Identification of functional groups

Question 1

How do we test for alcohols ie. to see if the alcohol is either primary/ secondary OR tertiary?

Answer 1

• Use acidified potassium dichromate (K2Cr2O7.)
• Primary and secondary alcohols: the potassium dichromate will turn from orange ( Cr₂O₇²⁻: dichromate ion) to green (Cr³⁺ - chromium ion.)
• Tertiary alcohols can’t be oxidised using dichromate, so, acidified potassium dichromate will remain orange!

Question 2

How do we test for an alcohol to see whether it is primary/ secondary alcohol?

Answer 2

• Use fractional distillation to collect product produced on oxidation (of alcohol by dichromate ions.)
• Then test the product to see whether it is an aldehyde/ ketone.
• Aldehyde = primary alcohol
• Ketone = secondary alcohol.

Question 3

How do we test for aldehydes/ ketones using Fehling’s solution?

Answer 3

• Add to aldehyde/ ketone in test-tube placed in hot water bath.
• If aldehyde: Fehling’s solution goes from blue SOLUTION to brick-red PRECIPITATE (Cu2O.)
• Ketones: Fehling’s solution will remain blue.

Question 4

How do we make Tollens’ reagant?

Answer 4

• Silver nitrate solution (in test-tube) = colourless.
• Add few drops of NaOH- pale, brown precipitate forms.
• Add few drops of dilute ammonia until precipitate dissolves.

Question 5

How do we test for aldehydes/ ketones using Tollens’ reagant?

Answer 5

• Add Tollens’ reagant to aldehyde/ ketone and place in hot water bath.
• Aldehydes: Tollens’ reagant reduced to silver precipitate; which coats inside of flask.
• Ketones: no silver precipitate formed.

Question 6

Why do we use a water bath to warm Fehling’s solution/ Tollens reagant when testing for aldehydes/ ketones rather than using a bunsen burner?

Answer 6

• Don’t use bunsen burner as aldehydes and ketones are flammable.

Question 7

What is the test for alkenes?

Answer 7

• Add bromine water to alkene and shake.
• If alkene is present, a colourless solution will be formed.
• Bromine water reacting with C=C double bond, colourless solution = dibromoalkane.

Question 8

What is test for carboxylic acid?

Answer 8

• Add carbonate to the substance in conical flask. Collect gas produced in a separate beaker filled with limewater.
• If substance = c.acid, the limewater will go from clear to cloudy as the CO2 is bubbled through it.

Question 9

Give an example of carbonate that could be used to test for carboxylic acids.

Answer 9

• Solid sodium carbonate.
• Sodium carbonate solution.

Question 10

Why is it important for further investigation to be carried out to prove substance is carboxylic acid rather than just reacting it with a carbonate?

Answer 10

• Because all acids will react with carbonate in same way.
• Further analysis will be needed to prove that it is a c. acid.