KK1. qualitative tests - carbon-carbon double bonds, hydroxyl and carboxyl functional groups

CARBON-CARBON DOUBLE BONDS

bromine water test

• Br₂ Used as a reagent to test for in organic compound

• Can be used to determine whether a compound contains double or triple bonds

• If organic compound is unsaturated, it will react with the bromine water to turn from light brown/orange to colourless

  • This is due to the formation of a colourless dibromoalkane

•  Test is specific for unsaturation and does not provide information about the type or location of the unsaturated bonds

Potassium permanganate test

• Used a reagent to test for the presence of unsaturated organic compounds

• Using alkaline KMnO_4(aq) involves the reduction of purple coloured KMnO_4(aq) to a dark green solutions to a brown precipitate

• If the organic compound is unsaturated, the purple KMnO_4(aq)  with be gradually decolourised and/or turned into a brown precipitate

• Brown precipitate formed in alkaline conditions is manganese dioxide

• Not specific for a type of unsaturation. Only indicates the presence of double or triple bonds in the organic compounds

HYDROXYL GROUPS

Acidified potassium dichromate test

• Adding acidified potassium dichromate (K₂Cr₂O₇/H⁺) can be used to qualitatively determine whether a primary or secondary alcohol is present in a solution

• When Cr^6+_(aq) ions are reduced to form Cr³⁺ ions, the solution turns from orange to green in the presence of a primary or secondary alcohol

• Will also show a positive result with aldehydes but not ketones

Lucas test

• Used to qualitatively determine whether a primary, secondary or tertiary alcohol is present

• When mixed with a Lucas solution (concentrated hydrochloric acid and zinc chloride), forms a cloudy liquid if alcohol is present

Acidified potassium dichromate followed by the silver mirror test

• If a small amount of acidified potassium dichromate is added to an excess of a primary alcohol, partial oxidation occurs to produce an aldehyde

• Silver mirror test (Tollens' test) is mixing the suspected oxidised primary alcohols with Tollens' reagent (ammoniacal silver nitrate solution)

• If aldehyde is present then a reduction reaction occurs and a silver mirror-like substance is formed

• Silver mirror test will show a positive result with aldehydes

  • Produced from the partial oxidation of primary alcohols

• Silver mirror test will show a negative result with ketones

  • Produced from the oxidation of secondary alcohols

CARBOXYL GROUPS

Acid-base indicator test

Indicator changes colour from its basic form to acidic form in the presene of carboxyl groups

Reaction with metal carbonates or metal bicarbonates

• When carboxyl groups react with carbonates or bicarbonates to produce carbon dioxide gas, which can be observes as effervescence or bubbling

• Type of neutralisation reaction with the carboxylic acid reacts with a base to form a salt, water and carbon dioxide

• Adding a solution of sodium carbonate or bicarbonate to the unknown compound has help identify the presence of a carboxyl group

Esterification test

• Unknown compound + alcohol +strong acid → ester

• Sweet, atomic smell of the ester can imply the presence of a carboxyl group

• May not provide a definite identification of the compound, only the presences of the functional group

• Additional tests such as spectroscopic analysis are required for a more accurate identification of specific compounds