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Part 1 : Modern analytical chemistry
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Analytical techniques Qualitative analysis: the detection of the presence but not the quantity of a substance in a mixture, for example, forbidden substances in an athlete’s blood. Quantitative analysis: the measurement of the quantity of a particular substance in a mixture, for example, the alcohol levels in a driver’s breath, or the toxic metal levels in a sample of river water. Structural analysis: a description of how atoms are arranged in molecular structures, for example, the determination of the structure of a naturally occurring or artificial product.
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Methods studied this year Infrared spectroscopy which is used to identify the bonds in a molecule. Nuclear magnetic resonance spectroscopy which is used to show the chemical environment of certain atoms (hydrogen, carbon, phosphorus and fluorine) in a molecule and so gives vital structural information.
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Example 1
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Example 2
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Example 3
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The electromagnetic spectrum
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Wavelength (λ) : the distance between two successive crests or troughs (in m) Frenquency (ν): the number of waves which pass a point every second (in Hz)
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Absorption and emission spectra
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Number of carbon atoms in the longest chainstem 1Meth- 2Eth- 3Prop- 4But- 5Pent- 6Hex- 7Hept- 8Oct- 9Non- 10Dec- Nomenclature of organic compounds
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Functional groups
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Substituent group
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Ester functional group
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Infrared (IR) spectroscopy Whatever the state of a molecule, the atoms are not fixed (or motionless), their bonds vibrate or bend. Therefore, a chemical bond can be thought as a spring.
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IR spectroscopy IR spectroscopy is the study of absorption of organic species for radiations between 800 and 1000 nm. The infrared spectrum of a species is shown in a graph of the transmittance T (in %) against the wavenumber σ (in cm -1 ). In IR spectroscopy, the frequency of radiation is often measured as number of waves per centimetre (cm -1 ), also called the wavenumber. The spectrum shows bands of absorption : as the vibration of a bond is excited, the molecule absorbs the radiation and therefore the transmittance falls, producing a peak or a band of absorption.
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Matching wavenumbers with bonds
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Different parts of an IR spectrum From 4000 to 1400 cm -1 the spectrum shows the characteristic bands or peaks of the bonds. From 1400 to 500 cm -1, this part, more complex, is typical of the compound and is often named as the « fingerprint » of the molecule.
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Structure of propanone
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Identification of an organic compound The formula of an organic molecule M is C 4 H 8 O. We know that the molecule is not a cyclic one. Which are the possible functional groups corresponding to the presence of one oxygen atom in the M molecule? Prove the presence of a double bond in the molecule, either between two carbon atoms or between one carbon atom and one oxygen atom. Justify using the octet rule.
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The IR spectrum of the molecule is shown below: Which extra information does this spectrum give us? Write down the structural formulae of the three possible molecules. The molecule comes from the oxidation of a second class alcohol. Conclude.
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