Organic Analysis Flashcards
How do you test for alkenes? What is the result?
Shake with bromine water, result is bromine water is decolourised (orange to colourless)
How do you test for haloalkanes? What is the result?
Add NaOH (aq) and warm, acidify with HNO3, add AgNO3(aq).
Result: precipitate of AgX (for Cl=white, for Br=cream, for I=yellow)
How do you test for alcohols? What is the result?
Add acidified K2Cr2O7 (potassium dichromate(VI)) and heat.
Result: colour change from orange to green for primary and secondary alcohols (note: no change for 30 alcohols).
How do you test for aldehydes? What is the result? (2 ways)
- Warm with Fehling’s solution, result: brick red ppt forms (from blue solution).
- Warm with Tollens’ reagent, result: “silver mirror” (Ag(s) ppt) forms.
How do you test for carboxylic acids? What is the result?
Add Na2CO3(aq), result: CO2(g) given off - effervescence
What is mass spectrometry? How does it work?
Used to find the relative molecular masses of organic compounds.
Compound is dissolved in solution, ionised by a high voltage supply (to mostly 1+ ions), accelerated by a negatively charged plate, becomes a beam of ionised molecules, reach detector and cause a current to flow. Time of flight used to work out m/z value and plot graph.
What does the x axis show on a mass spectrum? What does this effectively show and why?
Shows m/z value (mass divided by ionic charge). Since most ions are 1+, this effectively shows Mr.
Why are there multiple peaks when molecules are put into a mass spectrometer?
The molecular ion is shown as a peak, but it will
also FRAGMENT into smaller molecules, so these peaks are shown as well.
Also, due to isotopes of atoms, different peaks may be seen.
How does gas chromatography work and what does it tell you?
A stream of gas carries a mixture of vapours through a column packed with solids, different compounds move through at different speeds, so they are separated. The amount of each compound can then be measured.
What does GCMS stand for?
Gas chromatography linked to mass spectrometry
What might GCMS be used for?
Powerful chemical analysis - forensic work, measuring water pollution, drug testing on athletes, racehorses.
What is high resolution mass spectrometry?
Mass spectrometers which give Mr to 3d.p. or 4d.p. are called high resolution (low resolution is to nearest whole number)
What does high resolution mass spec allow you to do?
Distinguish between compounds that have the same Mr to the nearest whole number, but are made up of different atoms and therefore have different values of Mr to 3.d.p
Why do atoms and chemical bonds absorb infrared radiation?
They are constantly vibrating - they can absorb infrared radiation that is the same frequency as their frequency of vibration.
What effect does a stronger bond have on the frequency of vibration?
Vibrate faster (with higher frequency)
