Unit 3: Section 4 - Organic Analysis
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How do you test for alkenes? What is the result?
• Shake with bromine water Bromine water is decolourised (from orange to colourless)
How do you test for haloalkanes? What are the results?
• Add NaOH (aq) and warm • Acidify with HNO3 • Add AgNO3 (aq) • Result: precipitate of AgX (for 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 1° and 2° alcohols (no visible change for 3°)
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 Tollen’s 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
What does GCMS stand for?
Gas chromatography linked to mass spectrometry
What is high resolution mass spectrometry?
Mass spectrometers which give Mr to 3d.p. or 4d.p.
What does high level 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 3d.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)
What effect do heavier atoms have on the frequency of vibration?
Vibrate slower (with lower frequency)
How does infrared spectroscopy work?
• Every bond has a unique vibration frequency in the infrared region of the EM spectrum • Bonds absorb radiation that has the same frequency as their frequency of vibration • Infrared radiation emerged from a sample is missing the frequencies that have been absorbed -> this can be used to identify the compound’s functional group
What happens inside an infrared spectrometer?
• Beam of infrared radiation with a range of frequencies is passed through the sample • Radiation that emerges is missing frequencies that have been absorbed by the bonds in the sample • Graph is plotted of intensity against frequency of radiation
What do the troughs on an infrared spectrum show?
The frequencies where radiation has been absorbed - match to table to find out which bonds they represent
What is the fingerprint region?
• Area of the infrared spectrum below wavenumber of 1500cm^-1 • Many peaks caused by complex vibrations of the whole molecule • Unique to every compound, so can be used to identify compounds
How is the fingerprint region used to identify compounds?
• The fingerprint region’s pattern is matched to a database on a computer to identify the compound accurately