Identification of functional groups by test-tube reactions

Question 1

A test for an alcohol

Answer 1

Place about 1cm3 of potassium chromate (VI) solution in a test tube.
Add about 5cm3 of dilute sulfuric acid
Add a few drops of the organic compound
Heat in a hot water bath for a few minutes.
Orange -> green equals a primary or secondary alcohol

Question 2

A test for an aldehyde using fehlings

Answer 2

a) In a clean test tube mix together equal volumes of Fehling’s solution A and Fehling’s solution B. The resultant Fehling’s test reagent should be a clear dark blue solution.
b) Add 5 drops of this test reagent to about 1 cm3 of sodium carbonate solution in a test tube containing a few anti-bumping granules and then add about 1 cm3 of ethanal (or propanal) to this same test tube.
c) Warm the test tube gently for approximately two minutes in a beaker half- filled with hot water and then gradually bring the beaker of water to boiling and maintain this temperature for a few minutes.
d) Using the test tube holder, carefully lift the test tube out of the boiling water and allow its contents to stand for several minutes. Record your observations.

Question 3

Result of fehlings test

Answer 3

Red precipitate: aldehyde present

Question 4

A test for an alkene

Answer 4

A test for unsaturation (double bond)

a) To about 2 drops of cyclohexene in a test tube, add about 1 cm3 of bromine water and shake the contents of the tube vigorously from side to side.
b) Record your observations.

Question 5

Result of alkene test

Answer 5

Double c-c bond turns brown bromine water colourless - therefore an alkene

Question 6

A test for a carboxylic acid (simple)

Answer 6

Turns blue litmus paper red

Question 7

A test for a carboxylic acid

Answer 7

a) Place one small spatula measure of solid sodium hydrogencarbonate in a test tube and add to it about 2 cm3 of dilute ethanoic acid.
b) Record your observations.

Question 8

Result of test for carboxylic acid

Answer 8

Carboxylic acids will react with metal carbonates to produce a salt, water and carbon dioxide. Sodium carbonate is as good a choice as any. Effervescence will indicate the production of a gas and bubbling it through limewater will confirm that the gas is carbon dioxide.

Question 9

A test for an aldehyde using tollens

Answer 9

a) Place about 1 cm3 of silver nitrate in a clean test tube
b) Add one drop of sodium hydroxide solution to form a precipitate of silver oxide
c) Add dilute ammonia drop by drop until the brown precipitate just redissolves (this is now tollens reagent)
d) Add a few drops of organic compound
e) Heat in hot water bath for a few minutes

Question 10

Result of tollens test

Answer 10

formation of a silver mirror on the walls of the test tube.