Mass & infrared spectra Chem unit 2 Pt9
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What does the peak at the end of a mass spectrometry graph tell you? What is it called?
- Molecular ion peak (the M peak) - shows the molecular mass of the compound.
- (be careful, as there is small peak to the right of this, which is dues to presence of carbon-13 isotope)
What are the 5 processes involved in mass spectroscopy?
- Vaporisation - heated
- Ionisation - bombard electrons
- Acceleration - electric field
- Deflection - magnetic field
- Detection - electric plate detecting charge (photoelectric methods)
In a mass spectra, why are there many peaks for one compound? What is this called?
- Because the bombarding electrons break some of the molecular ions up into fragments.
- Fragments shown up in the spectrum are called fragmentation pattern.
- You can use these to identify the molecule and their structure.
When a molecule is fragmented in mass spectroscopy by bombarding electrons, what 2 things are created, and what does the spectrometer detect?
- into one free radical and one ion.
- The ions are shown up on spectrum but free radicals don't.
What are some of the common fragments from an organic compound and their mass?
- CH3 - 15
- C2H5 - 29
- C3H7 - 43
- OH - 17
Mass spectroscopy can be used to differentiate between similar molecules. eg. How can it identify whether its propanal or propanone?
- They both have same Mr but different structures so - different fragments!
- eg. Propanal has C2H5 fragment but propanone won't.
Only what molecules can absorb infrared radiation?
- Those that have bonds in molecule that when it absorbs IR, the bending or stretching vibrations causes a change in net dipole moment.
- (eg. O2 or N2 cannot because polarity will not change as it vibrates)
Why is the spectrum from infrared spectroscopy produced?
Different bonds absorb different wavelengths of IR.
What are the factors that influence what wavelength of IR is absorbed by a bond?
- Bond strength
- Bond length
- Mass of each atom involved in bond
- Rest of molecule also has an effect.
What shows up in a infrared spectrum for a carbonyl (C=O) bond?
Strong, sharp absorption around 1700cm-1
Infrared spectrum of alcohol (OH bond)?
Strong, broad absorption around 3200 - 3600cm-1
Infrared spectrum of carboxylic acids?
- Medium, broad absorption between 3200 - 3600 (OH bond)
- Strong, sharp absorption around 1700 (due to C=O bond)
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