3.6 Organic analysis Flashcards
How do you test for alkenes
What is the result
Shake with bromine water
Bromine water is decolourised
Orange - colourless
How do you test for halogenoalkanes
What is the result
Add NaOH(aq) and warm, acidify with HNO3
Add AgNO3(aq)
Result: ppt of AgX
Cl= white
Br=cream
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
No change for tertiary 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 spec 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, accelerated by a negatively charged plate, becomes a beam of ionised molecules, reach detector and cause a current to flow. TOF 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
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 3dp or 4dp 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, but are made up of different atoms and therefore have different values of Mr to 3dp
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)
