Testing For Functional Groups

Question 1

How would you test for an Alkene or alkyne

Answer 1

Bubble gas through, or add liquid to, a solution of bromine in hexane or water.
The orange/brown bromine rapidly decolourises, as a saturated.
R2C=CR2 + Br2 ==> BrR2C–CR2Br
RCCR + 2Br2 ==> Br2RC–CRBr2

Question 2

Test for Hydroxy group R–OH

Answer 2

Mix it with a few drops of ethanoyl chloride, test fumes with litmus and silver nitrate
Litmus turns red and a white precipitate with silver nitrate(aq) forms.
R–OH + CH3COCl ==> CH3COOR + HCl

Question 3

Test for primary alcohol

Answer 3

Distil with potassium dichromate(VI) and moderate conc. H2SO4
solution colour changes from orange to green.
R–CH2OH + [O] => R–CHO + H2O

3R–CH2OH + Cr2O72– + 8H+ ==>

2Cr3+ + 3R–CHO + 7H2O

Question 4

Secondary alcohol

Answer 4

Add ten drops of the alcohol to the 2cm3 of acidified potassium dichromate solution. Warm gently in hot water bath. Orange solution turns green as ketone.

Question 5

Tertiary alcohol

Answer 5

Add ten drops of the alcohol to the 2cm3 of acidified potassium dichromate solution. Warm gently in water bath. Nothing will happy for tertiary.

Question 6

Phenols (OH group is attached directly to aromatic ring)

Answer 6

Add a few drops of iron(III) chloride solution to a little of the phenol in water. Usually gives a purple colour.

Question 7

Carboxylic acids

Answer 7

Mix the carboxylic acid with water and add a little sodium hydrogencarbonate solid or solution. Fizzing, colourless gas gives white precipitate with limewater
RCOOH + NaHCO3 ==> RCOONa + H2O + CO2

Question 8

Phenols (using strong/weak bases)

Answer 8

Add a few drops of iron(III) chloride solution to a little of the phenol in water. Usually gives a purple colour.
Phenol is a weak acid, so it will only react with a strong base (NaOH) not a carbonate.

Question 9

Acid or Acyl Chloride

Answer 9

Add a few drops to water, test with litmus and silver nitrate solution. Litmus turns red and a white precipitate with silver nitrate.
RCOCl + H2O ==> RCOOH + HCl

Question 10

Acid Amide

RCONH2

Answer 10

Boil the suspected amide with dilute sodium hydroxide solution
Ammonia produced on boiling
RCONH2 + NaOH ==> RCOONa + NH3
Litmus paper test for ammonia

Question 11

Aliphatic amines (primary, where R = alkyl)
R–NH2

Answer 11

Dissolve in water and use red litmus paper. Red litmus turns blue.
Makes white clouds with HCl

Question 12

Aromatic amines

Answer 12

Dissolve the primary aromatic amine in dilute hydrochloric acid at 5oC and mix with sodium nitrite solution. It should be a clear solution with few, if any, brown fumes. Because nitrogen is formed.

Question 13

Aldehydes

Answer 13

Tollens’ reagent. (Silver nitrate + ammonia)
Add 5 drops of sodium hydroxide solution to 2 cm3 silver nitrate solution to form a light brown ppt. Add dilute ammonia until precipite dissolves.
Place in hot water bath and add ten drops of aldehyde.
A silver mirror forms on the walls of the test tube.

Question 14

Ketones

Answer 14

Tollens’ reagent. (Silver nitrate + ammonia)
Add 5 drops of sodium hydroxide solution to 2 cm3 silver nitrate solution to form a light brown ppt. Add dilute ammonia until precipite dissolves.
Place in hot water bath and add ten drops of ketone.
Nothing should happen

Question 15

Esters RCOOR

Answer 15

React unknown solution with saturated ethanolic hydroxylamine hydrochloride + 20% methanolic KOH and gently heated until boiling. Then mixture acidified with 1M HCl(aq) and FeCl3(aq) added dropwise.
Deep red or purple colour formed

Question 16

Carbonyl group (aldehydes/ ketone)

Answer 16

Add 2,4 DNP (dinitrophenylhydrazine) to 1cm3 HCl, 2cm3 water and 5cm3 methanol.
Shake unknown solution with this.
(Ketone or aldehyde) bright orange precipitate
Not either - nothing happens