Qualitative Analysis of Functional Groups
This lesson covers:
- Identifying key organic functional groups
- The positive and negative test results
- The chemical basis for each functional group test
Test for alkenes using bromine water
To confirm the presence of an alkene:
- Add orange bromine water to the sample and shake the mixture. Bromine undergoes an addition reaction with the double bond to form a colourless bromoalkane.
- Decolourisation of the orange bromine water confirms the sample is an alkene.
Test for halogenoalkanes using silver nitrate
To confirm the presence of a halogenoalkane:
- Add aqueous silver nitrate solution and ethanol to the sample and gently warm the mixture. The halide ion is oxidised to form an insoluble silver halide precipitate.
- Observe the colour of the precipitate formed and compare to the known precipitate colours of silver halides.
Halogenoalkane | Precipitate with AgNO3(aq) | Precipitate colour |
---|---|---|
Chloroalkane | AgCl | White |
Bromoalkane | AgBr | Cream |
Iodoalkane | AgI | Yellow |
Test for primary and secondary alcohols using acidified potassium dichromate
To confirm the presence of a primary or secondary alcohol:
- Add acidified orange potassium dichromate solution to the sample. Heat gently for a few minutes. The alcohol undergo an oxidation reaction to form carbonyl compounds.
- If the orange solution turns green, a primary or secondary alcohol was present in the sample.
Tertiary alcohols cannot be oxidised by potassium dichromate so the solution remains orange.
Test for carbonyl compounds using 2,4-DNPH
To confirm the presence of a carbonyl functional group:
- Dissolve 2,4-dinitrophenylhydrazine (2,4-DNPH) in sulfuric acid, water and methanol.
- Add the 2,4-DNPH solution to the sample and shake the mixture. 2,4-DNPH reacts with carbonyls to form an orange precipitate.
- Formation of an orange precipitate confirms the sample is an aldehyde or a ketone.
To identify the specific aldehyde or ketone, you can filter and purify the precipitate by recrystalisation, determine its melting point, and compare to known values.
Test for aldehydes using Tollens’ reagent or Fehling’s solution
Tollens' reagent and Fehling's solution can be used to distinguish between aldehydes and ketones. Aldehydes give positive test results with both reagents whereas ketones give negative results with both reagents.
To identify an aldehyde using Tollens’ reagent:
- Prepare Tollen's reagent, [Ag(NH3)2]+(aq), by mixing solutions of silver nitrate, sodium hydroxide and dilute ammonia.
- Add Tollens’ reagent to the sample and gently warm the mixture. Tollens’ reagent oxidises aldehydes to carboxylic acids as Ag+ ions are reduced to silver metal.
- Formation of a silver mirror inside the test tube confirms the presence of an aldehyde.
Ketones cannot be oxidised further by Tollens’ reagent so solution remains colourless.
To identify an aldehyde using Fehling's solution:
- Add blue Fehling's solution containing Cu2+ ions to the sample. Gently warm the mixture. Fehling’s solution oxidises aldehydes to carboxylic acids as Cu2+ ions ions are reduced to copper(I) oxide.
- Formation of a red precipitate (of Cu2O) confirms the presence of an aldehyde.
Ketones cannot be oxidised further by Fehling’s solution so solution remains blue.
Test for carboxylic acids using sodium carbonate
To confirm the presence of a carboxylic acid:
- Add a small spatula of solid sodium carbonate to the sample. Carboxylic acids react with carbonates to form carbon dioxide gas.
- Bubble any gas produced through limewater in a second test tube. The limewater will turn cloudy in the presence of carbon dioxide.
- If the limewater turns cloudy, a carboxylic acid was present in the sample.
Test for phenols using sodium hydroxide and a carbonate
To confirm the presence of a phenol:
- Add one small spatula of solid sodium hydroxide to a sample. Phenols are weakly acidic and are neutralised by bases to form colourless phenolate salts.
- If the solid sodium hydroxide dissolves to form a colourless solution, the sample may be a phenol.
- To confirm, add a small spatula of sodium carbonate to a separate sample. Carbonates are weaker bases than sodium hydroxide so they do not react with weak acids like phenols.
- A lack of effervescence confirms the sample is a phenol.
Test for CH3CO or CH3CH(OH) using iodine and sodium hydroxide
To confirm the presence of an ethanoate (CH3CO) or secondary alcohol group (CH3CH(OH)):
- Add alkaline iodine solution to the sample and gently warm the mixture. The CH3CO or CH3CH(OH) groups undergo a halogenation reaction, followed by hydrolysis to form a yellow precipitate of triiodomethane.
- Formation of a yellow precipitate (of CHI3) confirms the sample contains either an ethanoate or secondary alcohol functional group.
The yellow precipitate can be filtered, recrystallised, and its melting point compared to the known value for triiodomethane (119°C) to positively identify the presence of a CH3CO or CH3CH(OH) group.
Summary of functional group tests
Here is a summary table for the functional group tests:
Functional group | Reagent(s) | Positive result |
---|---|---|
Alkene | Bromine water | Decolourisation (orange to colourless) |
Halogenoalkane | Silver nitrate | Coloured precipitate formed: white (chloro), cream (bromo), yellow (iodo) |
Primary alcohol | Acidified K2Cr2O7, heat | Orange to green solution |
Secondary alcohol | Acidified K2Cr2O7, heat | Orange to green solution |
Carbonyl (aldehyde or ketone) | 2,4-DNPH | Orange precipitate formed |
Aldehyde | Tollens' reagent | Silver mirror formed |
Aldehyde | Fehling's solution, heat | Red precipitate formed |
Carboxylic acid | Na2CO3(s) | Cloudy limewater |
Phenol | NaOH(s) then Na2CO3(s) | Colourless solution with NaOH, lack of effervescence with Na2CO3 |
Ethanoate or secondary alcohol | Alkaline I2, heat | Yellow precipitate formed (CHI3) |