Alcohols and Spectroscopy

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Alcohols

  • An alcohol is an alkane in which one hydrogen is replaced by the OH functional group.
  • A primary alcohol is an alcohol where the carbon atom attached to the OH group is attached to 1 carbon atom.
  • A secondary or tertiary alcohol is where it is attached to 2 or 3 carbon atoms respectively. 
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Alcohols (cont)

  • Alcohols react with solid sodium to form hydrogen gas and a solid sodium oxide.
  • The Na substitutes for the alcoholic hydrogen.
  • You see effervescence, the sodium disappearing and a new solid forming
  • Example of sodium oxide is sodium propoxide (from propan-1-ol). This is ionic.
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Alcohols (cont)

  • When you react primary, secondary or tertiary alcohols with a halogen atom, a halogenalkane forms.
  • All alcohols react with phosphorus (V) chloride (solid), PCl5, to form the liquid chloroalkane, HCl gas and POCl3 (liquid).
  • All alcohols react with concentrated HCl (aqueous) to form the chloroalkane and water.
  • Both are substitution reactions. 
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Alcohols (cont)

  • Complete combustion of an alcohol forms carbon dioxide and water vapour.
  • This is the simplest oxidation reaction of alcohols. 
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Alcohols (cont)

  • For all the following oxidation reactions, the oxidising agent is acidified potassium dichromate.
  • As the dichromate ion (Cr2O7-) is reduced to chromium ion (Cr3+), it changes colour from orange to dark green.
  • The acid used to acidify the solution is sulphuric acid
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Alcohols (cont)

  • Primary alcohols that are oxidised and distilled immediately form aldehydes.
  • Dilute sulphuric acid should be used to acidify solution
  • Aldehydes form a red precipitate when boiled with Benedict's or Fehling's solution. 
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Alcohols (cont)

(http://wiki.chemprime.chemeddl.org/images/e/e6/Distillation_Apparatus.jpg)

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Alcohols (cont)

  • Primary alcohols that are oxided and heated under reflux before being distilled are oxidised further to form carboxylic acids.
  • Concentrated sulphuric acid should be used to acidify solution.
  • Carboxylic acids will neutralise large volumes of sodium carbonate solution. 
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Alcohols (cont)

                                   (http://upload.wikimedia.org/wikipedia/en/thumb/5/51/Reflux_labled.svg/200px-Reflux_labled.svg.png)

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Alcohols (cont)

  • When secondary alcohols are oxidised they always form ketones, which cannot be oxidised further.
  • Ketones form a yellow/orange precipitate with Brady's reagent. 
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Mass Spectra and Infrared Absorbtion

  • The mass spectrum of an organic compound tells us 2 main things.
  • The tallest peak, from the molecular ion, tells us the relative molecular mass of the compound.
  • The other peaks are from smaller fragement ions.
  • These ion peaks indicate likely functional groups.
  • A molecule can lose each part consecutively from either side to form an ion. Ethanol can lose the -H,     -OH, or -C2OH. It can also lose the CH3 from the opposite side
  • So there is a total of 5 peaks in the mass spectrum. 
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Mass Spectra and Infrared Absorbtion (cont)

  • Molecules absorb infrared radiation by vibrating with increased kinetic energy.
  • Different bonds have different vibrational frequencies, which depend on the bond strength.
  • A specific IR is required to make a certain bond vibrate, and can be seen in an absorbtion spectrum.
  • The unit of measurement for absorbance is wavenumber, a receprical of wavelength. 
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Mass Spectra and Infrared Absorbtion (cont)

  • Only molecules that change polarity as they vibrate (due to dipole movement in polar bonds) absorb IR.
  • So molecules without a dipole (oxygen and nitrogen) do not absorb IR.
  • Only molecules that can absorb IR are greenhouse gases
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