Unit 3: Section 4 - Organic Analysis

Question 1

How do you test for alkenes? What is the result?

Answer 1

• Shake with bromine water
  Bromine water is decolourised (from orange to colourless)

Question 2

How do you test for haloalkanes? What are the results?

Answer 2

• Add NaOH (aq) and warm
• Acidify with HNO3
• Add AgNO3 (aq)
• Result: precipitate of AgX (for Cl = white, Br = cream, I = yellow)

Question 3

How do you test for alcohols? What is the result?

Answer 3

• Add acidified K2Cr2O7 (potassium dichromate(VI)) and heat
  Result: Colour change from orange to green for 1° and 2° alcohols (no change for 3°)

Question 4

How do you test for aldehydes? What is the result? (2 ways)

Answer 4

1. Warm with Fehling’s solution, result: brick red ppt forms from blue solution
2. Warm with Tollen’s reagent, result: “silver mirror” (Ag(s) ppt) forms

Question 5

How do you test for carboxylic acids? What is the result?

Answer 5

• Add Na2CO3(aq)
• Result: CO2(g) given off - effervescence

Question 6

What is mass spectrometry? How does it work?

Answer 6

• 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 moleculesm reach detector and cause a current to flow
• Time of flight used to work out m/z value and plot graph

Question 7

What does the x axis show on a mass spectrum? What does this effectiely show and why?

Answer 7

• Shows m/z value (mass divided by ionic charge)
• Since most ions are 1+, this effectively shows Mr

Question 8

Why are there multiple peaks when molecules are put into a mass spectrometer?

Answer 8

• 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

Question 9

How does gas chromatography work and what does it tell you?

Answer 9

• 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

Question 10

What does GCMS stand for?

Answer 10

Gas chromatography linked to mass spectrometry

Question 11

What might GCMS be used for?

Answer 11

• Powerful chemical analysis - forensic work, measuring water pollution, drug testing on athletes, racehorses

Question 12

What is high resolution mass spectrometry?

Answer 12

Mass spectrometers which give Mr to 3d.p. or 4d.p.

Question 13

What does high level mass spec allow you to do?

Answer 13

Distinguish between compounds that hav 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.

Question 14

Why do atoms and chemical bonds absorb infrared radiation?

Answer 14

They are constantly vibrating - they can absorb infrared radiation that is the same frequency as their freuency od vibration

Question 15

What effect does a stronger bond have on the frequency of vibration?

Answer 15

Vibrate faster (with higher frequency)

Question 16

What effect do heavier atoms have on the frequency of vibration?

Answer 16

Vibrate slower (with lower frequency)

Question 17

How does infrared spectroscopy work?

Answer 17

• 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

Question 18

What happens inside an infrared spectrometer?

Answer 18

• 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

Question 19

What do the troughs on an infrared spectrum show?

Answer 19

The frequencies where radiation has been absorbed - match to table to find out which bonds they represent

Question 20

What is the fingerprint region?

Answer 20

• 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

Question 21

How is the fingerprint region used to identify compounds?

Answer 21

• The fingerprint region’s pattern is matched to a database on a computer to identify the compound accurately