Practical Methods

Question 1

Describe a Recrystallisation

Answer 1

1) dissolve compound in a minimum volume of warm solvent to create a saturated solution
2) allow solution to cool so the pure product crystallises out
3) use vacuum filtration to seperate the pure solid product
4) wash all equipment and the solid with distilled water and filtrate to ensure all impurities are removed and the yield is maximised

Question 2

Describe separation with a separation funnel

Answer 2

1) transfer impure solution to a separating funnel and shake well
2) leave to rest until the two distinct layers separate
3) tap off the bottom layer if it is aqueous discard it if it is organic transfer it to a new clean separating funnel
4) wash the remaining organic layer through with distilled water and repeat steps 2 and 3
5) add a drying agent such as anhydrous MgSO4 to the organic layer once again removing any aqueous layers
6) distil the remaining organic layer at the boiling point of the pure product in order to separate the pure dry organic product.
6)

Question 3

Determining melting points (electric heater)

Question 4

Determining melting points (oil bath or Thiele tube )

Question 5

TLC (thin layer chromatography)

Question 6

Gas Chromatography

Answer 6

stationary phase: a high BP liquid on an inert solid support along a capillary column
Mobile phase: inert gas such as helium
1) volatile gas is injected into the carrier gas which flows past the stationary phase
2) the gas interacts with the stationary phase causing it to slow down
3) compounds more soluble in the stationary phase slow down more
4) thus compounds seperate based on solubility and reach the detector at different times
5) the time for the gas to pass through the column is it s retention time

Question 7

Chemical test for unsaturated hydrocarbons

Answer 7

Bromine water
decolourises

Question 8

Chemical test for haloalkanes

Answer 8

silver nitrate and ethanol in 50C water bath
Cl: white precipitate dissolves in dilute ammonia
Br: cream precipitate dissolves in concentrate ammonia
I : yellow precipitate insoluble in ammonia

Question 9

chemical test for carbonyls

Answer 9

2,4-DNPH
orange precipitate
melting points of organic DNPH derivative compounds are determined and compared to a data base to identify specific carbonyls

Question 10

Chemical test for aldehyde

Answer 10

Warm Tollens
silver mirror forms

Question 11

Chemical test for primary or secondary alcohols

Answer 11

acidified potassium dichromate in hot water bath
orange to green colour change

Question 12

Chemical test for carboxylic acid

Answer 12

Na2CO3(aq)
effervescence of gas

Question 13

Chemical test for Phenol

Answer 13

Bromine water
bromine decolourises and a white precipitate forms

Question 14

Solvents for NMR

Answer 14

must be Deuterated to not show up on NMR
CDCl3 (deuterated chloroform)

Question 15

Standard reference for NMR

Answer 15

TMS (tetramethyl silane ) [Si(CH3)4]

Question 16

interpreting Gas Chromatography

Answer 16

retention times can be compared to known values to identify compounds
relative area under peaks give the concentrations of compounds in the solution

Question 17

how are is concentration determined from peak area in GC

Answer 17

1) different concentrations of standard solutions of know compounds are passed through gas chromatography
2) relative peak areas of these standard solutions are plotted agains concentration to give a calibration curve

3) relative area from unknown concentration are compared to the calibration curve to determine concentration