than a rigid bar, so it vibrates by stretching and bending, and each mode has its own characteristic frequency. When infrared radiation matches a bond's natural frequency the bond absorbs it, and the spectrometer records the frequencies absorbed as wavenumbers.

Each bond type absorbs within a characteristic range, and the shape matters as much as the position. O–H bonds give broad peaks because of hydrogen bonding, while carbon-oxygen double bonds give sharp, strong peaks, which is enough to tell an alcohol from a carboxylic acid from an aldehyde or ketone. Below 1500 wavenumbers lies the fingerprint region, a lot of small peaks unique to each compound, so comparing it with a database identifies the exact molecule rather than just the functional group. Unexpected peaks also reveal impurities left over from a synthesis.

The same bond vibrations explain global warming. The Earth absorbs short-wavelength ultraviolet radiation from the Sun and re-emits that energy as long-wavelength infrared. Greenhouse gases absorb the infrared by the same mechanism used in the laboratory: carbon dioxide through its two carbon-oxygen double bonds, methane through its four carbon-hydrogen bonds, and water vapour through its two oxygen-hydrogen bonds. The heat is trapped in the atmosphere, and that is the greenhouse effect. Without it the Earth would be too cold to support life, but human activity is raising the levels of these gases, and their absorption of infrared is a key cause of global warming.

When you identify a bond from a spectrum, quote both the bond and its wavenumber range, exactly as printed in the data booklet including the units. A wavenumber on its own, or a bond on its own, does not gain marks. For the fingerprint region, say that you compare the spectrum with a database of known spectra to find an exact match.

On global warming, say that the specific bonds absorb the infrared radiation emitted by the Earth. Do not say the gas absorbs ultraviolet, because that is ozone depletion, and do not say it reflects or blocks infrared. The required word is absorb.

A test-tube reaction names the functional group from a visible change. A mass spectrum gives the molecular mass from the molecular ion peak, and its fragments narrow that down to a structure. An infrared spectrum identifies the bonds present from the wavenumbers they absorb, and the fingerprint region identifies the compound itself. One unknown compound, three techniques, and in practice they are used together, because infrared absorptions can overlap and mass spectrometry confirms what infrared suggests.

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