Module 6: Section 3

Question 1

How do you test for alkenes?

Answer 1

Use bromine water:
Add 2cm^3 of substance you are testing into a test tube.
Add 2cm^3 of bromine water.
Shake.
Solution will decolourise if it is an alkene.

Question 2

How do you test for haloalkanes?

Answer 2

Use silver nitrate.
Add 5 drops of unknown substance to a test tube.
Add 1cm^3 of ethanol and 1cm^3 of AgNO3 (aq)
Observe colour of ppt.
Chloroalkane = white ppt
Bromoalkane = cream ppt
Iodoalkane = yellow ppt

Question 3

How do you test for phenols?

Answer 3

Use sodium hydroxide and a carbonate.
1. Add 2cm^3 of unknown substance to a test tube.
Add 1 small spatula of solid NaOH.
Acidic (Phenol) = solid will dissolve
Not acidic = nothing happens.

2. Add 2cm^3 of unknown substance to a test tube.
   Add 1 small spatula of carbonate.
   Phenol/Base = nothing happens
   Strong acid - effervescence.

Question 4

How do you test for carboxylic acids?

Answer 4

Use sodium carbonate.
Add 2cm^3 of unknown substance to a test tube.
Add a small spatula of solid carbonate.
If the solution fizzes, bubble the gas through limewater.
Carboxylic acid = solution will fizz and CO2 will be produced that turns limewater cloudy.
Not carboxylic acid = nothing happens.

Question 5

How do you test for a carbonyl group?

Answer 5

Use 2,4-DNPH / Bradys Reagant
Add 2,4-DNPH dissolved in methanol to unknown substance.
Shake the test tube.
Aldehyde/Ketone = Orange ppt

Question 6

How do you test for aldehydes.

Answer 6

Use Tollen’s Reagant.
Add silver nitrate solution to a test tube.
Add NaOH to form a light brown ppt.
Add dilute ammonia intil brown ppt disappears
Place test tube in hot water bath and add 10 drops of aldehyde or ketone.
Aldehyde = silver mirror forms as it can be reuced.
Ketone = nothing happens.

Question 7

How do you test for primary and secondary alcohols?

Answer 7

Use acidified potassium dichromate.
Add 10 drops of the alcohol to 2cm^3 of acidified potassium dichromate solution.
Warm the mixture gently.
Primary = orange -> green solution (forms an aldehyde)
Secondary = orange -> green solution (forms a ketone)
Tertiary = nothing happens.

Question 8

How does proton NMR with D2O change?

Answer 8

If an OH or NH proton is present it’ll swap with deuterium and the peak will disappear.