organic analysis Flashcards
(21 cards)
give two tests for distinguishing between aldehydes and ketones
● fehling’s solution
● tollen’s reagent
what would you observe when a ketone is added to tollen’s reagent?
warm with tollen’s: no visible change to colourless solution
what would you observe when an aldehyde is added to tollen’s reagent? why? (2)
● warm with tollen’s: colourless to silver mirror (Ag(s) ppt)
● Ag+ ions are reduced to Ag(s)
what would you observe when an aldehyde is added to fehling’s solution?
warm with fehling’s: clear blue solution produces a brick red ppt of copper oxide
what would you observe when a ketone is added to fehling’s solution?
warm with fehling’s: solution remains clear blue
how do you test for alkenes? what is the result? (2)
● shake with bromine water
● positive result: orange to colourless
how do you test for haloalkanes? what is the result? (4)
● add NaOH (aq) and warm (to release halide ion - nucleophilic substitution with OH)
● acidify with HNO3 (to remove impurities)
● add AgNO3 (aq)
● positive result: precipitate of AgX (for Cl = white, for Br = cream, for I = yellow)
how do you test for alcohol? what is the result? (2)
● add acidified potassium dichromate and heat
● positive result: orange to green
● no change for tertiary alcohols
why can’t you use acidified potassium dichromate to distinguish between an alcohol and an aldehyde? (2)
● the alcohol will be oxidised
● and give a colour change from orange to green
how do you test for carboxylic acids? what is the result? (2)
● add sodium carbonate
● result: CO2 given off - effervescence
what information does mass spectrometry provide?
the Mr of a sample
in electron impact ionisation, what does Mr equal?
m/z (the last peak)
in electrospray ionisation, what does Mr equal?
m/z - 1 (last peak - 1)
what does a high resolution mass spectrometer do? (2)
● determines the m/z of a peak to several decimal places
● can distinguish between compounds that have similar Mr but different empirical formulas
what does infrared spectroscopy tell you about?
functional groups
in infrared spectroscopy, what is the region below 1500cm-1 known as? (3)
● the fingerprint region
● unique to every compound
● can compare to a data base / known spectra to look for an exact match
what is the region above 1500cm-1 used to identify?
functional groups
what do broad OH peaks represent? (2)
● fats
● acids
what do smoother OH peaks represent?
alcohols
what do extra peaks in the fingerprint region indicate?
impurities in sample
how do you answer exam questions about ir spec?
● state the range with peaks
e.g.
● the infrared spectrum shows an absorption / peak at about ____ cm-1, which is in the range __ cm-1, showing that a ____ is present
