- Allows us to identify a functional group in a molecule.
- All molecules absorb infra-red radiation; making them either stretch or bend (vibrating).
- Different bonds absorb different wavelengths.
[A TABLE WILL BE ON THE BACK OF THE DATA SHEET WITH SPECIFICS]
- At AS only three peaks are needed to be recognised:
-At 3230 - 3500 (big trough for O-H bond). [alcohols]
-At 1680 - 1750 (peak for C=O bond). [aldehydes and ketones]
-At both (two peaks). [carboxylic acids]
- Must state when answering:
-The wavelength of any peaks.
-The functional group causing the peak.
-Any missing peaks that are significant (i.e. missing O-H trough).
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Infra-Red Spectroscopy (E.g.)
- Graph shows O-H stretch/trough.
- And also C=O stretch.
- (Others surpass AS level).
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- Can be used to:
-Determine abundance of each isotope of an element.
-Identify unknown compounds.
-Gain further knowledge about the structure and chemical properties of a molecule.
- Mass spectra of elements:
-Determines the mass of the isotopes present and abundance, using a m/z ratio (m being mass, z being charge).
-If there are say three peaks, the number of peaks repressents the three isotopes, the height equals the percentage abundance.
- Mass spectra of molecules:
-Used to ionise organic molecules and breaks the ions up into fragments; the positive ion formed is the M+ ion; which is used to identify the unknown molecule.
E.g. CH3OF + e- -> [CH3OH]+ + 2e- (m/z = 32)
-As the molecule is ionised, excess energy is transferred to make it vibrate.
-This causes bonds to weaken and break, causing it to fragment.
E.g. C2H5OH+ -> CH3 + CH2OH+
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Mass Spectrometry (Cont.)
- These fragment ions are broken up even smaller, forming a comparable fingerprint.
- The highest significant m/z value is the M+, the other peaks are fragmentation.
- The small peak to the right of the M+ is due to C-13 ion.
- The rest is manipulating the graph to determine what fragments have occured, so determining whether the molecule is an alkane or alcohol etc.
- N.B. Draw the displayed formula for the M+ and try to split at certain points.
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