Mass spectra an IR

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  • Created by: danni :)
  • Created on: 29-05-13 12:03

1. Which of the following equations for the fragmentation of ethanol, CH3CH2OH, caused by collision with a high-energy electron, e−, would give a peak at m/e = 15 in its mass spectrum?

  • CH3CH2OH + e− → CH3+ + •CH2OH + 2e−
  • CH3CH2OH + e− → CH3• + +CH2OH + 2e−
  • CH3CH2OH → CH3+ + +CH2OH + 2e−
  • CH3CH2OH → CH3• + ·•CH2OH
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2. It is suggested that IR spectroscopy can be used to show whether an alcohol has been oxidised to an aldehyde or to a carboxylic acid by comparing the IR spectra of the alcohol and its oxidation product. Which of the following statements is true?

  • You cannot tell to what the alcohol has been oxidised because these compounds do not absorb in the IR part of the spectrum.
  • The alcohol has been oxidised to an aldehyde if the broad O–H absorption at about 3300 cm–1 in the alcohol spectrum is replaced by a narrow C=O absorption at about 1700 cm–1 in the product spectrum
  • The alcohol has been oxidised to a carboxylic acid if it now has a broad C=O absorption at about 1700 cm–1 and has a narrow O–H absorption at 3300 cm–1 in the product spectrum.
  • You cannot tell whether the alcohol has been oxidised to an aldehyde or a carboxylic acid because both give an IR absorption at around 1700 cm–1.

3. Gases that absorb IR are

  • Inert gases
  • Greenhouse gases
  • Noble gases

4. Deuterium oxide is water that contains the stable hydrogen isotope 2H, usually shown as D. When dissolved in deuterium oxide, ethanol undergoes the following reaction: CH3CH2OH + D2O → CH3CH2OD + HOD The best way of showing this reaction has occured

  • is by determining the mass spectrum of the mixture and showing peaks at m/e values of 19 and 47
  • is by fractionally distilling the mixture and determining the amount of CH3CH2OD using a Geiger counter
  • is by determining the mass spectrum of the mixture and showing peaks at m/e values of 20 and 46
  • is by testing the product with phosphorus pentachloride and not getting steamy fumes

5. molecules that absorb IR

  • have bonds that can change polarity
  • have non-polar bonds
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