Chapter 17
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What is the molecular ion? How is it written?
• The ion formed when an organic compound loses an electron and forms a positive ion in a mass spectrometer • Is often written as M^+
Show how the molecular ion of propan-1-ol is formed.
• CH3CH2CH2CH2OH -> CH3CH2CH2CH2OH+ + e-
Where is the molecular ion peak found?
• It is the peak the furthest right on a mass spectrum
What peak can sometimes be found beyond the molecular ion peak, and why?
• The M+1 peak • It exists due to the fact that a small percentage of carbon atoms exist as carbon-13 (for example, a small proportion of propanol molecules will contain an atom of carbon-13, and so their molecular mass will be 61 instead of 60)
What is fragmentation?
• The breaking down of molecular ions into smaller pieces known as fragments in a mass spectrometer
What do molecular ions break down into? Give an example.
• In the simplest fragmentation, a molecular ion breaks down into a positively charged fragment ion and a radical • CH3CH2CH2CH2OH+ -> CH2OH+ + ·CH2CH3
What are the other peaks in a mass spectrum caused by?
• Fragment ions
What can mass spectrometry be used for, and how?
• To help identify molecules • From the fragment ions, as these will be different between molecules
What are the m/z of the hydroxide, methyl, ethyl, propyl and butyl fragment ions?
• 17 • 15 • 29 • 43 • 57
How do you show the difference between the m/z of the molecular ion and fragment ions?
• You show the molecular ion as M+, and the rest as m/z • The M+1 peak is also shown as M+1
What kind of motion do covalent bonds naturally have?
• They vibrate around a central point, either by bending or stretching
What does infrared radiation do to covalent bonds?
• It supplies them with more energy, so they bend/ stretch more
What restriction is there on the absorption of IR by covalent bonds?
• Bonds can only absorb radiation of the same frequency as their natural frequency
How is the absorption of IR by covalent bonds used by chemists?
• They use it to identify functional groups in organic molecules
List 3 things to be aware of when interpreting an infrared spectrum.
• The fingerprint region • Broad peaks • C-H peak
What is the fingerprint region?
• The fingerprint region is below 1500/cm, where there are several peaks • However, each molecule has a unique fingerprint region
How can the fingerprint region be used to identify a molecule?
• It can be ran through a database
What is a broad peak?
• A peak that is U-shaped rather than having a sharp point
Which peak is broad (but isn’t shown as broad on the data sheet)?
• OH for alcohols
How do you identify functional groups using infrared spectroscopy?
• First give the wavenumber range/s • Use to identify bond/s present • Using this information you can say the functional group/s (common trick is having an alcohol and a ketone as this can be mistaken for a carboxylic acid)
What is important about a C-H peak?
• It is usually not worth identifying as it is in almost all organic molecules • It is often confused with the OH bond from alcohols
What happens when IR radiation enters the atmosphere?
• It originally passes through the atmosphere and gets to the Earth’s surface, where most of it is absorbed • Some of it is re-emitted as longer wavelength IR radiation
What issue does the re-emission of IR radiation cause?
• Greenhouse gases (water, carbon dioxide and methane) have the same natural frequency as this IR radiation, and therefore absorb it • They then re-emit this radiation, which increases the temperature close to the Earth’s surface (global warming)
What are 2 uses of infrared spectroscopy?
• Breathalysers detect ethanol levels • Remote sensors monitor gases causing air pollution