3.6 Organic Analysis

Question 1

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

Answer 1

Shake in mixture with bromine water
Positive test is orange -> colourless

Question 2

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

Answer 2

*Add NaOH (aq) and warm (forms alcohol and X- ion)
*Add HNO3 and AgNO3
*Result : precipitate of AgX
*Chlorine is white, Bromine is cream, Ionine is yellow.

Question 3

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

Answer 3

•Add acidified K2Cr2O7 and heat
•Positive result is colour change from orange to green
•No change for tertiary alcohols

Question 4

What two methods are used to test for aldehydes ?

Answer 4

•Warm with Fehlings solution, positive is blue to red brick ppt
•Add equal volume of Tollens Reagent, positive result is a silver mirror forming.

Question 5

How do you test for carboxylic acids?

Answer 5

Add Na2CO3, positive result is CO2 being released (effervescence).

Question 6

What is mass spectrometry ? How does it work ?

Answer 6

•Used to find relative molecular masses of organic compounds
•Compound is dissolved in solution, ionised by high voltage to form +1 ions
•It is then accelerated by a negatively charged plate
•When they reach the plate/detector they cause a current
•Time of flight is used to work out m/z value.

Question 7

What does the x axis show on a mass spectrometry graph

Answer 7

The m/z ratio, since most ions are +1 it is effectively Mr.

Question 8

Why are multiple m/z peaks produced when a molecule is put into a mass spectrometer ?

Answer 8

•The molecular ion is shown as a peak, but it fragments into smaller molecules which are also shown as peaks
•Also, due to the isotopes of atoms, different peaks are seen.

Question 9

How does gas chromatography work ?

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 are separated
•The amount of each compound can then be measured