3.3.6 Organic Analysis

NameFunctional groupTestResult UnsaturationC=CAdd bromine water and shakeDecolourises Carboxylic acidRCOOH Add a metal hydrogencarbonate e.g. NaHCO 3 or Add a small piece of a reactive metal e.g. Mg Effervescence (carbon dioxide produced) Effervescence (hydrogen produced) AldehydeRCHO warm with either Tollen’s reagent or Fehling’s solution Silver mirror or grey/black precipitate of silver Blue solution produces a brick red precipitate of copper (I) oxide KetoneRCOR’ Tollen’s reagen or Fehling’s solution No reaction HalogenoalkanesR-X Warm with sodium hydroxide solution. Then acidify with nitric acid. Then add silver nitrate solution. White precipitate for chloroalkanes Cream precipitate for bromoalkanes Yellow precipitate for iodoalkanes Tertiary alcohol Secondary alcohol Primary alcohol RR’(OH)R’’ R(OH)R’ R-OH Heat under reflux with acidified potassium dichromate As above then distil the product and then test for Aldehyde/ketone No colour change Colour change from orange to green, ketone produced Colour change from orange to green, aldehyde produced

Mass spectrometry When an organic compound is vaporised and driven through a mass spectrometer, some molecules lose an electron each and are ionised. The resulting positive ion is called the molecular ion and is given the symbol M +.

The mass of the lost electron is negligible. The molecular ion has a molecular mass equal to the relative molecular mass of the compound. Molecular ions can be detected and analysed.

High resolution mass spectroscopy can measure the mass to 5 d.p. This can help differentiate between compounds that appear to have similar M r (to the nearest whole number). Precise masses of atoms: 1 H = ; 12 C = (by definition, as it is the standard); 16 O = ; 14 N =

For example, the following molecular formulae all have a rough M r of 60, but a more precise M r can give the molecular formula. M r = molecular formula = C 2 H 4 O 2 M r = molecular formula = C 3 H 8 O M r = molecular formula = CH 4 N 2 O

Infra-red spectrometry All molecules absorb infrared radiation. This absorbed energy makes covalent bonds vibrate more, with either a stretching or bending motion.

Every bond vibrates at its own unique frequency. The amount of vibration depends on: the bond strength the bond length the mass of each atom involved in the bond. Most bonds vibrate at a frequency between 300 and 4000 cm −1 – this is in the infrared part of the electromagnetic spectrum.

The absorbed energies can be displayed as an infrared spectrum. By analysing such a spectrum, we can determine details about a compound’s chemical structure. In particular, the spectrum indicates the presence of functional groups in the compound under investigation.

In a modern infrared spectrometer, a beam of infrared radiation is passed through a sample of the material under investigation. The beam contains the full range of frequencies in the infrared region. The molecules absorb some of these frequencies and the emerging beam is analysed to plot a graph of transmittance against frequency – this is the infrared spectrum of the molecule. The frequency is measured using wavenumbers, with units of cm −1.

An infrared spectrum is a bit like a graph. It is produced by a computer that analyses radiation that passes through completely uninterrupted (i.e. not having passed through any substances) and compares this with the radiation that has been passed through the substance being investigated. Absorption is shown by peaks (the dips!). These show where absorption of the radiation has occurred. These parts of the spectrum indicate to chemists the bonds present in the substance. When it comes to analysing spectra, you may find it helpful to annotate the actual diagram – for example, by labelling individual peaks.

The region below 1500cm -1 is known as the fingerprint region and unique to a compound. This can be compared to a data base to identify the substance. The absorption of infra-red radiation by bonds in this type of spectroscopy is the same absorption that bonds in CO 2, methane and water vapour in the atmosphere that maybe causing global warming.

Infrared spectroscopy has many everyday uses. It is used extensively in forensic science – for example, to analyse paint fragments from vehicles in hit-and-run offences. Other uses that rely on infrared spectrometry include: monitoring the degree of unsaturation in polymers quality control in perfume manufacture drug analysis air pollution monitoring