ࡱ> Y[Xq` bjbjqPqP 84::fffffff,D&> $Th`CfCffX___ff___ff_ 0GoF_yn0_L__&f_CCI$  zD,ffffff 2.12 Mass spectra and IR 2.12a Mass spectra The peak in the spectrum on the far right has the highest value of m/e and is called the molecular ion. This peak gives the Relative Molecular Mass of a compound. Below ethanol can be seen to have a relative molecular mass of 46.  INCLUDEPICTURE "http://www.drbateman.net/asa2sums/sum1.1/sum1.12.gif" \* MERGEFORMATINET  Some organic molecules (e.g. Ethanol ) are fragmented in a mass spectrometer. These fragments (which are parts or fragments of the molecular ion) can be seen in the peaks. The pattern of fragments can be used to work out the structure of original molecule. Above ethanol can be seen to contain a CH3 group, a C2H5 group and an OH group. An organic compound produces ions in a mass spectrometer. The ions generate pulses of electric current which are sent as signals to a computer (or chart plotter) to be displayed as a mass spectrum. On the spectrum the large peak on the right is the parent molecular ion and this indicates the relative molecular mass of the compound. In the spectrometer the molecules are fragmented into positive ions which form a pattern which depends on the structure of the molecule. e.g. ether  INCLUDEPICTURE "http://www.drbateman.net/asa2sums/sum5.5/sum5.51.gif" \* MERGEFORMATINET  Another compound of relative molecular mass 46 also contains carbon, hydrogen and oxygen has the mass spectrum below. Identify each fragment and the structural formula.  INCLUDEPICTURE "http://www.drbateman.net/asa2sums/sum5.5/new_pa2.gif" \* MERGEFORMATINET  Sketch the mass spectrum of propanal and propanone showing the masses of each fragment.  HYPERLINK "http://www.drbateman.net/asa2sums/sum5.5/task5.5aiiia.htm" Task 2.12a 2.12b Interpreting simple infra-red spectra The bonds in organic molecule absorb infra-red radiation. This happens when the frequency of the radiation matches the natural frequency of vibrations in the bonds. These might be stretching, or bending vibrations. A spectrometer shines infra-red light at a sample of an organic material and measures how much of the light is absorbed. A measure of the frequency (wavenumber) is displayed in the spectrum. Each bond has its own frequency (wavenumber) and this can be used to identify the bonds present in a compound. Bond stretching absorptions inwavenumber/cm-1 seen on spectrumC-H alkanes, alkenes, aldehydes2840 - 3095C=O aldehydes and ketones1680 - 1750C-Cl halogenoalkanes500 -1400O-H alcohols and carboxylic acids3200 - 3750N-H amines3300 - 3500Infra-red spectrum for propanone  INCLUDEPICTURE "http://www.rod.beavon.clara.net/propanoneIR.gif" \* MERGEFORMATINET  See p 234/235 Edexcel AS Chemistry , Fullick and McDuell for spectra for ethanol and oxidation products ethanal and ethanoic acid.  HYPERLINK "http://www.drbateman.net/asa2sums/sum5.5/task5.5aiiib.htm" Task 5.5aiiib 2.12c Molecule polarity and IR absorbtion A molecule must change its polarity as it vibrates to absorb IR radiation. Both dipoles in molecules and light are electromagnetic in nature. When a molecule absorbs infrared radiation, there is an interaction between a dipole moment set up during its vibration, and the electric dipole moment of light. A molecule can have vibrations that give a dipole if a molecule is bent, has more than two atoms, or has two different atoms. 2.12d identify greenhouse gases from formulae O2 and N2 do not absorb IR as they cannot form dipoles. all are permanent dipoles or form dipoles during bond vibrating so absorb IR. Task 2.12d Draw charge distribution diagrams for H20, CO2, NO, CH4 and O3 when they can absorb IR. 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