Chapter 16 - Organic Analysis
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how can you test for alkenes
add bromine water brown to colourless when alkene present
how can you test for tertiary alcohols
add acidified potassium dichromate if nothing happens (remains orange) then tertiary alcohol present
why does acidified potassium dichromate remain orange when a tertiary alcohol is added
because acidified potassium dichromate is an oxidizing agent, and tertiary alcohols cannot be oxidised.
how can you distinguish between aldehyde and ketones
add tollens reagent silver mirror forms for aldehydes or add Fehling’s solution brick red precipitate forms if aldehyde present
how can you test for a carboxylic acid
add sodium carbonate effervescence means acid is present
what is low resolution mass spectrometry useful for and why
for testing between samples of different substances because it isnt as expensive as high res spectrometry and still gives accurate results to 1dp
what is high resolution mass spectrometry useful for and why
for testing between samples of substances with different molecular formulae but same MR it gives the precise m/z ratio to 5dp
why are fragment peaks often bigger than the actual sample peaks
because fragmentation of molecules occurs very often leading to a greater ratio of fragments to whole molecules
what leads to fragmentation
when energetically unstable molecular ions dissociate
what is a molecular ion
a molecule with a single positive charge
how can mass spectra with fragmentation be useful
analysis can provide useful information about the structure of the molecule
how can you determine the m/z ratio of the whole molecule form a fragmentation spectrum
the highest m/z ratio is of the whole unfragmented molecule, not highest peak
whats the mr of a CH3+ fragment
15
whats the mr of a O+ fragment
16
whats the mr of a OH+ fragment
17
whats the mr of a CN+ fragment
26
whats the mr of a C2H3+ fragment
27
whats the mr of a C2H4+ or CO+ fragment
28
whats the mr of a C2H5+ or CHO+ fragment
29
whats the mr of a C3H7+ or C2H3O+ fragment
43
whats the mr of a C4H9+ or C3H5O+ fragment
57
whats the mr of a C5H11+ or C4H7O+ fragment
71
why do molecules absorb some IR light
because when their bonds vibrate at the same frequency as the wave, they absorb it this doesnt apply to molecules made of one element
how does IR spectroscopy work
a beam of IR is passed through a sample cell (with the sample in it) and a reference cell (with nothing in it). as the IR passes through the sample, some wavelengths of IR are absorbed by the bond’s vibrations so that the IR that comes out has lower transmittance for certain wavelengths the exiting waves are detected, and spectrums are produces which have peaks for the wavelengths have been absorbed